garbage collection tuning
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IBM Software Group
®
WebSphere® Support Technical Exchange
JVM Performance Tuning with respect to Garbage Collection(GC) policies for WebSphere Application Server V6.1 - Part 1
Giribabu ParamkushamAjay Bhalodia
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Objectives
� Understand Java5 Garbage Collection for IBM®
JVMs
� Selecting the Correct GC Policy
� Analyze GC output and provide suggestions
� Tuning Java™ heap for performance
� Questions/Answers
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What is Garbage Collection
� Responsible for allocation and freeing of memory
� Allocates objects using a contiguous section of Java heap
� Ensures the object remains on the heap as long as it is in use
� Reclaims objects that are no longer referenced
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Understanding Garbage Collection� Two main technologies used to remove the garbage:
� Mark Sweep Collector
� Copy Collector
� IBM uses a mark sweep collector
� or a combination for generational (gencon)
� Garbage Collection can be broken down into 3 steps
� Mark: Find all live objects in the system
� Sweep: Reclaim objects that are no longer referenced
� Compact: Converts many small holes into fewer large ones to
avoid fragmentation
� All steps are in a single stop-the-world (STW) phase
� Application “pauses” while garbage collection is running
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Why different GC policies?
� GC performance issues can take many forms
� Definition of a performance problem is user centric
�User requirement may be for:
• Very short GC “pause” times• Maximum throughput• A balance of both
� First step is to ensure that the correct GC policy has been selected for the workload type
� Second step is to ensure heap sizing is correct
� Third step is to look for specific performance issues
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Few but simple GC policies in IBM
IBM provides four simple GC “Policies”, optimized for scenarios
1. -Xgcpolicy:optthruput optimized for throughput (default gc policy)
2. -Xgcpolicy:gencon optimized for short lived objects to
reduce pause times while maintaining good throughput
3. -Xgcpolicy:optavgpause optimized for applications with
responsiveness criteria by greatly reducing STW times.
Reduction usually between 90% to 95%. Eg. Portal applications
4. -Xgcpolicy:subpools optimized for multi processor
systems (recommended for 16 or more processors)
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Comparing IBM gencon policy to SUN JVM GC collector
IBM J9:-Xmn (-Xmns/-Xmnx)
Sun:-XX:NewSize=nn
-XX:MaxNewSize=nn-Xmn<size>
Sun JVM Only:-XX:MaxPermSize=nn
Nursery/Young Generation Old Generation Permanent Space
JVM Heap
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 uses the normal mark/sweep (+compact) algorithm
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Sample verbosegc for optthroughput (default)
<af type="tenured" id="1" timestamp="Sun Mar 12 19:12:55 2006" intervalms="0.000">
<minimum requested_bytes="16" />
<time exclusiveaccessms="0.025" />
<tenured freebytes="23592960" totalbytes="471859200" percent="5" >
<soa freebytes="0" totalbytes="448266240" percent="0" />
<loa freebytes="23592960" totalbytes="23592960" percent="100" />
</tenured>
<gc type="global" id="3" totalid="3" intervalms="11620.259">
<refs_cleared soft="0" weak="72" phantom="0" />
<finalization objectsqueued="9" />
<timesms mark="74.734" sweep="7.611" compact="0.000" total="82.420" />
<tenured freebytes="409273392" totalbytes="471859200" percent="86" >
<soa freebytes="385680432" totalbytes="448266240" percent="86" />
<loa freebytes="23592960" totalbytes="23592960" percent="100" />
</tenured>
</gc>
<tenured freebytes="409272720" totalbytes="471859200" percent="86" >
<soa freebytes="385679760" totalbytes="448266240" percent="86" />
<loa freebytes="23592960" totalbytes="23592960" percent="100" />
</tenured>
<time totalms="83.227" />
</af>
Allocation request
details. And state of
heap before
collection
Heap occupancy
details after
running GC
Heap occupancy
details after the
request that
triggered the
allocation was
satisfied.
