This document discusses hotspot garbage collection (GC) tuning. It begins by explaining generational heap spaces and different GC algorithms like G1. It emphasizes that GC tuning goals and solutions are application-dependent. It then provides guidance on tuning for throughput versus pause time and describes how to monitor GC using tools like VisualVM. Specific GC algorithms like CMS and options for each are explained. Finally, it discusses G1 GC phases and tuning techniques like increasing heap size and survivor space.

1. Hotspot GC Tuning
Michał Warecki

2. Generational space

3. G1 Heap (Dynamic)
Eden
Survivor
Old
Humongous
Unused

4. Know your goal
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Different applications require different GC behavior
–
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No one-size-fits-all solution regarding GC and performance
User aspects:
–
–
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Throughput
Pauses
Engineering aspects:
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Footprint
–
Scalability

5. GC Benchmarks
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Hardware
–
Production like
●
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Data
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Production like in volumes and veracity
●
●
Why? Phantom bottlenecks
Why? Phantom bottlenecks i.e. caches
Tests
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No micro-benchmarks
–
Production like tests

6. Tuning for throughput
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Increase heap size
–
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Reduce overall need for GC
Let objects die in young generation
–
–
GC in old generation is more expensive than in
young generation
prevent promotion of medium lifetime objects into
old generation

7. Let objects die in young generation
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Increase young generation size
–
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Only limited by need for old generation size
Keep objects in survivor space
–
Increase survivors space
–
Raise occupancy threshold
–
Raise age threshold
●
●
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pro: prevents promotion of medium lifetime objects
con: needlessly copies around long lifetime objects
Use parallel young GC
–
increases throughput, if >>2 CPUs available

8. Tuning for minimal pause time
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Use parallel GC (parallel young and parallel compact)
–
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Reduces pause time, if >>2 CPUs available
Use concurrent GC (CMS)
–
–
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pro: mostly concurrent
con: fragmentation + more expensive young GC
Try out "G1"
–
Designed to limit pause time and frequency

9. Tuning CMS
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Avoid stop-the-world pauses
–
Reduce duration of full GC
–
Avoid full GC altogether

10. Prevent fallback to stop-the-world GC
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Increase heap size
–
●
Start CMS early, i.e. lower occupancy threshold
–
●
Defers the problems ("night time GC")
Reduces throughput because GC runs practically
all the time
Increase young generation size
–
Avoids fragmentation in the first place

11. Hotspot GC “packs”
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-XX:+UseSerialGC
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-XX:+UseParallelGC
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-XX:+UseParallelOldGC
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-XX:+UseConcMarkSweepGC
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-XX:+UseG1GC

12. Serial GC (STW)
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-XX:+UseSerialGC
–
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Single threaded young and old
generation
-XX:+UseParallelGC
–
Single threaded old generation

13. Parallel GC (STW)
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-XX:+UseParallelGC
–
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-XX:UseParallelOldGC
–
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Parallel young generation
Parallel young and old generation
-XX:+UseConcMarkSweepGC
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Parallel young generation

14. (Mostly) Concurrent GC
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-XX:+UseConcMarkSweepGC
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Mostly concurrent old generation
–
Young gen is still full STW

15. G1 GC
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-XX:UseG1GC
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Serial, Parallel and Concurrent

16. Scavenger Phases
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Scavenge
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Reference
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SoftReference
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WeakReference
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FinalReference
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PhantomReference
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JNI Weak Reference
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String Table

17. CMS phases (1/2)

18. CMS phases (2/2)
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Initial mark
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Concurrent mark
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Precleaning
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Remark
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Sweep
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Reset data structures

19. GC Monitoring
JVisualVM

20. GC Monitoring
Java Mission Control

21. GC Monitoring
Logs
-verbose:gc
-Xloggc:gclogs.txt
-XX:+PrintGCDetails
-XX:+PrintGCDateStamps
-XX:+PrintTenuringDistribution
-XX:+PrintGCApplicationConcurrentTime
-XX:+PrintGCApplicationStoppedTime

