moving to g1gc

Copyright 2015 Kirk Pepperdine

Moving to G1GC


About me

- Offer performance tuning services and training- created jPDM, a performance tuning methodology

- Write and speak about performance tuning- Co-founder of jClarity- engaged in building the first generation of

performance diagnostic engines- Java Champion since 2006


Questions To Be Answered

- How does the G1GC algorithm work- What does Java heap look like- What are the tools we can use the to help us

- want to engage in evidence based tuning- configure the collector to work better with

our application- tune our application to work better with the

collector- How does it compare to the other collectors


Generational heap is- 1 large contigous reserved space- specified with -mx (not a type-o)

- split into 5 (or 4) memory pools- Eden- Survivor (from, to)- Old- Perm (prior to Java 8)


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


Why another collector

- Scalability- pause time tends to be a function of heap size

- Difficult to tune- dozens of parameters some of which are very difficult

to understand how to use- -XX:PLABSize=????

- Completely unpredictable- well, maybe but that is a different talk


G1GC

- Designed to scale- break the pause time heap size dependency

- Easier to tune- fewer configuration options

- Predictable- offer pause time goals and have the collector tune it’s

self- Does it work?- lets see!!!


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

For -mx10G,Region size = 10240M/2048 = 5mNumber of regions = 10G/4m = 2560


Regions

- Placed in a free list- When used, tagged as- Eden, Survivor, Old, or Humongous

- Retuned to free list after being swept


Allocation

- mutator threads get 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


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


Garbage Collection Triggers

- Alloted number of Eden regions have been consumed- Unable to satisfy a Humongous allocation- Heap is full triggering a Full GC- Metaspace threshold is reached- full discussion beyond the scope of this talk


Garbage Collection

- Mark-Sweep/Mark GC combination- Mark-Sweep for young- (mostly) Concurrent-Mark for Old

- Sweep evacuates live objects in a region to another region

- automatic compaction- no need for fine grain free lists (expensive)

- Marked Old regions are swept by the Young collector- Most phases require threads to be at a safe-point


Safepoint

- A point in a threads execution where it can safely stop for JVM maintenance

- Cooperative effort- JVM signals it must perform some maintenance- mutator threads stop when they reach a safe

point- JVM carries out maintenance- mutator threads are restarted

- Measure TTSP vs time to perform maintenance


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


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


2015-07-13T02:17:35.909-0700: 47.896: [GC pause (G1 Evacuation Pause) (young)Desired survivor size 109051904 bytes, new threshold 3 (max 15)- age 1: 54191968 bytes, 54191968 total- age 2: 40340312 bytes, 94532280 total- age 3: 41274112 bytes, 135806392 total 47.896: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 33336, predicted base time: 56.24 ms, remaining time: 143.76 ms, target pause time: 200.00 ms] 47.896: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 89 regions, survivors: 13 regions, predicted young region time: 174.09 ms] 47.896: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 89 regions, survivors: 13 regions, old: 0 regions, predicted pause time: 230.33 ms, target pause time: 200.00 ms]47.943: [SoftReference, 0 refs, 0.0074530 secs]47.951: [WeakReference, 2 refs, 0.0038714 secs]47.955: [FinalReference, 1753 refs, 0.0058685 secs]47.961: [PhantomReference, 65 refs, 0.0025029 secs]47.963: [JNI Weak Reference, 0.0000256 secs], 0.0711540 secs] [Parallel Time: 43.5 ms, GC Workers: 23] [GC Worker Start (ms): Min: 47896.5, Avg: 47896.9, Max: 47897.3, Diff: 0.8] [Ext Root Scanning (ms): Min: 0.9, Avg: 1.5, Max: 3.3, Diff: 2.4, Sum: 35.4] [Update RS (ms): Min: 9.7, Avg: 10.9, Max: 11.8, Diff: 2.1, Sum: 251.0] [Processed Buffers: Min: 5, Avg: 11.3, Max: 26, Diff: 21, Sum: 260] [Scan RS (ms): Min: 2.2, Avg: 2.7, Max: 3.0, Diff: 0.8, Sum: 61.3] [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.2] [Object Copy (ms): Min: 25.2, Avg: 26.3, Max: 27.5, Diff: 2.3, Sum: 604.1] [Termination (ms): Min: 0.0, Avg: 0.2, Max: 0.3, Diff: 0.3, Sum: 4.2] [GC Worker Other (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 3.6] [GC Worker Total (ms): Min: 41.2, Avg: 41.7, Max: 42.3, Diff: 1.0, Sum: 959.7] [GC Worker End (ms): Min: 47938.5, Avg: 47938.6, Max: 47938.9, Diff: 0.4] [Code Root Fixup: 0.5 ms] [Code Root Migration: 0.5 ms] [Code Root Purge: 0.0 ms] [Clear CT: 1.0 ms] [Other: 25.6 ms] [Choose CSet: 0.0 ms] [Ref Proc: 22.7 ms] [Ref Enq: 0.5 ms] [Redirty Cards: 1.0 ms] [Free CSet: 0.4 ms] [Eden: 1424.0M(1424.0M)->0.0B(1632.0M) Survivors: 208.0M->208.0M Heap: 3003.2M(32.0G)->1706.3M(32.0G)] [Times: user=1.02 sys=0.03, real=0.07 secs]

