Tuning G1GC

A talk derived from my practical experiences with Tuning G1GC in a number of different production environments

Transcript

1. Copyright 2017 Kirk Pepperdine TUNING THE G1GC A GUIDE TO
2. Copyright 2017 Kirk Pepperdine HOW TO TUNE THE G1GC ▸ Set max heap size using -mx ▸default value is 1/4 of physical RAM ▸ Set pause time goal using -XX:MaxGCPauseMillis=200 (default value) jClarity
3. Copyright 2017 Kirk Pepperdine Questions? jClarity
4. Copyright 2017 Kirk Pepperdine jClarity I lift, you grab. … Was that concept just a little too complex, Martijn?
5. Copyright 2017 Kirk Pepperdine jClarity If only it was that easy!
6. Copyright 2017 Kirk Pepperdine jClarity ▸ Martijn Verburg, Kirk Pepperdine ▸ Authors of jPDM, a performance diagnostic model ▸ Co-founded jClarity ▸ Building the smart generation of performance diagnostic tooling ▸ Bring predictability into the diagnostic process ▸ Martijn über community person (delayed Java 9 for you all) ▸ Kirk Co-founded JCrete ▸ The hotest unconference on the planet ▸ Java Champion(s) OUR MARKETING SLIDE
7. Copyright 2017 Kirk Pepperdine THINGS WE NEED ▸ Java Heap ▸Regions ▸ Mark-Sweep (copy) ▸Young generational collection ▸Tenured Mark ▸Mixed collection ▸ Supporting Data Structures ▸ Collection set (CSet) ▸ Remembered Set (RSet) ▸ RSet refinement queue ▸ Tuning G1GC jClarity
8. Copyright 2017 Kirk Pepperdine THE G1GC HEAP jClarity ▸Reserved as a single contiguous region at JVM startup ▸Divide into 2048 to 4095 regions ▸ heap size is specified with -mx ▸ regions size is one of 1, 2, 4, 8, 16, or 32m ▸-XX:G1HeapRegionSize=<n> ▸example for -mx10G Region size = 10240M/2048 = 5m Number of regions = 10G/4m = 2560 regions
9. Copyright 2017 Kirk Pepperdine G1GC ALLOCATION jClarity All regions are put into a free region list
10. Copyright 2017 Kirk Pepperdine G1GC ALLOCATION jClarity Free Region List
11. Copyright 2017 Kirk Pepperdine ERGONOMICS jClarity Free Region List Regions allotted for Eden
12. Copyright 2017 Kirk Pepperdine G1GC ALLOCATION jClarity Free Region List MyDominObject mdo = new MyDomainObject Take region from free list
13. Copyright 2017 Kirk Pepperdine G1GC ALLOCATION jClarity Free Region List MyDominObject mdo = new MyDomainObject Label it as Eden
14. Copyright 2017 Kirk Pepperdine G1GC ALLOCATION jClarity Free Region List MyDominObject mdo = new MyDomainObject allocate
15. Copyright 2017 Kirk Pepperdine G1GC ALLOCATION jClarity Free Region List Continue until allotted Regions are full
16. Copyright 2017 Kirk Pepperdine G1GC ALLOCATION jClarity Free Region List Allocation failure triggers Young Gen collection
17. Copyright 2017 Kirk Pepperdine FORMING A CSET jClarity Free Region List Allocation failure triggers Young Gen collection CSet
18. Copyright 2017 Kirk Pepperdine YOUNG COLLECTION ▸ Mark/Sweep (copy) ▸Mix of parallel and serial phases jClarity 1. Place all young gen regions into a CSet 2. Calculate a root set for the CSet 3. Mark all live data in the CSet 4. Evacuate all live data to the ‘to’ space
 The ‘to’ space regions allocated from the free region list 5. Place eden regions back on free list 6. Ergonomics recalculates number of regions to allocate to Eden
19. Copyright 2017 Kirk Pepperdine G1GC SIZING jClarity Free Region List Head of List New allocation limit -XX:G1MaxNewSizePercentage=60, -XX:G1NewSizePercentage=5 Survivor regions
20. Copyright 2017 Kirk Pepperdine THE G1GC HEAP Eden Regions Unallocated Regions Young jClarity Survivor Regions aka: to space
21. Copyright 2017 Kirk Pepperdine TENURING Tenured Region Survivor Region aka: to space Eden Region Unallocated Region Young jClarity
22. Copyright 2017 Kirk Pepperdine YOUNG COLLECTION ▸ Mark/Sweep (copy) ▸Mix of parallel and serial phases ▸Place all young gen regions into a CSet ▸Calculation a root set for the CSet ▸Mark all live ▸Evacuate all live to survivor regions allocated from the free region list ▸Place eden regions back on free list ▸Ergonomics recalculates number of regions to allocate to Eden jClarity Time complexity!!!!
23. Copyright 2017 Kirk Pepperdine TENURING Tenured Region Survivor Region aka: to space Eden Region Unallocated Region Young jClarity CSet Roots are here Scan for roots is linear to the size of heap
