Memory Tuning In Java
Transcript of Memory Tuning In Java
© 2007 IBM CorporationConference materials may not be reproduced in whole or in part without the prior written permission of IBM.
Bangalore, IndiaAugust 6 - 9, 2007
IBM WEBSPHERETECHNICAL CONFERENCE
Session Number: W02Session Number: W02Tuning the Java Virtual Machine for Optimal Performance: Tuning the Java Virtual Machine for Optimal Performance:
Means and MethodsMeans and Methods
Rajeev Rajeev PalankiPalanki
IBM Java Technology CenterIBM Java Technology Center
[email protected]@in.ibm.com
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ObjectivesObjectives
• Have an insight into key aspects of JVM Runtime Performance and understanding of the means and methods to tune the JVM for optimal performance.
• At the end of this session you should have:
High level overview of the Java Virtual Machine (JVM) and its key components.
Understanding of different Garbage Collection schemas in the Sovereign & J9 Virtual Machines and their impact on JVM Runtime performance.
Knowledge about using verbosegc outputs effectively to improve application response times.
Introduction to Shared Classes Technology and performance gains.
Familiarity with debugging and profiling tools available for JVM and their effective usage.
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AgendaAgenda
• Overview of the Java Virtual Machine and its key components
• Garbage Collection Basics (Sovereign VM – 142 JDK)
• Profiling Garbage Collection: Verbosegc Outputs
• Garbage Collection Policies in Java 5.0
• Debugging and Analysis tools for Garbage Collection
• Introduction to Shared Classes Technology
• Real Time Java – A brief overview
• Q & A
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IBM Java Building BlocksIBM Java Building Blocks
Java SDK
Virtual Machine
Class Libraries JIT
Basic text slide
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IBM Java Building BlocksIBM Java Building Blocks
Java SDK
Virtual Machine
Class Libraries JIT
ORB XML
Security
Big decimalRAS
GC
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The Java Application StackThe Java Application Stack
Java Code
Platform
Native libraries
Java Class Libraries
ORBJava Class Extensions
Native Code
Execution Management (XM)Core Interface (CI)
Diagnostics Execution Engine Classloader
Lock Data Conversion
Storage
HPI (Hardware Platform Interface)JIT
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Building BlocksBuilding Blocks
The JDK is a key component in the Application Server Stack from a performance perspective
Operating system– Vendor specific operating environment– Specific hardware architecture (instruction set)
Java SDK
Virtual Machine
Class Libraries JIT
Application– “Write Once Run Anywhere”
– Class Libraries– Collection of well-defined code packages that assist developers’
creation of business applications (3 specifications) – Just-in-Time Compiler
– Code generator that converts bytecodes into machine language instructions at run time.
– Virtual Machine– Platform independent execution environment that abstracts
operating system specifics from the developer/user.
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IBM Java 5.0IBM Java 5.0
• IBM have totally rewritten and redesigned our VM for Java 5.0 – You may have heard this referred as J9
New Virtual Machine implementation (J9)New Garbage Collection Mechanism (Modron)New Just In Time Compiler (Testarossa)Shared Classes technology
• Just In Time Compiler (Testarossa)
Multiple optimization levelsRecompilation driven by sampling threadDynamic Profile Information CollectionProfiling thread helps determine “hotness” of methodsAsynchronous compilation
• Garbage Collection (Modron)
Uses a “type accurate” collectorIntroduces parallel compactorIntroduces “Generational GC” policy
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Garbage Collection OverviewGarbage Collection Overview
Garbage Collection (GC)
The main cause of memory–related performance bottlenecks in Java.
Two things to look at in GC: frequency and duration
Frequency depends on the heap size and allocation rateDuration depends on the heap size and number of objects in the heap
GC algorithm
Critical to understand how it works so that tuning is done more intelligently.
How do you eliminate GC bottlenecks?
Minimize the use of objects by following good programming practicesSet your heap size properly, memory-tune your JVM
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The (IBM) JVM Garbage Collector
• “The purpose of Garbage Collection is to identify Java Heap storage which is no longer being used by the Java application and so is available for reuse by the JVM”
•Key questions:
•Performance and Scalability: How quickly can you find garbage?