22. GC logs ParNew

23. GC logs ParNew

24. GC logs CMS

25. GC logs CMS

26. GC Logs G1 (wow!)
2013-09-29T13:17:21.424+0200: 10.978: [GC pause (young) (to-space overflow), 0.01560900 secs]
[Parallel Time: 9.9 ms]
[GC Worker Start (ms): 10978.3 10978.4 10978.4 10978.4 10978.5 10978.6 10978.6 10978.6
Avg: 10978.5, Min: 10978.3, Max: 10978.6, Diff: 0.3]
[Ext Root Scanning (ms): 2.3 2.0 2.2 2.2 2.0 1.9 1.6 2.4
Avg: 2.1, Min: 1.6, Max: 2.4, Diff: 0.8]
[Update RS (ms): 0.0 0.2 0.0 0.0 0.0 0.0 0.0 0.0
Avg: 0.0, Min: 0.0, Max: 0.2, Diff: 0.2]
[Processed Buffers : 0 26 0 0 0 5 0 0
Sum: 31, Avg: 3, Min: 0, Max: 26, Diff: 26]
[Scan RS (ms): 0.2 0.3 0.0 0.2 0.1 0.3 0.0 0.0
Avg: 0.1, Min: 0.0, Max: 0.3, Diff: 0.2]
[Object Copy (ms): 5.9 5.9 7.3 5.9 6.1 7.1 6.6 6.8
Avg: 6.5, Min: 5.9, Max: 7.3, Diff: 1.4]
[Termination (ms): 1.2 1.2 0.0 1.2 1.2 0.0 1.2 0.0
Avg: 0.7, Min: 0.0, Max: 1.2, Diff: 1.2]
[Termination Attempts : 1 2 1 2 2 1 1 1
Sum: 11, Avg: 1, Min: 1, Max: 2, Diff: 1]
[GC Worker End (ms): 10988.0 10988.0 10987.9 10988.0 10988.0 10987.9 10988.0 10988.0
Avg: 10988.0, Min: 10987.9, Max: 10988.0, Diff: 0.1]
[GC Worker (ms): 9.6 9.6 9.6 9.5 9.5 9.4 9.4 9.3
Avg: 9.5, Min: 9.3, Max: 9.6, Diff: 0.3]
[GC Worker Other (ms): 0.3 0.3 0.4 0.4 0.5 0.5 0.5 0.6
Avg: 0.4, Min: 0.3, Max: 0.6, Diff: 0.3]
[Clear CT: 0.7 ms]
[Other: 5.0 ms]
[Choose CSet: 0.1 ms]
[Ref Proc: 0.9 ms]
[Ref Enq: 0.0 ms]
[Free CSet: 1.2 ms]
[Eden: 400M(400M)->0B(377M) Survivors: 0B->23M Heap: 476M(500M)->157M(500M)]
[Times: user=0.07 sys=0.02, real=0.01 secs]

27. GC Logs G1
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Ext Root Scanning
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Update RS (Remembered Set)
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Scan RS
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Object Copy
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Termination
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GC Worker End
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GC Worker
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GC Worker Other
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Clear CT (Card table)
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Other (Choose CSet – Collection Set, Ref Proc, Ref Enq, Free
CSet)

28. G1 Remembered Sets
Ref: http://www.infoq.com/articles/tuning-tips-G1-GC

29. Useful options
-Xmn
-Xmx
-Xms
-XX:SurvivorRatio=X
-XX:MaxTenuringThreshold=XX
-XX:+UseConcMarkSweepGC
-XX:+CMSScavengeBeforeRemark
-XX:+CMSParallelRemarkEnabled
-XX:CMSInitiatingOccupancyFraction=XX
-XX:+UseCMSInitiatingOccupancyOnly
-XX:ParallelGCThreads=X
-XX:+UseGCTaskAffinity
-XX:+BindGCTaskThreadsToCPUs
-XX:ParGCCardsPerStrideChunk=XXXXX
-XX:+UseCondCardMark

30. Thanks!
Questions?