Yes, I know, the font is too smallthat will be fixed in a moment


2015-07-13T02:17:35.909-0700: 47.896: [GC pause (G1 Evacuation Pause) (young), 0.0711540 secs] [Eden: 1424.0M(1424.0M)->0.0B(1632.0M) Survivors: 208.0M->208.0M Heap: 3003.2M(32.0G)->1706.3M(32.0G)] [Times: user=1.02 sys=0.03, real=0.07 secs]

Captured using:-Xloggc:gc.log


2015-07-13T02:17:35.909-0700: 47.896: [GC pause (G1 Evacuation Pause) (young), 0.0711540 secs] [Parallel Time: 43.5 ms, GC Workers: 23] [GC Worker Start (ms): Min: 47896.5, Avg: 47896.9, Max: 47897.3, Diff: 0.8] [Ext Root Scanning (ms): Min: 0.9, Avg: 1.5, Max: 3.3, Diff: 2.4, Sum: 35.4] [Update RS (ms): Min: 9.7, Avg: 10.9, Max: 11.8, Diff: 2.1, Sum: 251.0] [Processed Buffers: Min: 5, Avg: 11.3, Max: 26, Diff: 21, Sum: 260] [Scan RS (ms): Min: 2.2, Avg: 2.7, Max: 3.0, Diff: 0.8, Sum: 61.3] [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.2] [Object Copy (ms): Min: 25.2, Avg: 26.3, Max: 27.5, Diff: 2.3, Sum: 604.1] [Termination (ms): Min: 0.0, Avg: 0.2, Max: 0.3, Diff: 0.3, Sum: 4.2] [GC Worker Other (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 3.6] [GC Worker Total (ms): Min: 41.2, Avg: 41.7, Max: 42.3, Diff: 1.0, Sum: 959.7] [GC Worker End (ms): Min: 47938.5, Avg: 47938.6, Max: 47938.9, Diff: 0.4] [Code Root Fixup: 0.5 ms] [Code Root Migration: 0.5 ms] [Code Root Purge: 0.0 ms] [Clear CT: 1.0 ms] [Other: 25.6 ms] [Choose CSet: 0.0 ms] [Ref Proc: 22.7 ms] [Ref Enq: 0.5 ms] [Redirty Cards: 1.0 ms] [Free CSet: 0.4 ms] [Eden: 1424.0M(1424.0M)->0.0B(1632.0M) Survivors: 208.0M->208.0M Heap: 3003.2M(32.0G)->1706.3M(32.0G)] [Times: user=1.02 sys=0.03, real=0.07 secs]