•Accuracy: Can you find all the garbage?
•Effectiveness: How much space can you make available?
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Garbage Garbage CollectionCollection: : IBM TechnologyIBM Technology• Concurrent mark
Most of the marking phase is done outside of ‘Stop the World’ when the ‘mutator’ threads are still active giving a significant improvement in pause time.
• Parallelizing the garbage Collection phases
The Mark and sweep workload is distributed across available processors resulting in a significant improvement in pause times
• Adaptive sizing of thread local heaps
Reduces the amount of Java Heap locking
• Incremental compaction
The expense of compaction is distributed across GCs leading to a reduction in (an occasional) long pause time.
• Java 5 technologies
Lazy sweep, parallel compaction, generational and accurate collection
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The JVM HeapsThe JVM Heaps
Size
Next
Size
Next
freelist
Nullfree storage
free storage
Native HeapJava Heap
‘Thread Stacks
Buffers
JIT Compiled Code
Motif structures
Free List
Xms - Active Area of Heap
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Allocation schemesAllocation schemes
• Two types of allocation
– Cache Allocation (for object allocations < 512 bytes), does not require Heap Lock. Each thread has local storage in the heap (TLH – Thread Local Heap) where the objects are allocated.
– Heap Lock Allocation (Heap Allocation occurs when the allocation request is more than 512 bytes, requires Heap Lock.
If size is less than 512 or enough space in the cachetry cacheAllocreturn if OK
HEAP_LOCKdo forever
If there is a big enough chunk on freelistTake itgoto Gotit
else manageAllocFailureIf any error
goto GetOutEnd do
Gotit:Initialize objectGet outHeap_UNLOCK
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Large Object AllocationLarge Object Allocation
• All objects => 64K are termed “large” from the VM perspective
• In practice, objects of 10MB+ in size are usually considered large
• The Large Object Area is 5% of the active heap by default.
• Any object is first tried to be allocated in the free list of the main heap – if there is not enough “contiguous” space in the main heap to satisfy the allocation request for object => 64K, then it is allocated in the Large Object Area (wilderness)
• Objects < 64K can only be allocated in the main heap and never in the Large Object Area
Active heap
LOA
Xmx
Users can identify the Java stack of a thread making an allocation request of larger than the value specified with the environment variable ALLOCATION_THRESHOLD
export ALLOCATION_THRESHOLD =5400This will give java stacks for object allocations of created than 5400 bytes.
Users can specify the desired % of the Large Object Area using the Xloration option (where n determines the fraction of heap designated for LOA.
(For example: Xloratio0.3 reserves 30% of the active heap for the Large Object Area)
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Sub poolsSub pools
• Subpools provide an improved policy of object allocation and is available from JDK 1.4.1 releases only on AIX.
– Improved time for allocating objects– Avoid premature GCs due to allocation of large objects– Improve MP scalability by reducing time under HEAP_LOCK – Optimize TLH sizes and storage utilization
• The subpool algorithm uses multiple free lists rather than the single free list used by the default allocation scheme.
• It tries to predict the size of future allocation requests based on earlier allocation requests. It recreates free lists at the end of each GC based on these predictions.
• While allocating objects on the heap, free chunks are chosen using a ″best fit″ method, as against the ″first fit″ method used in other algorithms.
• It is enabled by the –Xgcpolicy:subpool option.
• Concurrent marking is disabled when subpool policy is used.
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Garbage Collection Basics (142 JVM)Garbage Collection Basics (142 JVM)
• Garbage Collection is performed when there is:
An allocation failure in the heap lock allocationSpecific call to System.gc
• Garbage Collection is Stop the World (All other application threads are suspended during GC)
• Two main technologies used to remove garbage:
Mark Sweep CollectorCopy Collector
GC occurs in the thread that handled the request
Requested object allocation that caused allocation failureProgrammatically requested GC
• Thread must acquire certain locks required for GC
Heap LockThread queue lockJIT lock
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Object reclamation process for a Mark Sweep CollectorObject reclamation process for a Mark Sweep Collector
Obtain locks and suspend threads
Mark phaseProcess of identifying all objects reachable from the root set.All “live” objects are marked by setting a mark bit in the mark bit vector.
