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A Portable Mechanism for Thread
Persistence and Migration
Wei Tao
The School of Computing
University of Utah
Doctoral Dissertation Defense
January 3, 2001
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Outline
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Research objectives
Problem statement
Survey of related work
Our solution
– saving control state with an example
– synchronization
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Prototype implementation
– Performance
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Discussion
Research contribution
Future work
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Research objectives
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To investigate a general approach for thread
persistence and migration which is
– portable: applicable to any platforms and environments
– transparent: hiding most code from application users
– adaptable: general enough to be used by most frameworks
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To show effectiveness and viability of this
mechanism by
– prototype implementation
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To evaluate this mechanism by
– performance measurement
– definition of constraints
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The Java programming language
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Java supports basic forms of distribution
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Architecture-independent byte code with compact format
Dynamic loading and linking
Safety and security models enforcing type and access rules
Synchronized multiple threads of control providing concurrency
Thread migration requires externalizing and moving
execution states
– Java provides no mechanism for migrating execution states
» Java’s RMI is based on the RPC model
» Existing Java mobile agent systems transfer agent objects
without their associated control state
– Need a strategy to externalize thread’s state
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Externalizing Java threads
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A thread’s execution state includes
– Data component
» Values of local variables in the activation records
» In Java, it contains thread’s control stack and related data
– Code component
» Program’s executable code
» Pointers into this code (program counter values)
» In Java, it contains the byte code and a pointer into the code
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Object sharing and multithreading
– Deep or shallow copy
– Synchronization of multiple threads
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Possible solutions
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Checkpoint Java Virtual Machine
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Saves a memory image of JVM process in a flat file
Has platform specific data representation
Is inefficient and inflexible
Works only for complete programs
Modify Java Virtual Machine implementation
– Takes architecture-independent checkpoints in JVM
– Makes JVM save or restore the execution state when program
shuts down or restarts
– Negates the ubiquity benefit of standard JVM
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Define thread saving in application code
– Insert state saving code into either source code or byte code
– Platform independence
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State of the art
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Persistence in Java
– Persistent thread is a feature missing in existing persistence
systems in Java
» Object serialization
» Persistent Storage Engine (PSE)
» The JDBC interface
» Orthogonal persistence: PJAMA
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Java mobile agent systems
– Strong mobility is not supported in most Java mobile agent
systems
» IBM’s AGLETS
» ObjectSpace’s VOYAGER
» General Magic’s ODYSSEY
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State of the art (continued)
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Thread migration in Java
– Specialized thread migration or mobility systems
» DELTA EXECUTION: integrated with JESSICA
» MOBA: designed for Grid-based thread migration
» SUMATRA: integrated in its mobile agent system
– Extended JVM
» Provides new services in JDK for saving threads
» Not portable to standard JVMs
» No support for synchronization
– Source code level transformation
» Big performance overhead
» Cannot save complete execution state
» May extend the language
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Our approach
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Byte code level transformation
– Inserting byte code to save and rebuild control state by a class
file processor
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Special features
– Portability
» No modification to JVM or Java language
– Transparency
» Code are inserted automatically by the post-processor
– Adaptability
» Is a general strategy to be used with any persistence or
migration systems
– Low overheads
– Ability to save programs with complex synchronization states
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Design and implementation
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New APIs for thread externalization
– New exception classes: ShutdownException, MoveException
– New methods initiate externalization
» checkStop: throws ShutdownException if shutdown flag is set
» moveTo(host): starts to externalize by throwing MoveException
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ShutdownManager
– Provides public method to initiate shutdown: checkStop
– Provides methods to be inserted by the processor
– Manages the shutdown and restart process
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MigrationManager
– Provides public method to start migration: moveTo(host)
– Manages migration process when necessary
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Two exceptions
public class ShutdownException extends Exception {
private Stack record;
public void push(Object item) { … }
public Object pop() { … }
public Stack getRecord() { … }
}
public class MoveException extends ShutdownException {
private String host;
public MoveException(String host) { … }
public String getHost() { … }
}
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Saving control state
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Collapsing a control stack
– Wraps each statement that throws either exception with handlers
– In the handlers, saves local data and byte code label into the
exception and re-throws it
– Externalized thread state is saved or transmitted at top level
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Rebuilding a control stack
– Check the saved state for current thread at the beginning of each
processed method
– Execution jumps to the start of shutdown code if the saved state
is null (not a restart) or empty (restart process is complete)
– Otherwise the saved label is popped from the saved state record
– A switch instruction is used to dispatch to the proper code to
restore local data
– Execution jumps to the instruction with the saved label after
restoring local data
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A simple example: merge sort
public void mergeSort( int[ ] A, int p, int r )
throws ShutdownException {
if (p >= r) return;
int q = (p + r)/2;
mergeSort(A, p, q);
mergeSort(A, q+1, r);
merge(A, p, q, r);
}
public void merge( int[ ] A, int p, int q, int r )
throws ShutdownException {
ShutdownManager.checkStop( );
...