Captured using:-Xloggc:gc.log—XX:+PrintGCDetails


2015-07-13T02:17:35.909-0700: 47.896: [GC pause (G1 Evacuation Pause) (young)Desired survivor size 109051904 bytes, new threshold 3 (max 15)- age 1: 54191968 bytes, 54191968 total- age 2: 40340312 bytes, 94532280 total- age 3: 41274112 bytes, 135806392 total, 0.0711540 secs] [Parallel Time: 43.5 ms, GC Workers: 23] [GC Worker Start (ms): Min: 47896.5, Avg: 47896.9, Max: 47897.3, Diff: 0.8] [Ext Root Scanning (ms): Min: 0.9, Avg: 1.5, Max: 3.3, Diff: 2.4, Sum: 35.4] [Update RS (ms): Min: 9.7, Avg: 10.9, Max: 11.8, Diff: 2.1, Sum: 251.0] [Processed Buffers: Min: 5, Avg: 11.3, Max: 26, Diff: 21, Sum: 260] [Scan RS (ms): Min: 2.2, Avg: 2.7, Max: 3.0, Diff: 0.8, Sum: 61.3] [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.2] [Object Copy (ms): Min: 25.2, Avg: 26.3, Max: 27.5, Diff: 2.3, Sum: 604.1] [Termination (ms): Min: 0.0, Avg: 0.2, Max: 0.3, Diff: 0.3, Sum: 4.2] [GC Worker Other (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 3.6] [GC Worker Total (ms): Min: 41.2, Avg: 41.7, Max: 42.3, Diff: 1.0, Sum: 959.7] [GC Worker End (ms): Min: 47938.5, Avg: 47938.6, Max: 47938.9, Diff: 0.4] [Code Root Fixup: 0.5 ms] [Code Root Migration: 0.5 ms] [Code Root Purge: 0.0 ms] [Clear CT: 1.0 ms] [Other: 25.6 ms] [Choose CSet: 0.0 ms] [Ref Proc: 22.7 ms] [Ref Enq: 0.5 ms] [Redirty Cards: 1.0 ms] [Free CSet: 0.4 ms] [Eden: 1424.0M(1424.0M)->0.0B(1632.0M) Survivors: 208.0M->208.0M Heap: 3003.2M(32.0G)->1706.3M(32.0G)] [Times: user=1.02 sys=0.03, real=0.07 secs]

Captured using:-Xloggc:gc.log—XX:+PrintGCDetails-XX:+PrintTenuringDistribution


2015-07-13T02:17:35.909-0700: 47.896: [GC pause (G1 Evacuation Pause) (young)Desired survivor size 109051904 bytes, new threshold 3 (max 15)- age 1: 54191968 bytes, 54191968 total- age 2: 40340312 bytes, 94532280 total- age 3: 41274112 bytes, 135806392 total47.943: [SoftReference, 0 refs, 0.0074530 secs]47.951: [WeakReference, 2 refs, 0.0038714 secs]47.955: [FinalReference, 1753 refs, 0.0058685 secs]47.961: [PhantomReference, 65 refs, 0.0025029 secs]47.963: [JNI Weak Reference, 0.0000256 secs], 0.0711540 secs] [Parallel Time: 43.5 ms, GC Workers: 23] [GC Worker Start (ms): Min: 47896.5, Avg: 47896.9, Max: 47897.3, Diff: 0.8] [Ext Root Scanning (ms): Min: 0.9, Avg: 1.5, Max: 3.3, Diff: 2.4, Sum: 35.4] [Update RS (ms): Min: 9.7, Avg: 10.9, Max: 11.8, Diff: 2.1, Sum: 251.0] [Processed Buffers: Min: 5, Avg: 11.3, Max: 26, Diff: 21, Sum: 260] [Scan RS (ms): Min: 2.2, Avg: 2.7, Max: 3.0, Diff: 0.8, Sum: 61.3] [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.2] [Object Copy (ms): Min: 25.2, Avg: 26.3, Max: 27.5, Diff: 2.3, Sum: 604.1] [Termination (ms): Min: 0.0, Avg: 0.2, Max: 0.3, Diff: 0.3, Sum: 4.2] [GC Worker Other (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 3.6] [GC Worker Total (ms): Min: 41.2, Avg: 41.7, Max: 42.3, Diff: 1.0, Sum: 959.7] [GC Worker End (ms): Min: 47938.5, Avg: 47938.6, Max: 47938.9, Diff: 0.4] [Code Root Fixup: 0.5 ms] [Code Root Migration: 0.5 ms] [Code Root Purge: 0.0 ms] [Clear CT: 1.0 ms] [Other: 25.6 ms] [Choose CSet: 0.0 ms] [Ref Proc: 22.7 ms] [Ref Enq: 0.5 ms] [Redirty Cards: 1.0 ms] [Free CSet: 0.4 ms] [Eden: 1424.0M(1424.0M)->0.0B(1632.0M) Survivors: 208.0M->208.0M Heap: 3003.2M(32.0G)->1706.3M(32.0G)] [Times: user=1.02 sys=0.03, real=0.07 secs]