Sweep phaseSweep phase identifies all the objects that have been allocated, but no longer referenced.
Compaction (optional)Once garbage has been removed, we consider compacting the resulting set of objects to remove spaces between them.
Release locks and resume threads
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Mark Mark –– Sweep Sweep –– Compact AlgorithmCompact Algorithm
Root Set
Heap
reachable object
unreachable object
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Together we achieve: Some parallel processingTogether we achieve: Some parallel processing
• GC Helper threads
On a multiprocessor system with N CPUs, a JVM supporting parallel mode starts N-1 garbage collection helper threads at the time of initialization.
These threads remain idle at all times when the application code is running; they are called into play only when garbage collection is active.
For a particular phase, work is divided between the thread driving the garbage collection and the helper threads, making a total of N threads running in parallel on an N-CPU machine.
The only way to disable the parallel mode is to use the -Xgcthreads parameter to change the number of garbage collection helper threads being started.
• Parallel MarkThe basic idea is to augment object marking through the addition of helper threads and a facility for sharing work between them.
• Parallel BitWise SweepSimilar to parallel mark, uses same helper threads as parallel markImproves sweep times by using available processors.
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Concurrent MarkingConcurrent Marking
Designed to give reduced and consistent GC pause times as heap sizes increases.
Concurrent aims to complete the marking just before the before the heap is full.
In the concurrent phase, the Garbage Collector scans the roots by asking each thread to scan its own stack. These roots are then used to trace live objects concurrently.
Tracing is done by a low-priority background thread and by each application thread when it does a heap lock allocation.
Concurrent is enabled by the option: -Xgcpolicy:optavgpause
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Incremental CompactionIncremental Compaction
• Incremental compaction removes the dark matter from the heap and reduces pause times significantly
• The fundamental idea behind incremental compaction is to split the heap up into sections and compact each section just as during a full compaction.
• Incremental compaction was introduced in JDK 1.4.0; is enabled by default and triggered under particular conditions. (Called Reasons)
• -Xpartialcompactgc Option to invoke incremental compaction in every GC cycle.• -Xnopartialcompactgc Option to disable incremental compcation.• -Xnocompactgc Option to disable full compcation
The heap is divided into regionsThe heap is divided into regionsThe regions are further divided into sectionsThe regions are further divided into sectionsEach section is handled by one helper threadEach section is handled by one helper threadA region is divided into A region is divided into
(number of helper threads +1) or (number of helper threads +1) or 8 sections (whichever is less)8 sections (whichever is less)
The whole heap is covered in a few GC cycles.The whole heap is covered in a few GC cycles.
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Explicit Garbage CollectionExplicit Garbage Collection
Garbage collector is called only upon two conditions:
When an allocation failure occursGC explicitly called using System.gc
Don’t call System.gc() at all. It hurts more often than it helps. GC knows when it should run
The temptation to scatter System.gc() calls here, there, and everywhere is enormous. It does not make a good idea.TRUST ME !!!!!
Use –Xdisableexplicitgc to avoid running GC for each System.gc().
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Profiling Garbage Collection: Profiling Garbage Collection: VerbosegcVerbosegc outputoutput
The most indispensable tool for profiling GC activity is Verbosegc – from JVM runtime.
Enabled using –verbosegc on the java command line.
