Database store for a virtual heap6957237Abstract A database store method and system for a virtual persistent heap may include an Application Programming Interface (API) that provides a mechanism to cache portions of the virtual heap into an in-memory heap for use by an application. The virtual heap may be stored in a persistent store that may include one or more virtual persistent heaps, with one virtual persistent heap for each application running in the virtual machine. Each virtual persistent heap may be subdivided into cache lines. The store API may provide atomicity on the store transaction to substantially guarantee the consistency of the information stored in the database. The database store API provides several calls to manage the virtual persistent heap in the store. The calls may include, but are not limited to: opening the store, closing the store, atomic read transaction, atomic write transaction, and atomic delete transaction. Claims 1. A method for managing a virtual heap for a process executing within a virtual machine executing within a device, the method comprising: Description BACKGROUND OF THE INVENTION
BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which: FIG. 1a is a block diagram illustrating a device with virtual persistent heap and persistent store space located on the device according to one embodiment of the invention; FIG. 1b is a block diagram illustrating a device with virtual persistent heap and persistent store space located external to the device according to one embodiment of the invention; FIG. 1c is a block diagram illustrating a device with virtual persistent heap on the device and persistent store space located external to the device according to one embodiment of the invention; FIG. 1d is a block diagram illustrating a client device, proxy server, and server with persistent store space according to one embodiment of the invention; FIG. 1e is a block diagram illustrating a virtual heap and leases to local and remote resources according to one embodiment of the invention; FIG. 1f is a block diagram illustrating application virtual heaps and leases to system resources according to one embodiment of the invention; FIG. 2 is a block diagram illustrating virtual persistent heap architecture according to one embodiment of the invention; FIG. 3 is a state diagram illustrating the states of a page in a virtual persistent heap according to one embodiment of the invention; FIG. 4 is a flowchart illustrating a computation method for the in-memory heap page addresses according to one embodiment of the invention; FIG. 5a is a block diagram illustrating an application migration process with a stored state from a first process sent from a persistent store to a second process according to one embodiment of the invention; FIG. 5b is a block diagram illustrating an application migration process with a persistent store for each process according to one embodiment of the invention; FIG. 6 is a flowchart illustrating a method for migrating an application according to one embodiment of the invention; FIG. 7 is a block diagram illustrating virtual persistent heap architecture using cache lines according to one embodiment of the invention; FIG. 8 is a flowchart illustrating a computation method for the in-memory heap cache line addresses according to one embodiment of the invention; FIG. 9 is a block diagram illustrating a device with virtual heap, object nursery and garbage collector according to one embodiment of the invention; FIG. 10a is a flowchart illustrating garbage collecting a virtual heap according to one embodiment of the invention; FIG. 10b is a flowchart illustrating the processing of a nursery region in a virtual heap according to one embodiment of the invention; FIG. 10c is a flowchart illustrating garbage collection performed on one or more regions of a heap according to one embodiment of the invention; FIG. 11a is a flowchart illustrating an atomic read transaction from a persistent store for a process; FIG. 11b is a flowchart illustrating an atomic write transaction to a persistent store for a process; and FIG. 11c is a flowchart illustrating an atomic delete transaction from a persistent store for a process. Item numbers for objects in the Figures may be repeated in more than one Figure to signify objects that are substantially similar in the Figures. While the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are herein described in detail. It should be understood however, that drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS FIG. 1a—A device with virtual persistent heap on the device FIG. 1a illustrates an embodiment of a device 140 with virtual machine 101 and a virtual heap with persistence, referred to as a virtual persistent heap. In a virtual persistent heap, the entire heap may be made persistent. The virtual persistent heap may enable the checkpointing of the state of the computation of the virtual machine to a persistent storage such as a disk or flash device for future resumption of the computation at the point of the checkpoint. The Virtual Persistent Heap also may enable the migration of the virtual machine computation states from one machine to another. Both the data and computation state may be migrated. One embodiment may also provide for the suspension and resumption of an application, such as upon restarting a device after an intentional or unintentional shutdown of the device. In FIG. 1a, device 140 includes a client 101 and memory 115. Client device 140 may be a computer platform with operating system, such as a PC or laptop computer running an operating system such as Microsoft Windows 9x/NT, or a consumer or appliance device, for example, a cell phone or PDA. Client device 140 may include a service provider client, for example, a Jini client (not shown) for finding and leasing services on remote servers on a network. Client 101 may be a virtual machine such as a JVM or KVM. Client 101 may be used for running applications, for example, Java applications. One or more applications may be running on client 101, with one application typically executing and one or more applications suspended. Application 104 is shown as a currently executing application. Memory 115 may be integrated in or directly attached to client device 140. Memory 115 may be a volatile memory such as Direct Inline Memory Modules (DIMMs) or non-volatile storage device such as a flash memory, a hard disk, or removable disk such as a floppy disk. This embodiment may use persistent store space 120 in memory 115 to store the virtual heap 110 for application 104. Persistent store space 120 may also include virtual heaps (not shown) for one or more other suspended applications. Device 140 may comprises an operating system capable of executing the software to enable a virtual machine such as a JVM or KVM. The operating system may include a virtual memory manager (VMM) for managing virtual memory on device 140. The VMM may enable applications such as a virtual machine running on the device 140 to appear to have more physical memory than is actually present on the system by enabling virtual memory. The VMM may utilize storage such as a disk drive to set up a swap area. Sections of memory may be cached into a cache area in the physical memory for faster access by an application running on the device 140. Sections of memory may be flushed to the swap area on the storage when not actively in use by an application or to make room in physical memory for other sections of memory that may be in more immediate demand for direct access by an application. The sections of memory in the virtual memory on the device may be referred to as the heap for the application. Thus, a virtual machine running on device 140 may run on a heap in the virtual memory on the device. The virtual machine may execute in a portion of the memory space managed by the operating system on the device 140. In one embodiment, the memory space may be a virtual memory space managed by a VMM for the operating system. The virtual machine may comprise a virtual machine memory space for use by processes executing on the virtual machine. As used herein, "process" may refer to, but is not necessarily limited to: applications, applets, programs, tasks, subprocesses, threads, and drivers. The virtual machine memory space may be managed by a virtual machine virtual memory manager (VM VMM) as described herein. The VM VMM may allow processes executing on the virtual machine to use a virtual heap as described herein, and may also provide persistence for the virtual heap. The virtual heap may include an in-memory heap as described herein, which may reside in the virtual machine memory space. The virtual heap may also include a store heap as described herein. In one embodiment, the store heap may be resident in the virtual machine memory space. In another embodiment, the store heap may be resident in memory external to the virtual machine, such as on a storage device attached to the device 140 directly or via the Internet, but accessible using the VM VMM. The entire memory space, including the virtual machine memory space and the store heap, may be referred to as the virtual machine virtual memory space. The VM VMM as described herein may allow applications to execute on a virtual machine on device 140 that would otherwise require too much virtual machine memory space, and thus would not execute, by providing a virtual machine virtual memory space and a virtual heap for the application. The VM VMM and virtual heap as described herein may also allow multiple applications to run on a virtual machine by providing a store heap for each application, and allowing an application to be suspended by flushing its in-memory heap to its store heap, and a second application to be resumed by loading its in-memory heap into the virtual machine memory space from its store heap. As used herein, the definition of a heap may include an area of computer main storage (memory) that a process may use to store data in some variable amount that wont be known until the program is running. A heap may be reserved for data that is created at runtime, that is, when a program is executing. A heap may also include portions of code for the process. In