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IBM Software Group
JVM Performance Tuning with respect to
Garbage Collection(GC) policies for
WebSphere Application Server V6.1 - Part 1
Giribabu Paramkusham
Ajay 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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Generational and Concurrent GC (gencon)
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Comparing IBM gencon policy to SUN JVM GC collector
JVM Heap
Nursery/Young Generation
Old Generation
IBM J9:
IBM J9:
-Xmo (-Xmos/-Xmox)
Sun:
-XX:NewRatio=n
-Xmn (-Xmns/-Xmnx)
Sun:
-XX:NewSize=nn
-XX:MaxNewSize=nn
-Xmn<size>
Minor Collection:
Major Collection:
Permanent Space
Sun JVM Only:
-XX:MaxPermSize=nn
takes place only in the young generation, normally done through
direct copying very efficient
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>
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.
<time totalms="83.227" />
</af>
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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>
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Allocation request
details, time it took
to stop all mutator
threads.
Heap occupancy
details before
GC.
Details about the
scavenge.
Heap occupancy
details after GC.
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Benchmarking two GC policies
optavgpase
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gencon
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Benchmarking (cont…)
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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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Theoretical and Advised Max Heap Sizes
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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
Out Of Memory crash
100 MB
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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Heap Sizing for Generational GC
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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 Output
http://www-1.ibm.com/support/docview.wss?uid=swg21222488
GCMV - GC and Memory Visualizer (GCMV) from ISA
https://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
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Questions and Answers
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