Verbosegc redirection-Xverbosegclog: <path to file> filename
Verbosegc redirection to multiple files-Xverbosegclog:<path to file>filename#,X,Y(Redirect to Y files each containing X GC cycles)
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Understanding a typical Understanding a typical verbosegcverbosegc outputoutput
<AF[71]: Allocation Failure. need 65552 bytes, 3 ms since last AF><AF[71]: managing allocation failure, action=2 (142696/10484224)><GC(71): GC cycle started Fri Mar 19 17:59:06 2004<GC(71): freed 94184 bytes, 2% free (236880/10484224), in 12 ms><GC(71): mark: 5 ms, sweep: 0 ms, compact: 7 ms><GC(71): refs: soft 0 (age >= 32), weak 0, final 0, phantom 0><GC(71): moved 3095 objects, 188552 bytes, reason=1>
<AF[71]: managing allocation failure, action=3 (236880/10484224)><AF[71]: managing allocation failure, action=4 (236880/10484224)><AF[71]: managing allocation failure, action=6 (236880/10484224)>JVMDG217: Dump Handler is Processing a Signal - Please Wait.JVMDG315: JVM Requesting Heap dump fileJVMDG318: Heap dump file written to
/workarea/rajeev/gctests/heapdump.20040319.175906.8467.txtJVMDG303: JVM Requesting Java core fileJVMDG304: Java core file written to
/workarea/rajeev/gctests/javacore.20040319.175906.8467.txtJVMDG274: Dump Handler has Processed OutOfMemory.<AF[71]: insufficient heap space to satisfy allocation request><AF[71]: completed in 203 ms>
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When are GC messages printed out?When are GC messages printed out?
• The first two lines are put out just before the beginning of STW phase of GC.
• Rest of messages are printed out after the STW phase ends and threads are woken up. No messages are printed during GC.
• Heap shrinkage messages are printed before STW messages, but shrinkage happens AFTER STW phase!
• Heap expansion messages are correctly printed AFTER STW messages.
(STW = Stop The World)
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Things to look for in a Things to look for in a verbosegcverbosegc outputoutput
• Was it an Allocation Failure GC?
• What was the size of allocation request that caused AF?
• What were the total and free heap sizes before GC?
• What was the total pause time?
• Where was maximum time spent in GC?
• Are we doing a compaction in each GC cycle?
• What actions were taken by GC?
• Was GC able to meet allocation request in the end?
• What was the free heap size after GC?
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GC actions (JDK 142)GC actions (JDK 142)
• Look for lines of this type:managing allocation failure, action=<n> Where <n> is the numerical value of action taken.
Actions:
0 -> GC because of exhaustion of pinned free list.
1 -> Perform garbage collection without using wilderness
2 -> Garbage Collector tried to allocate out of wilderness and failed.
3 -> Expand the Java heap
4 -> Clear soft references
5 -> Steal space from transient heap (zOS only)
6 -> Couldn’t get enough space. Print appropriate messages.
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Using Using verbosegcverbosegc to set the heap sizeto set the heap size
• Use verbosegc to guess ideal size of heap, and then tune using –Xmx and –Xms.
• Setting –Xms: Should be big enough to avoid AFs from the time the application starts to the time it becomes ‘ready’. (Should not be any bigger!)
• Setting –Xmx:In the normal load condition, free heap space after each GC should be > minf (Default is 30%).There should not be any OutOfMemory errors.In heaviest load condition, if free heap space after each GC is > maxf (Default is 70%), heap size is too big.
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Example of Example of verbosegcverbosegc when heap is too smallwhen heap is too small
GC is too frequent
<AF[25]: Allocation Failure. need 65552 bytes, 1 ms since last AF><AF[25]: managing allocation failure, action=2 (319456/10484224)>
<GC(25): GC cycle started Sat Mar 20 15:32:50 2004<GC(25): freed 3968 bytes, 3% free (323424/10484224), in 11 ms><GC(25): mark: 5 ms, sweep: 0 ms, compact: 6 ms><GC(25): refs: soft 0 (age >= 32), weak 0, final 0, phantom 0><GC(25): moved 214 objects, 9352 bytes, reason=1>
<AF[25]: managing allocation failure, action=3 (323424/10484224)><AF[25]: managing allocation failure, action=4 (323424/10484224)><AF[25]: managing allocation failure, action=6 (323424/10484224)><AF[25]: warning! free memory getting short(1). (323424/10484224)><AF[25]: completed in 13 ms>
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Example of Example of verbosegcverbosegc when heap is too bigwhen heap is too big
GC is too long
<AF[29]: Allocation Failure. need 2321688 bytes, 88925 ms since last AF><AF[29]: managing allocation failure, action=1 (3235443800/20968372736)
(3145728/3145728)><GC(29): GC cycle started Mon Nov 4 14:46:20 2002<GC(29): freed 8838057824 bytes, 57% free (12076647352/20971518464), in
4749 ms><GC(29): mark: 4240 ms, sweep: 509 ms, compact: 0 ms><GC(29): refs: soft 0 (age >= 32), weak 0, final 1, phantom 0><AF[29]: completed in 4763 ms>
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Effect of wrong Effect of wrong ––XmsXms & & --XmxXmx settingssettingsToo small heap = Too frequent GC.