one embodiment, the process may be a Java process, and the code may be in Java classes. For example, a program may accept different amounts of input from one or more users for processing and then do the processing on all the input data at once. The heap may be pre-allocated for the process to use. The process manages its allocated heap by requesting a portion of the heap when needed, returning the portions when no longer needed, and doing occasional "garbage collecting," which makes portions of the heap available that are no longer being used and also may reorganize the available space in the heap to minimize fragmentation of the memory. In one embodiment, Java classes, including Java classes containing code for the process, may be garbage collected to remove classes (and thus process code) that are no longer in use. A heap may be portioned into blocks, sectors, pages, cache lines, or any other division of memory that meets the requirements of the heap management method. A "stack" may be defined as an area of memory that may be used for data whose size can be determined when the program is compiled. A stack may be managed substantially similarly to a heap except that portions of the stack may be taken out of storage in a certain order and returned in the same way. Client device 140 may also include volatile memory for loading and running client 101. Client 101 then may load and run applications in its memory space. In-memory heap 108 may be maintained in client 101 memory space, or alternatively may be maintained in memory external to client 101 memory space. In-memory heap 108 may include a portion of virtual heap 110 currently cached for use by application 104. In-memory heap 108 and virtual heap 110 may be divided into one or more sections. The sections may be sectors, pages, cache lines, or any other division of memory space. In-memory heap 108 may be used by application 104 for storing data and code currently being used in the execution of application 104. In-memory heap 108 may include a portion or all of virtual heap 110. If application 104 requires code or data that is not currently in heap 108, one or more sections of virtual heap 110 may be copied into in-memory heap 108. If there is insufficient room in in-memory heap 108, one or more sections of in-memory heap 108 may be removed from heap 108 before copying in the new sections. If one or more sections being removed from heap 108 are "dirty" (have been written to), the one or more dirty sections may be written to (flushed to) virtual heap 110 before being removed from in-memory heap 108. Application 104 may create new code and data in sections of in-memory heap 108. The new code and data in in-memory heap 108 may be flushed to virtual heap 110. The in-memory heap 108 may include a portion of the virtual heap 110 that is cached (acts as physical memory) for use by application 104. In one embodiment, the virtual heap address space may be divided into fixed size pages. Page-in and page-out operations may be used to move pages from the virtual heap 110 in store 120 to the in-memory heap 108 or to eject pages from the heap 108. Pages, page states and page addressing are further illustrated in FIGS. 2, 3 and 4. At certain times, a checkpoint for application 104 may be written to persistent store space 120. In this application, a checkpoint is a persistent store of the state of an application and its execution environment (such as the virtual machine state) at a point in time. After storing a checkpoint for application 104, persistent store space 120 may include an entire, up-to-date copy of the virtual heap 110. In one embodiment, persistent store space 120 may also contain an entire copy of the virtual heap for one or more other applications. In one embodiment, persistent store space 120 may include one or more versions of copies of the virtual heap (checkpointed states) for each application. The virtual persistent heap may allow the running of an application on a physical heap 108 that is much smaller than may otherwise be required. As an example, the virtual persistent heap 110 may be an order of magnitude larger than the physical, in-memory heap 108. In one embodiment, the virtual persistent heap may be maintained on non-volatile memory storage external to the device running the application, and portions of the heap for the current execution state of the application may be cached in and out of a physical heap resident in local memory on the device. For example, the device may connect to a server on the Internet, and the server may provide non-volatile storage space for the virtual persistent heap. In another embodiment, the external storage for the virtual persistent heap may reside on a non-volatile storage attached to the device, for example, a Flash card or hard disk drive. With persistence, an application may be checkpointed and suspended on a virtual machine, and a second application may then start execution on the virtual machine without ending the virtual machine process. This avoids the overhead of starting a new virtual machine for a new application. For example, a virtual machine may be launched on a system when one is required to run a first application. When a second application is launched, the web browser may not start a second virtual machine to run the second application, as is done in the prior art, but may instead checkpoint and suspend the first application, and then run the second application on the same virtual machine the first application was running on. The second application at some point may be checkpointed and suspended, and the first application may resume execution at the last checkpointed state prior to its suspension. In another example, a web browser may launch a virtual machine to run a first application. The web browser may keep the virtual machine active after the first application completes, and later use it to run a second application. In the prior art, terminating an application would have caused the virtual machine it was running on to terminate execution as well, requiring a new virtual machine to be launched for each application. The virtual persistent heap may enable the saving of the entire state of the virtual machine heap for possible future resumption of the computation at the point the save was performed, and may permit the migration of the computation to a different system. In one embodiment, the saved state of the virtual machine heap may also provide the ability to restart the virtual machine after a system crash or shutdown to a previously saved persistent state. This persistent feature may be useful for small consumer and appliance devices including Java-enabled devices, such as cellular phones and Personal Digital Assistants (PDAs), as these appliances may be shutdown and restarted often. The virtual persistent heap may include the entire address space of the virtual machine heap an application is using. Embodiments of the virtual persistent heap may include at least one of a caching method, a database store method, and a garbage collection method as described below. FIG. 1b—A Device with Virtual Persistent Heap External to the Device FIG. 1b illustrates an embodiment of the invention where a device 140 includes a client 101, and memory 117 external to the client stores the persistent store space 120 with virtual heap 110. Memory 117 may be on any device external to but coupled to client device 140. Examples of methods in which the devices may be coupled include, but are not limited to: wireless connection (cell phones, Wireless Access Protocol (WAP)), infrared (IrDA), Ethernet, Universal Serial Bus (USB), and phone/modem. The connection may be an Internet connection. Memory 117 may be a volatile memory such as Direct Inline Memory Modules (DIMMs) or non-volatile storage device such as a flash memory, a hard disk, or removable disk such as a floppy disk. This embodiment may use persistent store space 120 in memory 117 to store the virtual heap 110 for application 104. Persistent store space 120 may also include virtual heaps (not shown) for one or more other suspended applications on device 140. Persistent store space 120 may also include virtual heaps (not shown) for one or more applications running on devices other than device 140. The architecture and operation of in-memory heap 108 and virtual heap 10 as illustrated in FIG. 1b may be substantially similar to that described in FIG. 1a. In the embodiment illustrated in FIG. 1b, caching, checkpointing, and other reads or writes to virtual heap 110 may be performed over an external interface such as a network connection, rather than being performed over an internal interface such as a memory bus as in the embodiment illustrated in FIG. 1a. FIG. 1c—A Device with Virtual Persistent Heap Internal to the Device and a Persistent Store External to the Device FIG. 1c illustrates an embodiment of the invention where a device 140 includes a client 101 and memory 115. Memory 115 may include virtual heap 110. This embodiment may also include a memory 117 external to the client. Memory 117 may include persistent store space 120 for holding checkpoints 111 of virtual heap 110. Memory 117 may be on any device external to but coupled to client device 140. Examples of methods in which the devices may be coupled include, but are not limited to: wireless connection (cell phones, Wireless Access Protocol (WAP)), infrared (IrDA), Ethernet, Universal Serial Bus (USB), and phone/modem. The connection may be an Internet connection. Alternatively, memory 117 may be integrated in or directly attached to device 140. Memory 117 may be a volatile memory such as one or more memory modules (for example, Direct Inline Memory Modules (DIMMs)), or a non-volatile storage device such as a flash memory, a hard disk, or removable disk such as a floppy disk. This embodiment may use persistent store space 120 in memory 117 to store the checkpoints 111 of virtual heap 110 for application 104. Persistent store space 120 may also include checkpoints of virtual heaps (not shown) for one or more other suspended applications on device 140. Persistent store space 120 may also include checkpoints of virtual heaps (not shown) for one or more applications running on devices