Moving to g1 gc by Kirk Pepperdine.
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Kirk Pepperdine discusses G1GC, a garbage collector in Java designed to improve scalability and performance, particularly in Java 9. The document covers its architecture, heap management, memory reclamation, and garbage collection processes, while comparing G1GC to previous collectors like CMS. Additionally, it outlines configurations, tuning strategies, and benchmarks for optimizing Java applications, particularly Cassandra.
Moving to g1 gc by Kirk Pepperdine.
- 1. Copyright 2016 Kirk Pepperdine Moving to G1GC
- 2. Copyright 2016 Kirk Pepperdine About me - Write and speak about performance tuning - Offer performance tuning services and training - created jPDM, a performance tuning methodology - Co-founder of jClarity - building the first generation of performance diagnostic engines - Java Champion since 2006
- 3. Copyright 2016 Kirk Pepperdine G1GC will the be the default collector in Java 9 What impact might this have on your applications performance?
- 4. Copyright 2016 Kirk Pepperdine Questions To Be Answered - What does a Regional heap look like? - How does the current G1GC algorithm work? - How does performance compare to other collectors - What are the tools we can use the to help us; - engage in evidence based tuning - develop strategies so GC doesn’t interfere with with our application’s throughput - tune our application to work better with the collector
- 5. Copyright 2016 Kirk Pepperdine Generational Garbage Collection - Mark-Sweep Copy (evacuation) for Young - eden and survivor spaces - both serial and parallel implementations - Mark-Sweep (in-place) for Old space - Serial and Parallel with compaction - (mostly) Concurrent Mark-Sweep - incremential mode
- 6. Copyright 2016 Kirk Pepperdine Why another collector - Scalability - pause time tends to be a function of heap size - CMS is difficult to tune - dozens of parameters some of which are very difficult to understand how to use - -XX:TLABWasteTargetPercent=???? - Completely unpredictable - well, maybe but that is a different talk
- 7. Copyright 2016 Kirk Pepperdine G1GC - Designed to scale - break the pause time heap size dependency - Easier to tune (maybe) - fewer configuration options - Predictable - offer pause time goals and have the collector tune it’s self
- 8. Copyright 2016 Kirk Pepperdine A G1GC heap is - 1 large contigous reserved space - specified with -mx - split into ~2048 regions - size is 1, 2, 4, 8, 16, 32, or 64m eg. -mx10G, Region size = 10240M/2048 = 5m reduce to 4G Number of regions = 10G/4m = 2560
- 9. Copyright 2016 Kirk Pepperdine Regions - Free regions are kept on a free region list - When in use will be tagged as; - Eden, Survivor, Old, or Humongous
- 10. Copyright 2016 Kirk Pepperdine Allocation - Free regions are kept on a free region list - mutator threads acquire a region from region free list - tag region as Eden - allocate object into region - when region is full, get a new regions from free list Eden Eden Eden Eden
- 11. Copyright 2016 Kirk Pepperdine Humongous Allocation - allocation is larger than 1/2 a regions size - size of a regions defines what is humongous - allocate into a humoungous region - created from a set of contigous regions Eden Eden Eden Eden Humongous
- 12. Copyright 2016 Kirk Pepperdine Garbage Collection Triggers - Alloted number of Eden regions have been consumed - Unable to satisfy a Humongous allocation - regions fragmentation - may lead to full collection - Heap is full - full collection - Metaspace threshold is reached - full discussion beyond the scope of this talk
- 13. Copyright 2016 Kirk Pepperdine Garbage Collection - Young Gen is Mark-Sweep - Mostly Concurrent-Mark of Tenured - initial-mark included with Young-Gen collection - concurrent-root-region-scan - concurrent-mark - remark - cleanup - concurrent-cleanup - Mixed is mark Young, sweep Young and some tenured
- 14. Copyright 2016 Kirk Pepperdine Reclaiming Memory (detailed) - Mark Sweep Copy (Evacuating) Garbage Collection - Capture all mutator threads at a safepoint - Complete RSet refinement - Scan for GC Roots - Trace all references from GC roots - mark all data reached during tracing - Copy all marked data into a “to space” - Reset supporting structures - Release all mutator threads