Too big heap = Too much GC pause time. (Irrespective of amount of physical memory on the system)
Heap size > physical memory size = paging/swapping = bad for your application.
It is desirable to have the Xms much less than Xmx if you are encountering fragmentation issues. This forces class allocations, thread and persistent objects to be allocated at the bottom of the heap.
What about Xms=Xmx?It means no heap expansion or shrinkage will ever occur.
Not normally recommended.
It may be good for a few apps which require constant high heap storage space.
Hurts apps which show a varying load.
May lead to more fragmentation
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Mark Stack Overflow (MSO)Mark Stack Overflow (MSO)
• Verbosegc will contain the line:<GC(45): mark stack overflow>
• Is bad for performance.
• Caused by too many objects on the heap, especially deeply nested objects.
• Processing MSOs is expensive
• No solution is a silver bullet. Things to try:
Decrease the heap size!Use concurrent mark (-Xgcpolicy:optavgpause)Re-design the application.Increase GC helper threads (-Xgcthreads<n>)
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High pause times and system activityHigh pause times and system activity
In the event of pause times being usually acceptable with the exception of a few "abnormally high" spikes - we are likely to infer that the deviation was a result of some system level activity (heavy paging for ex) outside of the Java process.
Consideration: How many clock ticks our process actually spent executing instructions, not time spent waiting for I/O or time spent waiting for a CPU to become available for the process to run on?
Profile system level activity
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Headline Changes in Java 5.0 Garbage CollectorHeadline Changes in Java 5.0 Garbage Collector
Java Tiger: JSE 5.0
5.0 uses a completely new memory management framework
No pinned/dosed objectsStack Maps used to provide a level of indirection between references and heap5.0 VM never pins arrays, it always makes a copy for the JNI code
The GC is Type Accurate
New efficient parallel compactor
-Xgcpolicy:optavgpause includes concurrent sweeping (as well as marking)
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Garbage collection policies in IBM Java 5.0Garbage collection policies in IBM Java 5.0
• Four policies available:
Optthruput (default)Mark-sweep algorithmFastest for many workloads
optavgpause Concurrent collection and concurrent sweepSmall mean pauseThroughput impact
gencon The Generational HypothesisFastest for transactional workloads Combines low pause times and high throughput
subpoolsMark-sweep based, but with multiple freelistsAvoids allocation contention on many-processor machines Available on all PPC and S390 platforms
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Why different GC Policies?Why different GC Policies?• Availability of different GC policies gives you increased capabilities.• Best choice depends upon application behaviour and workloads.• Think about throughput, response times & pause times.
Throughput is the amount of data processed by the application
Pause time is the amount of time the garbage collector has stopped threads while collecting the heap.
Response time is the latency of the application – how quickly it answers incoming requests
Policy Option ( -Xgcpolicy )
Description
Optimize for throughput
Optthruput
(Default)
It is typically used for applications where raw throughput is more important than short GC pauses. The application is stopped each time that garbage is collected.
Optimize for pause times
Optavgpause Trades high throughput for shorter GC pauses by performing some of the garbage collection concurrently. The application is paused for shorter times.
Generational Concurrent
gencon Handles short lived objects differently than the longer lived.
Supool subpool Uses same algorithm similar to the default policy but employs allocation strategy suitable for SMPs.
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Runtime Performance TuningRuntime Performance TuningWhat policy should I choose for my J9 VMWhat policy should I choose for my J9 VM
Policy Considerations
optthruput I want my application to run to completion as quickly as possible.
optavgpause • My application requires good response times to unpredictable events.