other than device 140. The architecture and operation of in-memory heap 108 and virtual heap 110 may be substantially similar to that described in FIG. 1a. Periodically, a checkpoint 111 of virtual heap 110 for application 104 may be written to persistent store space 120. After storing a checkpoint for application 104, persistent store space 120 may include an entire, up-to-date copy of the virtual heap 110. Persistent store space 120 may also include checkpointed copies of the virtual heap for one or more other applications. Some embodiments may checkpoint one or more versions of virtual heap 110. For example, in the embodiment illustrated in FIG. 1c, multiple versions of checkpoint 111 for virtual heap 110 for application 104 may be stored in persistent store space 120. A method may be provided to select a checkpoint version from among one or more checkpointed version for resuming the application and/or virtual machine execution at a particular point. FIG. 1d—A Client-Server System with Persistent Store Space FIG. 1d is a block diagram illustrating a network including client system 100, gateway server 112, and server system 116 according to one embodiment of the invention. Server system 116 may include a service provider server 118, for example, a Jini or other network service connection system or compact service connection system server. Jini™ Sun Microsystems' Jini is an example of a Network Service Connection System (NSCS) that may be used with networked devices to locate and lease resources, herein referred to as services, on networked systems including servers, and to pass information to and from the services located on the networked systems. The Jini technology makes it possible for an application to discover and use local and remote services. A local service may be a service that is provided on the same device as the application. A remote service may be a service that is provided by a device other than the device the application is executing on. Furthermore, applications that use such local and remote services may obtain leases on the services. These leases may expire after a certain amount of time (or on demand). By modifying an application to use Jini when it accesses local and remote services (and to handle expiration and reactivation of a lease), the problem of maintaining the external state of a process during process migration may be addressed. The Jini system federates computers and computing devices on a network into what appears to the user as a single system. Each Jini technology-enabled device preferably has some memory and processing power. Devices without processing power or memory may be connected to a Jini system, but those devices may be controlled by another piece of hardware and/or software, called a proxy, that presents the device to the Jini system and which itself contains both processing power and memory. The Jini system is Sun Java technology-centered. The Jini architecture assumes that the Java programming language is the implementation language for components. The ability to dynamically download and run code is central to a number of the features of the Jini architecture. However, any programming language can be supported by a Jini system if it has a compiler that produces compliant bytecodes for the Java programming language. Services A service is an entity that can be used by a person, a program, or another service. A service may be a computation, storage, a communication channel to another user, a software filter, a hardware device, or another user. Services may be local or remote. A local service may be provided on the same device as the user of the service. A user of a service may be called a client, and the device client is accessing the service from may be called the client device. Thus, a client may access a service on the client device. A remote service may be provided on a device other than (external to) the client device. Examples of services include devices such as printers, displays, or disks: software such as applications or utilities; information such as databases and files: and users of the system, and translating from one word processor format to some other. Jini systems provide mechanisms for service construction, lookup, communication, and use in a distributed system. Services in a Jini system communicate with each other by using a service protocol, which is a set of interfaces written in the Java programming language. Services may be found and resolved using a lookup service. The lookup service may be the central bootstrapping mechanism for the system and may provide a major point of contact between the system and users of the system. In precise terms, a lookup service maps interfaces indicating the functionality provided by a service to sets of objects that implement the service. In addition, descriptive entries associated with a service allow more fine-grained selection of services based on properties understandable to people. A service is added to a lookup service by a pair of protocols called discovery and join; the service first locates an appropriate lookup service (by using the discovery protocol), and then it joins the lookup service (by using the join protocol). Service Leasing Access to many of the services in the Jini system environment is lease based. A lease is a grant of guaranteed access to a service over a period. A service may be a resource external to the virtual machine within which an application desiring a service is executing. A service may be a local service or a remote service. In Jini, a lease may be negotiated between the user of the service and the provider of the service as part of a service protocol: a service is requested for some period, and access is granted for some period, presumably considering the request period. If a lease is not renewed before it is freed—because the service is no longer needed, the client or network fails, or the lease is not permitted to be renewed—then both the user and the provider of the service may conclude the service can be freed. Leases may be exclusive or non-exclusive. Exclusive leases insure that no one else may take a lease on the service during the period of the lease; nonexclusive leases allow multiple users to share a service. In one embodiment, an application may establish one or more leases to local and/or remote services external to the application. In one embodiment, an application may establish one or more leases to system code that give the application access to resources external to the application such as system resources. System code for accessing an external resource may be referred to as a system service. A lease on system code for accessing an external resource may be referred to as a leased system service. For example, an application may establish leases to a system services that give the application access to system drivers for accessing communications ports in the system. In one embodiment, interactions between services and applications may be stateless. For example, each interaction request may be handled by the receiver using information included with the request. Jini and JavaSpaces™ The JavaSpaces technology package provides a distributed persistence and object exchange mechanism for code written in the Java™ programming language. Objects are written in entries that provide a typed grouping of relevant fields. Clients can perform simple operations on a JavaSpaces server to write new entries, lookup existing entries, and remove entries from the space. Objects in JavaSpaces are stored in Java Serialization Format. Server JavaSpaces provide persistent object storage replacing traditional file system storage persistence models. JavaSpaces servers provide network service connection system clients such as Jini clients access to a persistent and shareable object store. Network Service Connection System for Small Footprint Devices A consumer or appliance device with a small amount of memory may be referred to as a "small footprint device." A Compact Network Service Connection System (CNSCS) may be provided for use with small footprint network client devices (PDAs, cell phones, etc.) to locate and lease services on networked systems including servers, and to pass information to and from the located services and resources. The CNSCS may be designed specifically for use with small footprint network client devices that may be too "small" (not have enough resources such as memory) to support a system such as Jini. The CNSCS may be embodied as a self-contained message-passing system that may operate among similar small systems, and may be bridged to a complete Jini federation using a bridging server. Examples of such a CNSCS is described in U.S. Provisional Patent Application No. 60/208,011 to Slaughter, Saulpaugh, Traversat, Abdelaziz, Duigou, Joy, and Pouyoul, titled "DISTRIBUTED COMPUTING ENVIRONMENT", filed May 26, 2000, which is hereby fully incorporated by reference in its entirety, and in U.S. Provisional Patent Application No. 60/209,430 to Slaughter, Saulpaugh, Traversat, Abdelaziz, Duigou, Joy, and Pouyoul, titled "DISTRIBUTED COMPUTING ENVIRONMENT", filed Jun. 2, 2000, which is hereby fully incorporated by reference in its entirety, and in U.S. Provisional Patent Application No. 60/209,140 to Slaughter, Saulpaugh, Traversat, Abdelaziz, Duigou, Joy, and Pouyoul, titled "DISTRIBUTED COMPUTING ENVIRONMENT", Jun. 2, 2000, which is hereby fully incorporated by reference in its entirety. CNSCS clients are typically small footprint devices that may include small display screens and keyboards. CNSCS clients may be mobile or non-mobile devices. Examples of mobile CNSCS clients may include, but are not limited to: cell phones, palmtop computers, notebook computers, Personal Digital Assistants (PDAs), desktop computers, and printers. An example of a non-mobile CNSCS client may be a light switch comprising a simple chip capable of receiving a simple set of commands (on/off) and of transmitting a simple set of status messages (on/off status). A CNSCS client may include core CNSCS software and one or more client applications. A CNSCS client may connect to a "fixed" network through a variety of paths. Examples of connection methods may include, but are not