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
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Scalability

5. GC Benchmarks
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Hardware
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Production like
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Data
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Production like in volumes and veracity
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Why? Phantom bottlenecks
Why? Phantom bottlenecks i.e. caches
Tests
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No micro-benchmarks
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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
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Increase survivors space
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Raise occupancy threshold
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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
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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
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Reduce duration of full GC
–
Avoid full GC altogether

10. Prevent fallback to stop-the-world GC
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Increase heap size
–
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Start CMS early, i.e. lower occupancy threshold
–
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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
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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
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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?