• A degradation in performance is acceptable as long as GC pause times are reduced.
• My application is running very large java heaps.
• My application is a GUI application and user response times are important.
gencon • My application allocates many short lived objects
• The java heap space is fragmented
• My application is transactions based
Subpool I have scalability concerns on large multiprocessor machines.
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Memory Management / Garbage Collection Memory Management / Garbage Collection How the IBM J9 Generational Garbage Collector WorksHow the IBM J9 Generational Garbage Collector Works
JVM Heap
Nursery/Young Generation Old Generation Permanent Space
Sun JVM Only:-XX:MaxPermSize=nn
IBM J9:-Xmn (-Xmns/-Xmnx)Sun:-XX:NewSize=nn-XX:MaxNewSize=nn-Xmn<size>
IBM J9:-Xmo (-Xmos/-Xmox)Sun:-XX:NewRatio=n
• Minor Collection – takes place only in the young generation, normally done through direct copying very efficient
• Major Collection – takes place in the new and old generation and usesthe normal mark/sweep (+compact) algorithm
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Nursery/Young GenerationNursery/Young Generation
Nursery/Young Generation
Allocate Space Survivor SpaceSurvivor Space Allocate Space
• Nursery is split into two spaces (semi-spaces)Only one contains live objects and is available for allocationMinor collections (Scavenges) move objects between spacesRole of spaces is reversed
• Movement results in implicit compaction
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Sample Sample verbosegcverbosegc output for output for gencongencon
<af type="nursery" id="35" timestamp="Thu Aug 11 21:47:11 2005" intervalms="10730.361"><minimum requested_bytes="144" /><time exclusiveaccessms="1.193" /><nursery freebytes="0" totalbytes="1226833920" percent="0" /><tenured freebytes="68687704" totalbytes="209715200" percent="32" >
<soa freebytes="58201944" totalbytes="199229440" percent="29" /><loa freebytes="10485760" totalbytes="10485760" percent="100" />
</tenured><gc type="scavenger" id="35" totalid="35" intervalms="10731.054">
<flipped objectcount="1059594" bytes="56898904" /><tenured objectcount="12580" bytes="677620" /><refs_cleared soft="0" weak="691" phantom="39" /><finalization objectsqueued="1216" /><scavenger tiltratio="90" /><nursery freebytes="1167543760" totalbytes="1226833920" percent="95" tenureage="14" /><tenured freebytes="67508056" totalbytes="209715200" percent="32" >
<soa freebytes="57022296" totalbytes="199229440" percent="28" /><loa freebytes="10485760" totalbytes="10485760" percent="100" />
</tenured><time totalms="368.309" />
</gc><nursery freebytes="1167541712" totalbytes="1226833920" percent="95" /><tenured freebytes="67508056" totalbytes="209715200" percent="32" >
<soa freebytes="57022296" totalbytes="199229440" percent="28" /><loa freebytes="10485760" totalbytes="10485760" percent="100" />
</tenured><time totalms="377.634" />
</af>
Allocation request details, time it took to stop all mutator threads.
Heap occupancy details before GC.
Heap occupancy details after GC.
Details about the scavenge.