limited to: wireless connection (cell phones, Wireless Access Protocol (WAP)), infrared (IrDA), Ethernet, and phone/modem. CNSCS clients may connect to a network through gateways. The gateways provide the client devices with access to CNSCS servers on the network. A gateway may include a proxy CNSCS server. When connected, a CNSCS client "finds" a proximity network on which the client can run one or more applications from the network. A CNSCS client may also connect to a network to remotely access files on a server. A mobile CNSCS client may send out broadcast messages using whatever physical interface it has (IRDA, WAP, proprietary connection, etc). All that is required of the device is that it can send and/or receive messages. Some devices may only have to receive messages. For example, a CNSCS capable light switch may only need to receive and act on messages (ON message, OFF message, or TOGGLE message). More sophisticated CNSCS clients may send out a message to join a CNSCS federation. Message Capable Networking in CNSCS A distributed computing facility can be built upon a messaging layer. Furthermore, a messaging layer (providing both reliable and unreliable messages) can be built upon the socket networking classes provided in an embedded Java platform. TCP, UDP, and IP are examples of message capable protocols that may be leveraged by CNSCS. Other more specialized protocols such as the Wireless Application Protocol (WAP) are also capable of supporting CNSCS messages. WAP is tuned for networks with high latency and low bandwidth. CNSCS messages also work well with other network drivers such as IrDA (Infrared Data Association) and Bluetooth beneath the transport. The only required portion of CNSCS for a device (above the basic networking protocol stack) is a thin messaging layer, and all additional facilities are optional. CNSCS Spaces A CNSCS Space may be smaller and simpler than a JavaSpace. Some CNSCS Spaces are transient, while others are persistent. Transient spaces may be used as rendezvous mechanisms for peer-to-peer communication (Palm Pilot IrDA, for example). Server CNSCS Spaces may provide persistent object storage, replacing traditional file system storage persistence models. CNSCS Space servers provide CNSCS clients access to a persistent (and potentially shared) object store. In one embodiment of a CNSCS, the objects stored in a CNSCS Space (and sent in a message) may be represented in XML (eXtensible Markup Language). XML may be used as the means of representing objects because it is sufficiently rich, as well as being an Internet standard. A persistent CNSCS Space is a directory containing XML representations of objects. Because XML is used to represent the space and its objects, Internet search facilities can be leveraged to find spaces and objects within those spaces and Java and non-Java objects created in C++ or any other object-oriented language may be stored and retrieved from a CNSCS Space or placed in a message. XML object representations are language independent. In one embodiment, only an object's data is represented in XML, not its code. This means that Java and non-Java applications can send and receive objects from each other. Classes (with encapsulated bytecode) may be stored in a CNSCS Space or passed in a message. XML class representations may not be supported in all platforms due to security and size constraints. In one embodiment, threads may be compiled into XML to enable virtual machine migration to a CNSCS Space. In this model, the CNSCS Space may be used as a persistent heap/virtual machine store. A Java virtual machine understands the structure of a Java object, so in one embodiment, CNSCS may provide JVM extensions for compiling a Java object to XML, and for decompiling XML into a Java object. In some embodiments, the CNSCS may provide other extensions for compiling and decompiling other object types into XML or other messaging languages. Space Searching A CNSCS client may not need a browser. Instead, a search may be offloaded to a server that performs the actual search using a front-end proxy that parses the results for the client. Hence, CNSCS Space queues may be Internet document searches triggered by messages sent to a proxy. CNSCS Leasing As in Jini, access to many of the services in the CNSCS system environment may be lease based. A lease is a grant of access to a service. In one embodiment, an application may establish one or more leases to local and/or remote services external to the application. In one embodiment, an application may establish one or more leases to system code that give the application access to resources external to the application such as system resources. The leasing mechanism may allow clients to obtain leases for objects in a CNSCS Space. In one embodiment, the CNSCS leasing mechanism may use time-based leasing. In another embodiment, clients may make claims on Java objects, and register a callback method that may be invoked when another client desires a lease that is incompatible with current leaseholders. There may be several levels of CNSCS leases. A first level may not return a copy of the Java object when a lease is obtained, but simply registers an interest in this object being kept in the CNSCS Space. A second level does return a copy of the Java object when a lease is obtained at this level, but there could be multiple clients accessing the object. A third level does return a copy of the Java object when a lease is obtained at this level, and there are other clients are prohibited from accessing the object. In one embodiment, interactions between processes and services provided through leases may be stateless. For example, each interaction request may be handled by the receiver using information included with the request. Returning to FIG. 1d, service provider server 118 may include a persistent store space 120. Client system 100 may be a computer platform with operating system, such as a PC or laptop computer running Windows 9×/NT, or a virtual machine, for example, a JVM or KVM, sitting on top of a computer platform or executing on a consumer or appliance device, for example, a cell phone or PDA. Client system 100 may include a service provider client 102, for example, a Jini or CNSCS client, for finding and leasing services on remote servers. Client system 100 may be used for running applications and applets, for example, Java applications and applets. One or more applications may be executing on client system 100. Application 104 is shown as a currently executing application, and application 106 is shown as a suspended application. Application 104 may access an "in-memory" heap 108. Persistent store space 120 may include a virtual heap 110 for application 104. Persistent store space 120 may also include a virtual heap (not shown) for application 106. Client system 100 may broadcast and receive messages using whatever physical I/O interface it has (IRDA, WAP, Ethernet, modem, proprietary connection, etc). Client system 100 may access services on one or more servers on the network including server 116. In one embodiment, the service provider client 102 may connect to servers on the network through gateway server 112. A gateway 112 may include a proxy service provider server 114. When connected, the service provider client 102 may find server 116 on which the client 102 may provide, via lease or otherwise, persistent store space 120 for virtual heap space for one or more applications including applications 104 and 106. Checkpoints for applications may be stored in persistent store space 120. Thus, persistent store space 120 may be a service that may be leased by an application using a service provider system such as Jini or CNSCS. A lease may be established for a leasable service 125 on server 124. The lease may be established for application 104 by service provider client 102. Service provider 102 may establish the lease through service provider proxy server 114. In one embodiment, leases to services and/or resources on client device 100 may also be established. The architecture and operation of in-memory heap 108 and virtual heap 110 as illustrated in FIG. 1d may be substantially similar to that described in FIG. 1a. In the embodiment illustrated in FIG. 1d, caching, checkpointing, and other reads or writes to virtual heap 110 may be performed over a network connection, for example, over the Internet, rather than being performed over an internal interface such as a memory bus as in the embodiment illustrated in FIG. 1a. FIG. 1e—A Virtual Heap and Leases to Local and Remote Resources FIG. 1e is a block diagram illustrating a virtual heap 148 for an application and leases to local and remote resources according to one embodiment of the invention. Virtual heap 148 may include one or more pages of application code and data 152. Virtual heap 148 may also include one or more pages of system code and data 154. Pages 152 may include a lease to a service 164 that may include information describing the lease relationship with a service 166. Service 166 may be a local or remote service. In one embodiment, the lease may be established using an NSCS such as Jini. In another embodiment, the lease may be established using a CNSCS. Pages 152 may also include a lease to system code 156. In one embodiment, the lease may be established using an NSCS such as Jini. In another embodiment, the lease may be established using a CNSCS. The lease to system code 156 may give the application access to a system resource 162 by leasing native method 158. Native method 158 may be system code that invokes one or more system native code functions 160 for accessing system resource 162. For example, system resource 162 may be a bus port such as a USB port. Code 160 may be the native language driver for the USB port. Native method 158 may include the code necessary to invoke various functions provided by the native language USB driver. In one embodiment, when the application is checkpointed, the system code and data pages 154 may not be checkpointed. When the application code and data pages 154 are checkpointed, service lease 164 and system resource lease 156 may be checkpointed. In one embodiment, the information checkpointed for a lease (system