Captured using:-Xloggc:gc.log—XX:+PrintGCDetails-XX:+PrintTenuringDistribution-XX:+PrintReferenceGC


2015-07-13T02:17:35.909-0700: 47.896: [GC pause (G1 Evacuation Pause) (young)Desired survivor size 109051904 bytes, new threshold 3 (max 15)- age 1: 54191968 bytes, 54191968 total- age 2: 40340312 bytes, 94532280 total- age 3: 41274112 bytes, 135806392 total 47.896: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 33336, predicted base time: 56.24 ms, remaining time: 143.76 ms, target pause time: 200.00 ms] 47.896: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 89 regions, survivors: 13 regions, predicted young region time: 174.09 ms] 47.896: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 89 regions, survivors: 13 regions, old: 0 regions, predicted pause time: 230.33 ms, target pause time: 200.00 ms]47.943: [SoftReference, 0 refs, 0.0074530 secs]47.951: [WeakReference, 2 refs, 0.0038714 secs]47.955: [FinalReference, 1753 refs, 0.0058685 secs]47.961: [PhantomReference, 65 refs, 0.0025029 secs]47.963: [JNI Weak Reference, 0.0000256 secs], 0.0711540 secs] [Parallel Time: 43.5 ms, GC Workers: 23] [GC Worker Start (ms): Min: 47896.5, Avg: 47896.9, Max: 47897.3, Diff: 0.8] [Ext Root Scanning (ms): Min: 0.9, Avg: 1.5, Max: 3.3, Diff: 2.4, Sum: 35.4] [Update RS (ms): Min: 9.7, Avg: 10.9, Max: 11.8, Diff: 2.1, Sum: 251.0] [Processed Buffers: Min: 5, Avg: 11.3, Max: 26, Diff: 21, Sum: 260] [Scan RS (ms): Min: 2.2, Avg: 2.7, Max: 3.0, Diff: 0.8, Sum: 61.3] [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.2] [Object Copy (ms): Min: 25.2, Avg: 26.3, Max: 27.5, Diff: 2.3, Sum: 604.1] [Termination (ms): Min: 0.0, Avg: 0.2, Max: 0.3, Diff: 0.3, Sum: 4.2] [GC Worker Other (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 3.6] [GC Worker Total (ms): Min: 41.2, Avg: 41.7, Max: 42.3, Diff: 1.0, Sum: 959.7] [GC Worker End (ms): Min: 47938.5, Avg: 47938.6, Max: 47938.9, Diff: 0.4] [Code Root Fixup: 0.5 ms] [Code Root Migration: 0.5 ms] [Code Root Purge: 0.0 ms] [Clear CT: 1.0 ms] [Other: 25.6 ms] [Choose CSet: 0.0 ms] [Ref Proc: 22.7 ms] [Ref Enq: 0.5 ms] [Redirty Cards: 1.0 ms] [Free CSet: 0.4 ms] [Eden: 1424.0M(1424.0M)->0.0B(1632.0M) Survivors: 208.0M->208.0M Heap: 3003.2M(32.0G)->1706.3M(32.0G)] [Times: user=1.02 sys=0.03, real=0.07 secs]