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Diagnostic tool for garbage collectorDiagnostic tool for garbage collector• Diagnostic tool for optimizing parameters affecting the
garbage collector while using IBM JVM
• Reads the “verbosegc” output and produces textual and graphical visualizations and related statistics
– Frequency of garbage collection cycles– Time spent in different phases of garbage collection– Quantities of heap memories involved in the
process– Characteristics of allocation failures– Mark Stack Overflows
• Built in parsers for JVM 1.5, 1.4.2 , 1.3.1 & 1.2.2
• Prerequisite – JFreeChart libraries (freely downloadable)
• http://www.alphaworks.ibm.com/tech/gcdiag
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Screen ShotsScreen Shots
Starting the tool and selecting verbosegc input file and appropriate JVM parser
Extract GCCollector.zip
Place jfreeChart-0.9.21.jar and jcommon-0.9.6.jar in the lib directory of the GCCollector folder
Execute GCCollector.bat (which will spawn a GUI)
Select verbosegc file for analysis
Select appropriate JVM parser
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Duration of GC Cycles Duration of GC Cycles –– Graphical ViewGraphical View
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Graphical view of heap usageGraphical view of heap usage
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Information for specific cycleInformation for specific cycle
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Choosing time rangeChoosing time range
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Extensible Verbose Tool KitExtensible Verbose Tool KitAnalyzing your verbose GC outputAnalyzing your verbose GC output
• EVTK: Verbose GC visualizer and analyzerAvailable through ISA
IBM Support Assistant v3.0.2https://www14.software.ibm.com/webapp/iwm/web/preLogin.do?source=isa
Reduces barrier to understanding verbose output
Visualize GC data to track trends and relationshipsAnalyze results and provide general feedbackExtend to consume output related to applicationBuild plug-able analyzers for specific application needs
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Starting the EVTKStarting the EVTK
In the Toolssection of ISA, select the Java product plug-in to display the available tools
Click on the name of a tool to start that tool
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EVTK usage scenariosEVTK usage scenarios
• Investigate performance problemsLong periods of pausing or unresponsiveness
• Evaluate your heap sizeCheck heap occupancy and adjust heap size if needed
• Garbage collection policy tuningExamine GC characteristics, compare different policies
• Look for memory growthHeap consumption slowly increasing over timeEvaluate the general health of an application
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Extensible Verbose Toolkit overviewExtensible Verbose Toolkit overview
• The Extensible Verbose Toolkit (EVTK) is a visualizer for verbose garbage collection output
The tool parses and plots verbose GC output and garbage collection traces (-Xtgcoutput)
• The tooling framework is extensible, and will be expanded over time to include visualization for other collections of data
• The EVTK provides
Raw view of dataLine plots to visualize a variety of GC data characteristicsTabulated reports with heap occupancy recommendationsView of multiple datasets on a single set of axesAbility to save data as an image (jpeg) or comma separated file (csv)
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Plotting data with the EVTKPlotting data with the EVTK
Use File > Opento open a new
input file
The VGC Datamenu allows you to choose what data to display
The Line plot tab contains the data visualization
Use File > Addto add multiple input files to a single data set for comparison
and aggregated display
The Axespanel supports customized units and pan-and-zoomRight-click on the
plot and use the context menu to
export data
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 52
Reports and recommendationsReports and recommendations
• Report contents can be configured using VGC menu options
Occupancy recommendations tell you how to adjust heap size for better performanceSummary information is generated for each input in the datasetGraphs included for all GC display data
• Can export as HTML by right-clicking and using the context menu The Report tab contains the
report for the current dataset
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 53
Types of graphsTypes of graphs
• The EVTK has built-in support for over forty different types of graphs
These are configured in the VGC Data menu
Options vary depending on the current dataset and the parsers and post-processors that are enabled
• Some of the VGC graph types are:
• Note: Different graph types and a different menu are available for TGC output
• Free tenured heap (after collection)• Tenured heap size• Tenure age• Free LOA (after collection)• Free SOA (after collection)• Total LOA• Total SOA
• Used total heap• Pause times (mark-sweep-compact
collections)• Pause times (totals, including exclusive
access)• Compact times• Weak references cleared• Soft references cleared
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 54
Heap usage and occupancy Heap usage and occupancy recommendationrecommendation
This graph shows heap usage after garbage
collection; it jumps up to around 60M and stays there
The summary report shows that mean heap occupancy is 98% and that the application is spending over a third of its time doing garbage collection
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 55
EVTK EVTK –– Heap VisualizationHeap Visualization
Heap occupancy
Pause times
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 56
EVTK EVTK -- Comparison & AdviceComparison & Advice
Compare runs…
Performance advisor…
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 57
Java 5 Shared ClassesJava 5 Shared Classes
• Available on all platforms.
• Feature enabled using the –Xshareclasses flag.
• Static class data caches in shared memoryShared between all IBM Java VMs
All application and bootstrap classes shared
Cache persisted beyond lifetime of any JVM, but lost on shutdown/reboot
• Provides savings in footprint and start up times.
• Target: Server environments where multiple JVMs exist on the same box.