resource or service lease) may include enough information to re-establish the lease if necessary. In one embodiment, leases to system resources and services may be stateless—no record of previous interactions between the application and the service or resource may be kept, and each interaction request between the application and the service or resource may be handled based entirely on information that comes with it. Being stateless may simplify the checkpointing of the leases because no current or past state information needs to be checkpointed for the leases. If the application needs to be migrated to another device, or if the application is suspended for some reason, then the leases held by the application may be cancelled, releasing the services and/or resources held by the leases. When the application is resumed (locally or on another device), then the lease information from the checkpointed state of the application may be used to re-establish the leases to services and/or system resources. In one embodiment, an application may migrate to a device with a different system and native language than the system and native language of the device from which it is migrating. In this embodiment, the lease to system resource 156 may be reestablished to a method 158 in the system code 154 of the device to which the application migrated. Native code functions 160 for accessing system resource 162 may be in the native code of the new device. In one embodiment, the application may migrate to a device that does not have system resource 162. In this case, the application may be notified that the lease to the system resource cannot be re-established. In one embodiment, the application may migrate to a device that does not have access to service 166. In one embodiment, the application may be notified that the lease to the service cannot be re-established. In one embodiment, when it is discovered that service 166 is not available, an alternate service may be searched for to fulfill the functionality of service 166, and, if found, a lease may be established to the alternate service. FIG. 1f—A Virtual Heap and Leases to System Resources FIG. 1f is a block diagram illustrating a virtual heap 148a and 148b for two applications with leases to system resources 162a and 162b according to one embodiment of the invention. In this embodiment, a heap 164 external to virtual heaps 148a and 148b may be used to store system code and data that may be shared among two or more applications. Virtual heaps 148a and 148b may each include one or more pages of application code and data 152a and 152b. Pages 152a and 152b may include leases to system code 156a and 156b that may give the application access to system resources 162a and 162b respectively by leasing shared native methods 158a and 158b. Native methods 158a and 158b may include system code that may invoke one or more native code functions 160 for accessing system resources 162a and 162b. Some system resources may be shareable and others may require exclusive access privileges. In one embodiment, if a native method in heap 154 allows shared access, two or more applications may hold leases to the same native method, and thus the same system resource, simultaneously. FIG. 2—Virtual Persistent Heap Architecture FIG. 2 is a block diagram illustrating a virtual persistent heap architecture according to one embodiment of the invention. Application 104 may be executing in client system 100. Application 104 may be using in-memory heap 108. Persistent store 120 may reside on a server on the network to which client system 100 has access, or alternatively may be located in a local non-volatile memory on the system application 104 is executing on. Page table 122 may reside on the same system as application 104 or alternatively may reside on another system on the network. The persistent store 120 may include an entire copy of the virtual heap 110 (virtual memory) for application 104. The "in-memory" heap 108 may include a portion of the virtual heap 110 that is cached (acts as physical memory). In one embodiment, the virtual persistent heap is a page-based heap. The virtual heap address space is divided into fixed size pages. Page-in and page-out operations are used to move pages from the persistent store 120 to the in-memory heap 108 and to eject pages from the in-memory heap 108. In this application, the terms "physical heap" or "heap" may be used to indicate the heap structure in memory 108. This is only a portion of the total virtual heap 110 saved in the persistent store 120. The term "virtual heap" may be used to indicate the entire heap image saved in the store 120. The "in memory" heap address space may be viewed as the physical memory. The "in store" heap address space may be viewed as the virtual memory. The store 120 may be segmented into two or more disjoint virtual heaps. Each checkpointed application such as application 104 has its own virtual heap space reserved in the store. In exemplary persistent store 120, a virtual heap space exists for application 106 and application 104, and two unused virtual heap spaces exist to allow for two more applications. Paging provides a simple model to move data from the persistent store 120 to the in-memory heap 108 in virtual machine 100. In one embodiment, a page table 122 and offset based address translation may be used to convert virtual heap 110 references into in-memory heap 108 references. Relatively small pages may be used to reduce heap waste. In one embodiment, a paging-based approach may enable page protection mechanisms and support for DMA and block I/O devices. In one embodiment, object-caching granularity may be implemented instead of paging. Object-caching granularity is possible if objects can efficiently be moved in the heap. A consideration in embodiments using object handles is the memory footprint constraint. The object handle area may take more memory space than related structures such as handle tables in embodiments using pages. Using page handles rather than object handles may give the ability to tune the implementation to fit the memory footprint requirement of a targeted device. The page size determines the amount of space required for the handle table. Larger memory environments may use smaller pages. Smaller memory environments may need to use larger pages. Larger objects may be broken up and stored across multiple pages, allowing portions of objects to be cached in an out of the in-memory heap 108. This may allow devices with limited memory resources to support objects, and therefore applications, that may not be supportable with object caching granularity. With object caching granularity, the entire object may have to be cached into in-memory heap 108. One embodiment may use a page-in and page-out approach to bring pages from the virtual heap 110 into the in-memory heap 108. For embodiments in which persistent store 120 is comprised in a flash memory device, paging-out may use a scatter/gather object phase to only write updated objects to increase the life of the flash device. In one embodiment, this may be combined with a log-based approach to guarantee atomicity on store transactions. In a paging-based system; the page size may be increased to reduce the page table 122 size. Increasing the page size may permit the grouping of multiple objects into a single page. In this case, a single page table entry may play the role of multiple object handle entries (one handle for each object in the page). Grouping objects into a single table entry may allow the reduction of the memory footprint required for a handle table, as there may be fewer handles. Updating a single object in the page may require the writing of the entire page (all objects in the page). Alternatively, reducing the page size allows fewer objects to be stored in a page, thus reducing paging granularity. This approach may increase the page table size. The page size may be adjusted accordingly based upon memory constraints on the device on which the paging-based system is implemented. In one embodiment, the virtual heap 110 may be divided into a fixed number of pages. In one embodiment, to aid in efficient address translation, the application virtual heap size (i.e. the Kernel plus all user pages) may be a fixed multiple of the size of the in-memory heap 108. This allows each application virtual heap store to start at a multiple heap size offset in the persistent store 120. In this embodiment, the address translation includes subtracting a fixed heap size multiple. Since a virtual machine may not have access to a hardware translation mechanism, the address translation may be simplified so it can be efficiently performed in software. An offset based schema may be used to convert a virtual heap address into an in-memory heap address. All object references in the virtual heap 110 and the in-memory heap 108 may be kept as virtual heap addresses. In one embodiment, there may be an address translation to convert the virtual heap address into the physical heap location. The CPU of the system or CPU layer of the virtual machine may perform address translation from the virtual heap address space into the in-memory heap location via a Page Table 122 entry. The Page table 122 maintains the mapping of the virtual heap 110 page into the heap 108. For each virtual heap 110 address reference (read or write), code may be inserted (read/write barriers) to verify the validity of the address (i.e. check if the corresponding page is resident in the heap), and to translate it into the in-memory heap 108 reference. The process of converting heap addresses is illustrated in FIG. 4. In some embodiments, the virtual machine CPU layer, for example, the Java CPU layer, may provide access to hardware Memory Management Unit (MMU) address translation functions, allowing object handle in directions to be done in hardware. In one embodiment, an object in the virtual address space may maintain references to other objects via a valid or invalid address. A valid address may mean the corresponding page is resident in the in-memory heap 108. An invalid address may mean the corresponding page is not resident in the in-memory heap 108. Page Table 122 In one embodiment, page table 122 is not persistent, but is a "live" structure. The page table may be reinitialized whenever a new application is restarted. In one embodiment, the page table 122 may include one or more of the following entries for a page of the active application virtual heap 110:
In one embodiment, as shown in FIG. 2, there may be one entry in the page table 122 for each page of the active application virtual heap 110. This embodiment may simplify the location of an entry for a page in the page table 122. In another embodiment, there may be one entry in the page table for each page currently cached in in-memory heap 108. This embodiment may reduce the size of the page table 122. Read-only/static core virtual machine objects may be located into pinned and read-only system pages (objects may be tagged by the primary class loader). These classes are typically not loaded twice. Read/write core virtual machine objects may be located into user pages. In one embodiment, read/write core virtual machine objects may be "colored" as system pages. All user objects may be allocated in user pages. In one embodiment, an application may establish one or more leases to system objects that may give the application access to resources external to the application such as system resources. In one embodiment, system pages in a heap may include system objects (code and/or data) that are currently leased. In one embodiment, the leases for the system objects may be contained in the application virtual heap 110. In embodiments that allow the running of only one application at a time, each application virtual heap may contain its own set of system pages. In these embodiments, system pages are not shared among applications. In embodiments running more than one application at a time, system pages may be shared among applications. These embodiments may have a system segment in the persistent store 120 to checkpoint static and read-only pages that can safely be shared among applications. FIG. 3—The States of a Page in a Virtual Persistent Heap FIG. 3 is a state diagram illustrating the states of a page in a virtual persistent heap according to one embodiment of the invention. A page may be in one of the following states:
A page fault occurs when a reference is made to a page not resident in the in-memory heap 108. The page fault may induce a caching of the page from the virtual heap 110 in store 120 to the heap 108. When a page fault occurs, the following conditions may be encountered:
In one embodiment, when looking for candidate pages to be evicted, more than one page may be selected for eviction, since it is likely that another page may need to be evicted soon. In one embodiment, a free page threshold may be used to induce this behavior. In one embodiment, a standard LRU (Last Recently Used) method may be used to select pages for eviction (page out). In other embodiments, other methods, for example, Least Frequently Used (LFU), may be used to select pages for eviction. If a page is dirty, the page may be checkpointed to the store 120, or alternatively a shadow copy is made, before being freed from the heap 108. In one embodiment, non-dirty pages may be evicted before dirty pages. Page Checkpointing As previously described, pages may be brought into the heap 108, modified and checkpointed to the virtual heap 110 in store 120 when they are evicted. In one embodiment, pages may be checkpointed when there are evicted. Alternatively, pages may be checkpointed when they remain in the heap 108. For instance, if checkpointing can be performed asynchronously (an executing thread does not have to be frozen), then pages may be checkpointed whenever convenient with minimum overhead to look for dirty pages. In embodiments with a single threaded virtual machine environment using a simple bytecode count as a time sharing quantum for switching between threads, pages to be checkpointed may be searched for whenever a thread synchronization or a context switch occurs. On a thread context switch, dirty pages may be scanned for and placed in a checkpoint queue. In another embodiment, a mechanism to interrupt the running thread may be used to provide an opportunity to search for and checkpoint pages. The flush bit in the Page Table 122 may be used to mark pages that are in the checkpoint queue. Further writes may be prevented to the page while the page is in the queue or in the process of being checkpointed. In one embodiment, the thread may be blocked until the page is written. In this embodiment, the checkpoint queue may be reordered to prioritize pages that have a blocked thread. In another embodiment, a copy of the page may be made to let the thread "go" on the shadow copy. A recently checkpointed page may not be put back into the checkpoint queue right away. System pages may have a different checkpoint strategy than user pages. For instance, checkpointing system pages may freeze the entire virtual machine. System pages may therefore be more selectively checkpointed than user pages. Store Checkpoints and Consistency Having pages checkpointed individually may be insufficient to maintain the consistency of the virtual heap 110 in store 120. For instance, if two pages have changed in the heap 108, but only one page has been checkpointed, and the system crashes, the checkpointed virtual heap 110 in store 120 may end up in an inconsistent state. When the system restarts with an inconsistent store 120, the application may crash due to incorrect pointer locations. There is no guarantee that pages put in the checkpoint queue will be checkpointed to the store 120 (the system may crash at any time). In one embodiment, in order to capture a consistent virtual machine state, the set of changes made to the store 120 may be combined into an atomic operation. An atomic operation is an operation may comprise a series of steps that are not finalized until all of the steps in the atomic operation are confirmed to have successfully completed. The atomic operation allows all of the steps in the operation to be undone if any of the steps in the operation do not complete successfully. The process of undoing all of the steps in an atomic operation may be referred to as "rolling back" the operation. In the example above, if one of a series of two or more checkpoints in a checkpoint queue are not completed when recovering a crashed system, the system may be "rolled back" to a previous checkpointed state. In one embodiment, a transaction-based API to allow the client system 100 to issue checkpoint requests may be provided. Using the API, the client system 100 may tell the store:
In one embodiment, the client system 100 may have only one outstanding store transaction at a time. Each successive store checkpoint may be encapsulated in a different-transaction. When a store checkpoint is issued, client system 100 execution may need to be stopped for as short a time as possible in order to save non-heap virtual machine structures. One embodiment may provide for pre-flushing of dirty pages by allowing dirty pages to be checkpointed independently of the store checkpoint. Thus, when a store checkpoint is issued, all heap 108 pages may have already been saved (pre-flushed) into store 120. Thus, the only structures that may need to be stored are a few dirty system pages and the virtual machine non-heap structures. In one embodiment, the store may verify that all states have been correctly written to the store 120 when the checkpoint transaction is committed. If one or more writes failed or did not complete, the store may abort the transaction and roll back to the last committed checkpoint. In one embodiment, if the checkpoint fails, but client system 100 is still running, the client system 100 may continue to run under restrictions, such as no more paging allowed, and also warning the user that the store has been lost. In another embodiment, the client system 100 may be stopped when the checkpoint fails. In one embodiment, an application level checkpointing API may be provided to inform the application 104 that the checkpointing failed. The client system 100 may verify that any heap or non-heap changes are correctly recorded as part of the store transaction. The store may verify that all changes have been made persistent (written to non-volatile storage such as disk or flash memory) and the store is left in a consistent state. The client system 100 may rely on the store to guarantee transaction ACID properties. ACID is the acronym used to describe the following properties of a transaction:
In one embodiment, the store may only maintain one checkpoint per application (the last successfully committed checkpoint). In another embodiment, the store may maintain one or more checkpoints per application, and the client system 100 may select which checkpoint to use when restarting an application. Since each application store is kept in a different persistent store 120 virtual heap segment, the current application heap segment may be "touched" for a checkpoint, while other application segments are untouched. In one embodiment, a store management User Interface (UI) may be provided, and may allow the removal of an application's corrupted virtual heap. When to Commit a Store Checkpoint? In one embodiment, in order to keep the heap 108 and client system 100 states very closely synchronized with the store 120, a store checkpoint may be issued any time a change is made. This embodiment may not be practical due to the high incidence of store checkpointing that may degrade performance. To avoid performance degradation, the heap 108 and client system 100 state may be loosely synchronized with the store 120 state, and store checkpoints may only be issued under certain conditions. The following are examples of conditions that may be used in deciding when to issue a store checkpoint:
FIG. 4 is a flowchart describing a computation method for in-memory heap page addresses according to one embodiment of the invention. In one embodiment, the translation of a virtual heap page reference to an in-memory heap page reference may include the following steps. In step 300, the store page ID may be determined. An example of a method for determining the store page ID is:
In step 300, first, the application ID may be multiplied by the virtual heap size to get the base address of the virtual heap for the application. Second, the base address of the virtual heap may be subtracted from the virtual heap page reference address to produce an address offset from the base address. Third, the address offset may be shifted to remove the bits containing the in-page address information. For example, if a page comprises 256 addressable bytes, the address may be shifted 8 bits. In one embodiment, the result of the shift is the page ID for the virtual heap page reference. In step 302, the location of the page in the heap may be determined from the page table:
If the page is not resident, a page fault may be issued to bring the page into the heap. In step 304, the in-memory heap address may be computed. An example of a method for computing the in-memory heap address is:
First, the heap page ID produced in step 302 may be multiplied by the page size to produce the base address of the in-memory heap page. The original virtual heap page reference address may then be ANDed with a page size bit mask to produce the bits containing the in-page address information. The in-page address information may then be added to the base address of the in-memory heap page to produce the in-memory heap address. FIGS. 5a and 5b—Application Migration The embodiments of an application migration processes as illustrated in FIGS. 5a and 5b, and other embodiments not illustrated, may provide for migrating Java applications from one machine to another on a network or between devices when at least one of the devices may not be connected to a network. In other embodiments, non-pure Java applications and/or non-Java applications from one machine to another on a network or between devices when at least one of the devices may not be connected to a network. In order to handle the problem of migrating the external state of an application, migratable applications may use a Network Service Connection System such as Jini or a Compact Network Service Connection System (CNSCS) for accessing resources external to the applications, referred to as services. Services may be local (on the device within which the application is running) or remote (on other devices connected to the device via the network). Local services may include system resources on the device within which to the application is running. These local or remote services may be leased by an application using an NSCS or CNSCS. Thus, in one embodiment, the external state of the application may be represented by one or more leases to local and/or remote services, including system resources. Other embodiments may use other methods for accessing external resources that allow for the preservation of external state during migration. In one embodiment, each application on a system is separated from other applications, and is thus migratable separately from other applications. In one embodiment, each application on a system may have an in-memory heap serving as "physical" memory that is being used for the current execution of the application, a virtual heap that may include the entire heap of the application including at least a portion of the runtime environment of the virtual machine, and a persistent heap or store where the virtual heap may be checkpointed. In one embodiment, the virtual heap and the persistent heap may be combined in one memory (the virtual heap may serve as the persistent heap). In another embodiment, the virtual heap may be checkpointed to a separate, distinct persistent heap. The combination of the in-memory heap, the virtual heap, and the persistent store may be referred to as the "virtual persistent heap." In yet another embodiment, there may be sufficient memory available for the in-memory heap so that a virtual heap is not required to run the application; in this embodiment, only an in-memory heap and a persistent heap on the store may be present for an application. One embodiment of a method for migrating an application may include.
The application resuming execution in the persistent heap on the node where it migrated. In one embodiment, since processes that migrate away from a node may migrate back after minor state changes on the node where they migrated (e.g. updated a page of a document), a versioning mechanism may be used whereby nodes where an application once lived may cache a previous state, and thus may avoid sending over the network a state that hasn't changed. Information on the current leases for the application may also be packaged and sent to the new node where the application is to migrate. The information may be used in reestablishing the leases on the new node. In one embodiment, the lease information may be maintained in a gate structure. Example of gate structures for a CNSCS is described in U.S. Provisional Patent Application No. 60/208,011 to Slaughter, Saulpaugh, Traversat, Abdelaziz, Duigou, Joy, and Pouyoul, titled "DISTRIBUTED COMPUTING ENVIRONMENT" filed May 26, 2000, which was previously fully incorporated reference in its entirety, and in U.S. Provisional Patent Application No. 60/209,430 to Slaughter, Saulpaugh, Traversat, Abdelaziz, Duigou, Joy, and Pouyoul, titled "DISTRIBUTED COMPUTING ENVIRONMENT", filed Jun. 2, 2000, which was previously fully incorporated by reference in its entirety, and in U.S. Provisional Patent Application No. 60/209,140 to Slaughter, Saulpaugh, Traversat, Abdelaziz, Duigou, Joy, and Pouyoul, titled "DISTRIBUTED COMPUTING ENVIRONMENT", filed Jun. 2, 2000, which was previously fully incorporated by reference in its entirety. In addition, a user interface (UI) may be provided to manage application checkpoints. Functions the UI may allow the user to perform may include, but are not limited to, the following;
FIG. 5a is a block diagram illustrating an embodiment of an application migration process where the original application 104a and the migrated application 104b may use the same virtual heap 110 in persistent store 120. In FIG. 5a, the in-memory heap 108 for application 104a executing on client system 100 is checkpointed to persistent store 120. The checkpointing may be performed as an atomic transaction. The store checkpoint may include one or mom of the following states that may be made permanent to the store;
Any current leases to external services (for example, services leased via an NSCS such as Jini or a CNSCS) may be expired. In one embodiment, expiration of current leases may be required prior to migration. In one embodiment expiration of current leases is not required before checkpointing the application. The checkpointed persistent state of the application 104 stored in persistent store 120, including user pages, system pages, and the current state of non-heap structures, is packaged and sent to the client system 130 where the application 104a is to migrate. In this step, a transaction mechanism may be used, where a process's entire persistent state is copied atomically and committed as having migrated on both the sending and receiving client systems. In one embodiment, since processes that migrate away from a client system may be expected to migrate back after only relatively minor state change on the client system where they migrated (e.g. updated a page of a document), a versioning mechanism may be used whereby nodes on which an application once lived may cache previous states and avoid sending over the network a state that hasn't changed. The packaged and sent state is received and reconstituted on client system 130, and application 104b resumes running on the new client system. A new in-memory heap 108b may be allocated for application 104b including the checkpointed user and system pages. The current state of non-heap structures on client system 130 may be set from the received checkpointed state. Required leases of services may be reestablished for the application. The application 104b then may resume running in the heap 108b on the client system 130 where it migrated. Application 104b may continue to use persistent store 120 for its virtual heap or may establish a new virtual heap in another persistent store on client system 130 or on another server providing persistent store space on the network. FIG. 5b illustrates an embodiment where client 100 comprises a persistent store 120a used by application 104a to store virtual heap 110a. When migrating application 104a to client 130, a checkpointed state of application 104a may be sent to client 130. On client 130, a new virtual heap 104b may be established in persistent store 120b, a new in-memory heap 108b may be established, and application 104b may resume executing on client 130 from the checkpointed state of application 104a. FIG. 6—A Method for Migrating an Application FIG. 6 is a flowchart describing a method for migrating processes, including applications, from one client system to another according to one embodiment of the invention. Client systems may be "real" systems such as Windows 9×/NT systems or virtual machines such as Java Virtual Machines running on top of other systems. In one embodiment, each application may be independent from other applications on the system, and thus may be migratable separately from other applications. In one embodiment, each application on a particular client system will have an in-memory heap where it executes and a persistent heap where it can be checkpointed before being migrated. In step 320, application 104 executing on client system 100 is checkpointed to its persistent heap 110 in persistent store 120. The store checkpoint may include one or more of the following states that may be made permanent to the store:
A "user page" includes application-specific data or executable code. A "system" page includes operating system and/or virtual machine data or executable code that is not application-specific. In step 322, current leases to services (for example, services leased via an NSCS such as Jini or a CNSCS) may be expired on client system 100. In one embodiment, all current leases must be expired before migration. In step 324, the most recently checkpointed persistent state of the application 104 in persistent heap 120 is packaged and sent to the client system 130 where the application 104 is to migrate. In step 326, the packaged checkpointed state of application 104 is received on the client system 130. In one embodiment, a transaction mechanism may be used, where a process's entire persistent state is copied atomically and committed as having migrated on both the sending and receiving client systems in step 328. In step 330, the received packaged state is reconstituted on the client system 130 where the application 104 is migrating. Required leases of local and/or remote services may be re-established for the application 104 in step 332. In one embodiment, one or more of the leases expired in step 322 maybe re-established. In one embodiment, the received packaged state include information describing the one or more leases expired in step 322, and this information on the leases may be used in step 332 in re-establishing the leases. In one embodiment, the re-established leases may include leases to system resources on the device to which the application is migrating. In step 334, the application 104 then may resume running using heap 108 on the client system 130 where it migrated. The migrated application 104 may continue to use the virtual heap 110 that was used by application 104 on client system 100 prior to migration, as illustrated in FIG. 5a. Alternatively, a new virtual heap 110 may be established for application 104 on client system 130, as illustrated in FIG. 5b. FIG. 7—Virtual Persistent Heap Architecture Using Cache Lines A feature of the virtual persistent heap is the method used to cache portions of the virtual persistent heap into the "physical", in-memory heap. The virtual persistent heap may include a caching mechanism that is effective with small consumer and appliance devices that typically have a small amount of memory and that may be using flash devices as persistent storage. The caching strategy may achieve a lesser amount of caching and may improve locality among elements of the virtual persistent heap that are cached in the physical heap, thus reducing caching overhead. FIGS. 2 through 5 illustrate embodiments that use a "page" as a level of granularity for the virtual persistent heap caching mechanism. FIG. 7 is a block diagram illustrating an embodiment of a virtual persistent heap architecture substantially similar to the embodiment illustrated in FIG. 2. Application 104 may be executing in client system 100. Application 104 may be using in-memory heap 108. Persistent store 120 may reside on a server on the network to which client system 100 has access, or alternatively may be located in a local non-volatile memory on the system that application 104 is executing on. Cache table 122 may reside on the same system as application 104 or alternatively may reside on another system on the network. The embodiment illustrated in FIG. 7 includes a caching mechanism in which the virtual persistent heap is divided into cache lines. A cache line is the smallest amount of virtual persistent heap space that can be loaded or flushed at one time. Caching in and caching out operations are used to load cache lines into the heap or to flush dirty cache lines into the store. In general, the definition of a "page" as used in FIGS. 2 through 5 includes a cache line. In other words, a cache line is a size of page. In other embodiments, object granularity may be used in a virtual persistent heap. In these embodiments, caching in and caching out operations may be performed on objects that may be created by the application. A level of object locality in a cache line may be achieved to reduce heap waste by is the use of object caching nurseries and a generational garbage collector as described below. Different cache line sizes may be used for different regions of the heap. Cache lines may provide a natural path for porting the virtual persistent heap to cache based Memory Management Unit (MMU) architectures, and may allow address translation from the virtual persistent heap to the heap to be performed in hardware. In one embodiment, all heap references may be kept in one address space (the virtual persistent heap address space). The address translation is therefore simplified, and may require no "swizzling" of virtual references into in-memory heap references when manipulating objects in the heap. In one embodiment, having a single address space may allow a single garbage collector to run on the virtual persistent heap space. If a single address space is not used, two or more garbage collectors, may be required, for example, one running on the virtual persistent heap and one running on the in-memory heap. The term "virtual persistent heap" may be used to refer to the entire virtual heap image saved in the persistent store. The terms in-memory heap or heap may be used to refer to the portion of virtual heap currently cached in memory. The term cache line may be used to refer to the smallest caching-in and caching-out granularity. A cache line corresponds to the smallest amount of data that can be loaded or flushed from the in-memory heap at one time. The in-memory heap and the virtual persistent heap may be divided into fixed size cache lines, or alternatively the heaps may be divided into groups of cache lines with differing cache line sizes. The virtual persistent heap size may be a multiple of the maximum in-memory heap size, and an offset-based schema, such as that illustrated in FIG. 4, may be used to convert virtual persistent heap addresses into in-memory heap addresses. In one embodiment, references in the virtual persistent heap and the in-memory heap structure may be kept as virtual persistent heap addresses. There may be no updates to physical heap references when heap references are manipulated. The address translation from the virtual persistent heap address space to the in-memory heap location may be done using a cache table entry. In a cache line based system, the cache line size may be increased to reduce the cache table size. Increasing the cache line size may permit the grouping of multiple objects into a single cache line. In this case, a single cache table entry may play the role of multiple object handle entries (one handle for each object in the cache line). Grouping objects into a single cache table entry may allow the reduction of the memory footprint required for a handle table, as there may be fewer handles. Updating a single object in the cache line may require the writing of the entire cache line (all objects in the cache line). Alternatively, reducing the cache line size allows fewer objects to be stored in a cache line, thus reducing caching granularity. This approach may increase the cache table size. The cache line size may be adjusted accordingly based upon memory constraints on the device on which the cache line based system is implemented. On each virtual persistent heap reference (read or write), read/write barriers may be used to verify the validity of the address (i.e. to check if the corresponding cache line is resident in the heap), and to translate it into the current heap location. In one embodiment, objects in the virtual persistent heap may maintain references to other objects via a valid or invalid address. A valid address may mean the corresponding address is resident in the in-memory heap. An invalid address may mean the corresponding address is not resident in the in-memory heap. Caching Considerations for Flash Devices Using cache line addressing, reads may be done at a very small granularity (for example, 2 bytes). Bringing a cache line into the in-memory heap, rather than a single object, means that more objects may be brought into the heap than needed. For example, a cache line may include two objects, only one of which may be required by the application. Caching the cache line in the in-memory heap may cache both the required object and the non-required object. This may be exacerbated if there is bad object locality (i.e., if unrelated objects are in the same cache line). If the cache line is too big, many objects read in may never be referenced. Cache lines may also waste heap space if the lines are not full. For example, an average object size for an application may be 50 bytes, and a cache line size may be 4 Kbytes. If 40 objects are resident in a 4 Kbyte cache line in this example, approximately half of the cache line may be unused. Flash memory writes are typically destructive, and are therefore preferably minimized. Flash devices may use relatively large block writes (for example, 128 Kbytes). In one embodiment, the cache line size may be a multiple of the flash block write size for optimum performance. In one embodiment, the cache line size may be equal to the block write size. In one embodiment, a cache line flush may write the entire line (all objects in the line). From the above, it is evident that cache lines for reads may be small and cache lines for writes may be large. For example, read cache lines may be 4 Kbytes and write cache lines may be 128 Kbytes. To satisfy both requirements, different nursery regions in the heap may be used to combine objects with different flushing policies. Scatter/gather operations may also be used to combine dirty objects into cache I/O buffers, so that only updated objects are written, allowing for larger writes. Caching may provide a simple scheme to load and flush data between the store and the in-memory heap. In one embodiment, a cache table and offset based address translation may be used to convert virtual persistent heap references into in-memory heap references. Successive caching and garbage collection compaction cycles may improve spatial locality so that cache lines may contain related objects. This may help reduce heap waste and improve performance due to less caching. Smaller cache line regions may also be used to reduce heap waste. Flushing to a flash device may include a scatter/gather step to combine dirty objects into preallocated cache I/O buffers, so that a minimum number of writes are performed. In one embodiment, only dirty objects are written to increase the life of the flash. This may be combined with a log-based store and atomicity for store transactions to maintain the consistency of the image of the virtual persistent heap stored in the persistent device. Using | ||||||