Garbage Collection Tuning

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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

IBM Software Group

WebSphere® Support Technical Exchange 2

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

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>

</gc>




</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>

<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>


</gc>

<nursery freebytes="1167541712" totalbytes="1226833920" percent="95" />




</tenured>


</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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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

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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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 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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Questions and Answers

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Garbage Collection Tuning