}
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Merge sort: transformed code
public void merge( int[ ] A, int p, int q, int r )
throws ShutdownException {
… // retrieve the saved record of current thread to se
if( se != null && !se.isEmpty()) {
goto r
}
try {
ShutdownManager.checkStop( );
} catch (ShutdownException se) {
se.pushInt( r ); se.pushInt( q );
se.pushInt( p ); se.pushObject( A );
throw se;
}
r : ...
}
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Merge sort: transformed code
public void mergeSort(int[ ] A, int p, int r) throws ShutdownException {
… // retrieve the saved record of current thread to se
if( se != null && !se.isEmpty( ) ) {
int pc = se.popInt( );
switch( pc ) {
case 0: q = se.popInt( );
t1 = (int[])se.popObject( );
t2 = se.popInt( );
t3 = se.popInt( ); goto r1
case 1: q = se.popInt( );
t1 = (int[])se.popObject( );
t2 = se.popInt( );
t3 = se.popInt( ); goto r2
case 2: q = se.popInt( );
t1 = (int[])se.popObject( );
t2 = se.popInt( );
t3 = se.popInt( ); t4 = se.popInt( ); goto r3
}
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Transformed code (continued)
if (p >= r) return;
int q = (p + r)/2;
t1 = A; t2 = p; t3 = q;
r1: try {
mergeSort( t1, t2, t3 );
} catch (ShutdownException se) {
se.pushInt( q ); se.pushInt( 0 ); se.pushInt( r ); se.pushInt( p ); se.pushObject( A ); throw se;
}
t1 = A; t2 = q + 1; t3 = r;
r2: try {
mergeSort( t1, t2, t3 );
} catch (ShutdownException se) {
se.pushInt( q ); se.pushInt( 1 ); se.pushInt( r ); se.pushInt( p ); se.pushObject( A ); throw se;
}
t1 = A; t2 = p; t3 = q; t4 = r;
r3: try {
merge( t1, t2, t3, t4 );
} catch (ShutdownException se) {
se.pushInt( q ); se.pushInt( 2 ); se.pushInt( r ); se.pushInt( p ); se.pushObject( A ); throw se;
}
}
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Problems with multi-threading
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Inconsistent state
– A thread releases all locks it holds after shutdown, possibly
leaving the unlocked objects in inconsistent states
– Threads should be prevented from accessing those resources
with inconsistent states
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Indefinite blocking
– No threads should be blocked indefinitely because of other
threads’ shutdown
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Missed signals
– A thread should not send signals to threads that have already
shut down
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Nested monitor lockouts
– Threads should be restarted in a correct order
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Synchronizing shutdown
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Saving synchronization state
– Locking information is collected during shutdown process
– Object wait set information is collected at execution time
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Before a thread releases a lock during shutdown
process, it
– sets the locked object to be dirty
» A thread starts shutdown when trying to access a dirty object
– interrupts all threads that are waiting for the lock
» All waiting threads start shutdown after receiving interruption
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Notify signal is never missed due to shutdown
– Wait threads will not start shutdown until the owner thread starts
shutdown and interrupts them
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Synchronizing restart
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Threads should be restarted in a specific order
– The threads in wait set of a lock should restart before the thread
who owned the lock at shutdown time
» A wait thread acquires the lock and jumps to execute wait()
» It releases the lock when entering wait state, allowing other
threads to acquire the lock
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A central barrier object is used to block new
computation
– A thread should not start new computation until all other threads
have finished the restart process
– Threads in wait state when shutdown occurred would wait again
– Other threads are blocked by a central barrier controlled by
shutdown manager
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Prototype implementation
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Thread persistence
– All active threads in a program are saved and restarted
– A shutdown manager is created when a program starts
– Experimental frameworks
» Java serialization
» Persistent storage engine
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Thread migration
– A thread migrates transparently with control state
» resumes execution from exactly where it left
– Experimental frameworks
» Java sockets
» ObjectSpace’s VOYAGER
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Using serialization
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Persistent types
– Classes implement java.io.Serializable or java.io.Externalizable
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Object sharing
– A complete representation of the entire object graph is serialized
– Objects inside the object graph maintain their sharing status
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Non-serializable types
– Declaring a field to be transient prevents it from being serialized
» Transient objects become null after serialization
» They may need to be newly created after deserialization
– Local variables with non-serializable types are saved as null
» Define a corresponding wrapper class for each nonserializable types that need to be recreated
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Using persistent storage engine
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PSE has the following advantages over serialization
– Better performance and reliability
– Database management features such as transactions