• Multiple sharing strategiesStandard Classloaders (including Application Classloader) exploit this feature when enabled.
API to extend Customer Classloaders available.
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 58
Shared Classes Shared Classes –– Start up timesStart up times
Sharing helps speed up JVM startupStartup Improvements with Shared Classes and AOT
0
5
10
15
20
25
30
eclipse 3.2.2 tomcat 5.5 WAS 6.1
Seco
nds
default
-Xshareclasses(Java 5.0)"
-Xshareclasses(Java 6.0)
Lower is better
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 59
What does realWhat does real--time mean?time mean?
• Real-time = predictability of performanceHard - Violation of timing constraints are hard failuresSoft - Timing constraints are simply performance goals
• Constraints vary in magnitude (microseconds to seconds)
• Consequences of missing a timing constraint:from service level agreement miss (stock trading)to life in jeopardy (airplanes)
• Real-fast is not real-time, but Real-slow is not real-good
• Need a balance between predictability and throughput
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 60
WebSphereWebSphere Real Time (WRT)Real Time (WRT)
• 1.0 generally available in August 2006.
• WRT is a Java runtime providing highly predictable operationReal-time garbage collection (Metronome)Static and dynamic compilationFull support for RTSJ (JSR 1)Java SE 5.0 compliantBuilt and rigorously tested on a RT Linux OS with IBM Opteron Blades
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 61
RealReal--Time Garbage CollectionTime Garbage CollectionThe Metronome Garbage CollectorThe Metronome Garbage Collector
• UtilizationPercentage of time dedicated to the application in a given window of time
Application
Collector
Time
Application receives a minimumpercentage of time to run
10ms
• Metronome uses a 10ms sliding window to measure utilization
Utilization is user selectable via command line
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 62
Visual Performance Visual Performance AnalyserAnalyser
• Eclipse based performance visualization kit– Profile Analyzer– Code Analyzer– Pipeline Analyzer
• Profiler Analyzer provides a powerful set of graphical and text-based views that allow users to narrow down performance problems to a particular process, thread, module, symbol, offset, instruction or source line.
– Supports time based system profiles
• Code Analyzer examines executable files and displays detailed information about functions, basic blocks and assembly instructions.
• Pipeline Analyzer displays the pipeline execution of instruction traces.
• http://www.alphaworks.ibm.com/tech/vpa
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 63
Profile Analyzer ViewsProfile Analyzer Views
• The following are important views within Profile Analyzer:
Basic Blocks Call-Graph Callers/Descendants Compiler Listing Console Counters Database Connections Disassembly/Offsets Disassembly Comparison Java/Classes Hierarchy Profile Comparison Profile Details Profile Resources Resolved Calls Sample Distribution Chart Source Code Symbol Distribution Temporal Profiling
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 64
Temporal Profiling ViewTemporal Profiling View
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 65
Counters : In the Process Hierarchy View Counters : In the Process Hierarchy View ( ( View Window View Window --> Show View > Show View --> Others > Others --> Profile Analyzer > Profile Analyzer --> Counters)> Counters)
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 66
SummarySummary
• Take Aways from the Session:
High level overview of the Java Virtual Machine (JVM) and its key components.
Understanding of different Garbage Collection schemas in the Sovereign & J9 Virtual Machines and their impact on JVM Runtime performance.
Knowledge about using verbosegc outputs effectively to improve application response times.
Introduction to Shared Classes Technology and performance gains.
Familiarity with debugging and profiling tools available for JVM and their effective usage.
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 67
References & Further ReadingReferences & Further Reading
• www.ibm.com/developerworks/java/library/j-ibmjava2
• developers.sun.com/learning/javaoneonline/2007/pdf/TS-2023.pdf
• http://www.ibm.com/developerworks/java/library/j-ibmjava4/
• http://www-128.ibm.com/developerworks/java/library/j-rtj1/index.html
• https://www-950.ibm.com/events/IBMImpact/Impact2007/3977.pdf
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 68
Questions ?Questions ?
2007 WebSphere Technical Conference (Bangalore, India)© 2007 IBM Corporation 69
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