JVM Performance Tuning with respect to Garbage Collection

Transcription

®
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
WebSphere® Support Technical Exchange
IBM Software Group
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
2
IBM Software Group
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
3
IBM Software Group
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
4
IBM Software Group
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
5
IBM Software Group
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)
6
IBM Software Group
Generational and Concurrent GC (gencon)
7
IBM Software Group
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
8
IBM Software Group
9
IBM Software Group
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>
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>
10
IBM Software Group
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.
Details about the
scavenge.
Heap occupancy
details after GC.
11
IBM Software Group
Benchmarking two GC policies
optavgpase
gencon
12
IBM Software Group
Benchmarking (cont…)
13
IBM Software Group
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
14
IBM Software Group
Theoretical and Advised Max Heap Sizes
15
IBM Software Group
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
16
IBM Software Group
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
17
IBM Software Group
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
18
IBM Software Group
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%
19
IBM Software Group
Heap Sizing for Generational GC
20
IBM Software Group
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
21
IBM Software Group
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
22
IBM Software Group
Garbage collection and Memory Visualizer (GCMV)
23
IBM Software Group
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.
24
IBM Software Group
25
IBM Software Group
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
26
IBM Software Group
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/
27
IBM Software Group
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/
Learn about other upcoming webcasts, conferences and events:
http://www.ibm.com/software/websphere/events_1.html
Join the Global WebSphere User Group Community: http://www.websphere.org
Access key product show-me demos and tutorials by visiting IBM Education Assistant:
http://www.ibm.com/software/info/education/assistant
View a Flash replay with step-by-step instructions for using the Electronic Service
Request (ESR) tool for submitting problems electronically:
http://www.ibm.com/software/websphere/support/d2w.html
Sign up to receive weekly technical My support emails:
http://www.ibm.com/software/support/einfo.html
28
IBM Software Group
Questions and Answers
29

JVM Performance Tuning with respect to Garbage Collection

Transcription

Similar documents

Lowest Prices Guaranteed

How to index IBM Connection in Portal environment

Business Partner Sessions & Reception 27 October 2014

Hardware Support

IBM SurePOS 100 Express features

bridging business and IT

} ERP TRANSFORMATION FOR EFFICIENCY, EFFECTIVENESS AND GROWTH CHANGING MARKET DYNAMICS TRANSFORMATION APPROACH

A3RMT - IBM Joint Brochure - A3 Remote Monitoring Technologies

Lotusphere Comes To You 20 years of Lotus Notes and a look

Ponani Gopalakrishnan. Ponani Gopalakrishnan.