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Same APIs and byte code transformations as used
in the object serialization
Two implementation differences
– The captured states are saved with the objects to a persistent
store in PSE instead of a file
– The object of a class implements java.lang.Runnable is saved
» New thread objects are created using saved Runnable objects
» A look-up table is used to swizzle old references to the
corresponding new thread objects
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Using Java sockets
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Class MobileThread
– serves as a super class for all mobile threads
– has a local field keeping the thread’s control state
– provides method send( ) to send out the thread
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Each server which may host mobile threads
– maintains a server socket with specified address and port.
– creates a migration manager for each client socket
– manages a local name service to identify local resources
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A migration manager
– receives object streams and recreates mobile thread objects
– starts the received mobile threads
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Integrating with VOYAGER
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Voyager
– Voyager is a Java distributed object system which supports
object mobility and mobile agents
– Objects are transferred, but not the control states
– Programmer must manually encode control states
» forced to adopt event style programming
» migration cannot occur deep in recursion, iteration …
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Supporting control migration in Voyager agents
– Use the additional APIs to initiate externalization
– Insert externalization code via post-processor
– Enable agents to migrate to a new host in any control context
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Performance measurements
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Four benchmarked applications
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Bank: average time on 10 threads with 30,000 transactions
Crew: total time on two readers and two writers
Buyer agent: total time of 50 repetitions of the bid cycle
Tower of Hanoi: total time of 20 disks
Experiment setup
– Environment:
» Java SDK 2 on Microsoft Windows98 (PII 300MHz)
– Measurement:
» Run time: Averaging 10 executions of each program
» Space: total file size in bytes for all processed class files
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Normalized run time
300
250
200
% 150
Original
Transformed
State Saving
100
50
0
Bank Crew Buyer Hanoi
Programs
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Normalized class file sizes
250
200
150
%
Original
Without Synch
With Synch
100
50
0
Bank Crew Buyer Hanoi
Programs
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Performance analysis
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Run time overhead for normal execution depends on
– percentage of processed methods
– percentage of instructions which may initiate shutdown
– total run time of original code
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Run time overhead for state saving depends on
– how deep in the call chain the shutdown instruction is
– how many local data at each level needs to be saved
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Space overhead depends on
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percentage of processed methods
percentage of instructions which may initiate shutdown
how many local data needs to be saved at each instruction
how intense synchronization is used in the program
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Discussion
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Limitations of language level transformation
– System level thread migration
– Class and interface initializers
– Finally clauses
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Constraints for class files
– Unsafe type
– Unstructured lock
– Other restrictions
» Requesting shutdown at prohibited spots
» Call back of native methods
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Research contributions
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Designed a language level rewriting approach to externalize
running threads
Analyzed the correctness of code transformation using
denotational semantics
Implemented a Java byte code processor for thread
persistence and migration on different persistence and
migration frameworks
Presented a mechanism to preserve synchronization when
saving multiple threads
Measured and analyzed performance overheads
Discussed the limitations of language level approach
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Future work
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Formal analysis
– Byte code transformation
– Synchronization
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Framework enhancements
– Saving a group of related threads and synchronization
– Processing byte code during load time
– Enabling state saving and restoration for native methods
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More applications
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Fast activation of applications or applets
Recoverable exceptions
Distributed transactions
Load balancing among heterogeneous platforms
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
The full dissertation is available at
– http://www.cs.utah.edu/~tao/research/index.html
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The prototype JTHREAD is available at
– http://www.cs.utah.edu/~tao/research/Jthread.html
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