Captured using:-Xloggc:gc.log—XX:+PrintGCDetails-XX:+PrintTenuringDistribution-XX:+PrintReferenceGC-XX:+PrintAdaptiveSizePolicy


What is this 1 record telling us?

- Initial state of the Java heap- what our application did to the heap

- Final state of Java heap- what the GC algorithm did to the heap- how long did the whole process take- breakdown of phases

- Need to use 100s of these records to make decisions


Scan for Roots

- Find all pointers external to the memory pool in question

- Use RSet to track all incoming pointers from one region

- prevent a heap scan for roots- uses cards, card marking- card is 1 word that tracks references into 512 bytes

- mark bit dirty when reference is updated


RSet

- Collection card tables- one card table for pointers

originating from each region- Dirty cards are placed in a

concurrent refinement queue- refinement threads will build the

RSet- aim is to reduce the cost of scan

for roots


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


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


Reclaiming Memory (detailed)

- Mark Sweep Copy (Evacuating) Garbage Collection- Capture all mutator threads at a safepoint- 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


Parallel Phases

- external root scanning- updating remembered sets- scan remembered sets- code root scanning- object copy- string dedup


Serial Phases

- code root fixup- code root migration- clear CT- choose CSet- Reference processing- redirty cards- free CSet


GC Log Entry Basics

2015-07-13T02:17:35.909-0700: 47.896: [GC pause (G1 Evacuation Pause) (young)…….., 0.0711540 secs]……..…….. [Eden: 1424.0M(1424.0M)->0.0B(1632.0M) Survivors: 208.0M->208.0M Heap: 3003.2M(32.0G)->1706.3M(32.0G)] [Times: user=1.02 sys=0.03, real=0.07 secs]


Parallel Phases

[Parallel Time: 43.5 ms, GC Workers: 23] [GC Worker Start (ms): Min: 47896.5, Avg: 47896.9, Max: 47897.3, Diff: 0.8] [Ext Root Scanning (ms): Min: 0.9, Avg: 1.5, Max: 3.3, Diff: 2.4, Sum: 35.4] [Update RS (ms): Min: 9.7, Avg: 10.9, Max: 11.8, Diff: 2.1, Sum: 251.0] [Processed Buffers: Min: 5, Avg: 11.3, Max: 26, Diff: 21, Sum: 260] [Scan RS (ms): Min: 2.2, Avg: 2.7, Max: 3.0, Diff: 0.8, Sum: 61.3] [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.1, Diff: 0.1, Sum: 0.2] [Object Copy (ms): Min: 25.2, Avg: 26.3, Max: 27.5, Diff: 2.3, Sum: 604.1] [Termination (ms): Min: 0.0, Avg: 0.2, Max: 0.3, Diff: 0.3, Sum: 4.2] [GC Worker Other (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 3.6] [GC Worker Total (ms): Min: 41.2, Avg: 41.7, Max: 42.3, Diff: 1.0, Sum: 959.7] [GC Worker End (ms): Min: 47938.5, Avg: 47938.6, Max: 47938.9, Diff: 0.4]


Serial Phases

[Code Root Fixup: 0.5 ms] [Code Root Migration: 0.5 ms] [Code Root Purge: 0.0 ms] [Clear CT: 1.0 ms] [Other: 25.6 ms] [Choose CSet: 0.0 ms] [Ref Proc: 22.7 ms] [Ref Enq: 0.5 ms] [Redirty Cards: 1.0 ms] [Free CSet: 0.4 ms]