24. Copyright 2017 Kirk Pepperdine ALLOCATORS VS COLLECTORS ▸Pay me now or pay me later ▸ Allocator can allocate fast ▸The collector has more work ▸ Longer pause times ▸ Allocators help the collector ▸Shorter pause times ▸Slower application throughput jClarity
25. Copyright 2017 Kirk Pepperdine RSET ▸Each Young gen region has an RSet ▸ Tracks pointers from Tenured regions into target region ▸ Requires a write barrier ▸ Has an associated space complexity ▸Complex implemetation solves this ▸RSet maintenance costs are non-linear to size of the region ▸ Mutations place an unwanted burden on the mutator threads jClarity
26. Copyright 2017 Kirk Pepperdine RSET REFINEMENT QUEUE No threads are working Number of RSets that can be processed run less than 10% of the pause time goal All refinement threads are working G1UpdatingPauseTimePercent > 10% Application threads are involved in RSet refinement. Slow mutation rates which acts as back pressure jClarity
27. Copyright 2017 Kirk Pepperdine COLLECTING TENURED ▸ Triggered at IHOP=45% ▸ Tenured regions are marked ▸region liveliness is calculated by the mark 23.615: [GC pause (Metadata GC Threshold) (young) (initial-mark) 23.649: [GC concurrent-root-region-scan-start] 23.698: [GC concurrent-root-region-scan-end, 0.0484826 secs] 23.698: [GC concurrent-mark-start] 23.889: [GC concurrent-mark-end, 0.1907474 secs] 23.889: [GC remark 23.889: [Finalize Marking, 0.0002867 secs] 23.889: [GC ref-proc, 0.0001423 secs] 23.889: [Unloading, 0.0094320 secs], 0.0123489 secs] 23.902: [GC cleanup 504M->500M(1024M), 0.0015885 secs] 23.903: [GC concurrent-cleanup-start] 23.903: [GC concurrent-cleanup-end, 0.0000172 secs] jClarity
28. Copyright 2017 Kirk Pepperdine BUILDING A CSET REVISITED ▸ CSet contains all eden and survivor regions ▸a mixed collection will add a subset of tenured regions to the CSet ▸ CSet size is controlled by ▸the pause time goal (200ms default) ▸minimum collection percentages (10%) ▸other factors jClarity
29. Copyright 2017 Kirk Pepperdine MARK AND CALCULATE LIVELINESS Tenured regions jClarity
30. Copyright 2017 Kirk Pepperdine SORT BY LIVELINESS jClarity
31. Copyright 2017 Kirk Pepperdine SORT BY LIVELINESS jClarity free eligible for collection not eligible -XX:G1MixedGCLiveThresholdPercent=85
32. Copyright 2017 Kirk Pepperdine ELIGIBLE REGIONS jClarity
33. Copyright 2017 Kirk Pepperdine MIXED COLLECTION COUNT jClarity -XX:G1OldCSetRegionThresholdPercent=10 -XX:G1HeapWastePercent=10 mixed 1 mixed 2 mixed 3 mixed 8 …
34. Copyright 2017 Kirk Pepperdine HUMONGOUS ALLOCATIONS Tenured Region Humungous Region (start) Humungous Region (cont) Survivor Region aka: to space Eden Region Unallocated Region Tenured Young jClarity
35. Copyright 2017 Kirk Pepperdine HUMONGOUS ALLOCATIONS Humungous Region (start) Humungous Region (cont) jClarity ▸Allocation that is 1/2 the size of a region ▸Fragmented heaps may not have enough space to satisfy an allocation ▸ dip into reserved space ▸ trigger a young gen collection with and initial-mark ▸commit more regions to the heap ▸ worst case can trigger a Full GC
36. Copyright 2017 Kirk Pepperdine RESERVED SPACE Tenured Region Humungous Region (start) Humungous Region (cont) Survivor Region aka: to space Eden Region Unallocated Region Tenured Young jClarity Reserved
37. Copyright 2017 Kirk Pepperdine HOW TO GET A GC LOG jClarity -Xloggc:gc.log -XX:+PrintGCDetails -XX:+PrintTenuringDistribution -XX:+PrintGCDateStamps -XX:+PrintGCApplicationStoppedTime -XX:+PrintGCApplicationConcurrentTime -XX:+PrintReferenceGC
38. Copyright 2017 Kirk Pepperdine jClarity DON’T LIKE READING GC LOGS? I’ve got your back
39. Copyright 2017 Kirk Pepperdine WHY IS IT TAKING SO LONG??? jClarity Lets look at a GC Log using Censum
40. Copyright 2017 Kirk Pepperdine CONTROLLING OBJECT COPY COSTS volume of reachable objects jClarity Occupancy Age
41. Copyright 2017 Kirk Pepperdine TEXT TITLE TEXT ▸ Body Level One ▸ Body Level Two ▸ Body Level Three ▸ Body Level Four ▸ Body Level Five Java Performance Tuning Workshop www.jClarity.com @kcpeppe @karianna @jclarity