- 15. Copyright 2016 Kirk Pepperdine RSet - Track all external pointers to a region - GC roots for the region - Expensive to update - mutations recored to a refinement queue - update delegated to refinement threads
- 16. Copyright 2016 Kirk Pepperdine RSet Refinement - Refinement queue is divided into 4 regions - White: no refinement threads are working - Green: number of cards that can be processed without exceeding 10% of pause time - Yellow: all refinement threads are working to keep up - Red: Application threads are involved in refinement
- 17. Copyright 2016 Kirk Pepperdine CSets - Set of all regions to be swept - Goal is to keep pauses under MaxGCPauseMillis - controls the size of the CSet - CSet contain - all Young regions - selected Old regions during mixed collections - number / mixed GC ratio
- 18. Copyright 2016 Kirk Pepperdine Heap after a Mark/Sweep - all surviving objects are copied into (to) Survivor regions - Eden and (from) Survivor regions are returned to free regions list Humoungous Survivor
- 19. Copyright 2016 Kirk Pepperdine Promotion to Old - Data is promoted to old - from survivor when it reaches tenuring threshold - to prevent survivor from being overrun - pre-emptive or reactive Humongous Survivor Old
- 20. Copyright 2016 Kirk Pepperdine Parallel Phases - external root scanning - updating remembered sets - scan remembered sets - code root scanning - object copy - string dedup
- 21. Copyright 2016 Kirk Pepperdine Serial Phases - code root fixup - code root migration - clear CT - choose CSet - Reference processing - redirty cards - free CSet
- 22. Copyright 2016 Kirk Pepperdine Starting a (mostly) Concurrent Cycle - Scheduled when heap occupancy reaches 45% - initial-mark runs inside a Young collection - mark calculates livelyness - used for CSet inclusion decisions Eden Eden Eden Eden Humoungous Survivor Survivor Old OldOld Old Old Old Old Old OldOld Old Old Old Old Old
- 23. Copyright 2016 Kirk Pepperdine Flags (you want to use) -XX:+UseG1GC -mx4G -XX:MaxGCPauseMillis=200 -Xloggc:gc.log -XX:+PrintGCDetails -XX:+PrintTenuringDistribution -XX:+PrintReferenceGC" -XX:+PrintGCApplicationStoppedTime -XX:+PrintGCApplicationConcurrentTime"
- 24. Copyright 2016 Kirk Pepperdine Flags (you might want to use) -XX:G1HeapRegionSize=1 -XX:InitiatingHeapOccupancyPercent=45 -XX:+UnlockExperimentalVMOptions" -XX:G1NewSizePercent=5 -XX:+UnlockDiagnosticVMOptions -XX:+G1PrintRegionLivenessInfo -XX:SurvivorRatio=6 -XX:MaxTenuringThreshold=15
- 25. Copyright 2016 Kirk Pepperdine Flags (you should think twice about using) -XX:G1MixedGCCountTarget=8 -XX:+UnlockExperimentalVMOptions" -XX:G1MixedGCLiveThresholdPercent=85/65
- 26. Copyright 2016 Kirk Pepperdine Flags (you should never use) -XX:+UnlockExperimentalVMOptions" -XX:G1OldCSetRegionThresholdPercent=10 -XX:G1MaxNewSizePercent=60 -XX:G1HeapWastePercent=10 -XX:G1RSetUpdatingPauseTimePercent=10
- 27. Copyright 2016 Kirk Pepperdine Things that give the G1 grief - RSet refinement - too much overhead to put work on mutator thread - affects application throughput - high rates of mutation place pressure on RSet refinement - will affect Young parallel phase and remark times - Object copy - not much to say here (unfortunately)
- 28. Copyright 2016 Kirk Pepperdine Things that give the G1 grief - Humongous allocations - definition controlled by region size - bigger region yields bigger RSet refinement costs - Floating garbage - “dead” objects in other regions keep dead objects alive - negative impact on object copy costs - more aggressive ripeness settings - most costly collections
- 29. Copyright 2016 Kirk Pepperdine Tuning Cassandra (benchmark) - Out of the box tuned for using CMS - exceptionally complex set of configurations - Reconfigured - to run G1 - given fixed unit of work which should ideally be cleared in 15 minutes Goal: Configure G1 to maximize MMU
- 30. Copyright 2016 Kirk Pepperdine Cassandra throughput running @ 100% CPU 0 17500 35000 52500 70000 1 2 3 4 5 6 7 8 9 10 11 12 CMS G1GC
- 31. Copyright 2016 Kirk Pepperdine Run times 00:12:35 00:14:40 00:16:45 00:18:50 00:20:55 1 2 3 4 5 6 7 8 9 10 11
- 32. Copyright 2016 Kirk Pepperdine Weak Generational Hypothesis Rate Time
- 33. Copyright 2016 Kirk Pepperdine Performance Seminar ` www.kodewerk.com Java Performance Tuning, June 2-5, Chania Greece