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Sample verbosegc for gencon<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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Benchmarking two GC policiesoptavgpase gencon
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Tuning Java heap for performance
� Maximum possible Java heap sizes
� The “correct” Java heap size
� Fixed heap sizes vs. Variable heap sizes
� Heap Sizing for Generational GC
� 32 bit Java processes have maximum possible heap size
� 32 bit architecture has addressable space of 2^32
� Which is 4GigaBytes
� Determined by the process memory layout
� 64 bit processes do not have this limit
� Limit exists, but is so large it can be effectively ignored
� Addressability usually between 2^44 and 2^64
� Which is 16+ TeraBytes
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Things to consider when moving to 64 bit
� Moving to 64bit removes the Java heap size constraint
� However, ability to use more memory is not “free”
�64bit applications perform slower
• More data has to be manipulated• Cache performance is reduced
�64bit applications require more memory
• Java Object references are larger• Internal pointers are larger
� Major improvements to this in Java 6.0 due to compressed pointers
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The “correct” Java heap size
� GC will adapt heap size to keep occupancy between 40% and 70%
� Heap occupancy over 70% causes frequent GC cycles
• Which generally means reduced performance
� Heap occupancy below 40% means infrequent GC cycles, but cycles longer than they need to be
• Which means longer pause times than necessary
• Which generally means reduced performance
� The maximum heap size setting should therefore be above 40% larger than the maximum occupancy of the application. For example 43%:
� Maximum occupancy + 43% means occupancy at 70% of total heap
• Eg. For 70MB occupancy, 100MB Max heap required, which is 70MB + 43% of 70MB
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Trade-off: Heap Size vs. Performance
Heap size Occupancy GC overhead Time taken
100 MB Out Of Memory crash
110 MB 89% 77% 30s
120 MB 82% 37% 9s
130 MB 75% 20% 9s
140 MB 69% 14% 8s
200 MB 49% 9% 7s
400 MB 24% 4% 7s
800 MB 12% 4% 7s
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Fixed heap sizes vs. Variable heap sizes
� Should the heap size be “fixed”?
� i.e. Minimum heap size (-Xms) = Maximum heap size (-Xmx)?
� Dependent on application
� For “flat” memory usage, use fixed
� For widely varying memory usage, consider variable
� Variable provides more flexibility and ability to avoid
OutOfMemoryErrors
� Variable Heap Sizes
� GC will adapt heap size to keep occupancy between 40% and 70%
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Sizing the Nursery
� “Copying” from Allocate to Survivor or to Tenured space is expensive
� Physical data is copied (similar to compaction which is also expensive
� Ideally “survival” rates should be as low as possible
� Less data needs to be copied
� Less tenured/global collects that will occur� The larger the nursery:
� the greater the time between collects
� the less objects that should survive
� Recommendation is to have a nursery as large as possible
� Whilst not being so large that nursery collect times affect the application responsiveness
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Tools - GCMV Features
� Parse and plot IBM verbose GC logs
� Analyses heap usage, heap size, pause times, and many other properties
� Compare multiple logs in the same plots and reports
� Handles optthruput, optavgpause, and gencon GC modes.
� Many views on data
� Reports
� Graphs
� Tables
� Can save data to
� HTML reports
� JPEG pictures
� CSV files
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Garbage collection and Memory Visualizer (GCMV)
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Tools – PMAT Features
� The following features are included:
� GC analysis
� GC table view
� Allocation failure summary
� GC usage summary
� GC duration summary
� GC graph view
� GC pattern analysis
� Zoom in/out/selection/center of chart view
� Option of changing chart color.
� The tool can also parse IBM, SUN and HP JVM
� Handles optthruput, optavgpause, and gencon GC modes.
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Summary
� How the GC works and various GC policies in 1.5 IBM JDK.
� How to select the right GC Policy based on application scenario
� How to interpret verbose GC output
� Tuning Java Heap for performance
� Available Tools to parse gc data and provide recommendations
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References� IBM JDK 5.0: Java Diagnostics Guide
http://publib.boulder.ibm.com/infocenter/javasdk/v5r0/index.jsp
� Analyzing Verbose GC Outputhttp://www-1.ibm.com/support/docview.wss?uid=swg21222488
� GCMV - GC and Memory Visualizer (GCMV) from ISAhttps://www14.software.ibm.com/webapp/iwm/web/preLogin.do?source=isa
� IBM Pattern Modeling and Analysis Tool for Java Garbage Collector (PMAT): http://www.alphaworks.ibm.com/tech/pmat
� Garbage Collection policies (Part 1)http://www.ibm.com/developerworks/java/library/j-ibmjava2/
� Garbage Collection policies (Part2) http://www.ibm.com/developerworks/java/library/j-ibmjava3/
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Additional WebSphere Product Resources
� Discover the latest trends in WebSphere® Technology and implementation, participate in technically-focused briefings, webcasts and podcasts at: http://www.ibm.com/developerworks/websphere/community/
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� Join the Global WebSphere User Group Community: http://www.websphere.org
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