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


Starting a (mostly) Concurrent Mark

4167.445: [G1Ergonomics (Concurrent Cycles) initiate concurrent cycle, reason: concurrent cycle initiation requested]2015-10-17T21:13:17.981-0400: 4167.445: [GC pause (G1 Evacuation Pause) (young) (initial-mark)Desired survivor size 4194304 bytes, new threshold 1 (max 1)- age 1: 1585104 bytes, 1585104 total 4167.459: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 984, predicted base time: 3.98 ms, remaining time: 0.00 ms, target pause time: 1.00 ms] 4167.459: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 1 regions, survivors: 1 regions, predicted young region time: 0.90 ms] 4167.459: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 1 regions, survivors: 1 regions, old: 0 regions, predicted pause time: 4.88 ms, target pause time: 1.00 ms], 0.0347452 secs]

No need to squint, a bigger version is coming

-XX:+PrintAdaptiveSizePolicy


Initiating Heap Occupancy Percent reached

4167.445: [G1Ergonomics (Concurrent Cycles) initiate concurrent cycle, reason: concurrent cycle initiation requested]


Piggyback an initial-mark onto the young collection

4167.445: [GC pause (G1 Evacuation Pause) (young) (initial-mark)……,0.0347452 secs]


CSet construction

4167.459: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 984, predicted base time: 3.98 ms, remaining time: 0.00 ms, target pause time: 1.00 ms]


CSet construction


4167.459: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 1 regions, survivors: 1 regions, predicted young region time: 0.90 ms]


CSet construction


4167.459: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 1 regions, survivors: 1 regions, predicted young region time: 0.90 ms]

4167.459: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 1 regions, survivors: 1 regions, old: 0 regions, predicted pause time: 4.88 ms, target pause time: 1.00 ms]


2.172: [GC pause (Metadata GC Threshold) (young) (initial-mark) [rest of young record removed]

2.177: [GC concurrent-root-region-scan-start]2.179: [GC concurrent-root-region-scan-end, 0.0016985 secs]

2.179: [GC concurrent-mark-start]2.184: [GC concurrent-mark-end, 0.0045815 secs]

2.184: [GC remark 2.184: [Finalize Marking, 0.0001992 secs] 2.184: [GC ref-proc2.184: [SoftReference, 0 refs, 0.0000104 secs]2.184: [WeakReference, 16 refs, 0.0000089 secs]2.184: [FinalReference, 14 refs, 0.0000082 secs]2.184: [PhantomReference, 0 refs, 1 refs, 0.0000116 secs]2.184: [JNI Weak Reference, 0.0000045 secs], 0.0000561 secs] 2.184: [Unloading, 0.0030695 secs], 0.0034609 secs] [Times: user=0.03 sys=0.00, real=0.01 secs]

2.187: [GC cleanup 35M->15M(256M), 0.0004233 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 2.188: [GC concurrent-cleanup-start]2.188: [GC concurrent-cleanup-end, 0.0000148 secs]


Common Failure Conditions

[GC pause (young) (to-space exhausted), 0.1709670 secs]

- Collection ran out of reserved space

- protect against temporary overflows

- -XX:G1ReservedPercent=10

- Heap is too small

[GC concurrent-mark-reset-for-overflow]

- Global marking stack filled

- Collection started too late


Tools for evidence based tuning

- GC logs- Log file parser/visualization tool- HPJMeter- GCViewer

- Censum- Flags- dozens of flags- most of them you don’t want to touch!!!!


Flags (you want to use)

-XX:+UseG1GC-mx4G-XX:MaxGCPauseMillis=200

-Xloggc:gc.log-XX:+PrintGCDetails-XX:+PrintTenuringDistribution-XX:+PrintReferenceGC"-XX:+PrintGCApplicationStoppedTime-XX:+PrintGCApplicationConcurrentTime"


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


Flags (you should think twice about using)

-XX:G1MixedGCCountTarget=8

-XX:+UnlockExperimentalVMOptions"-XX:G1MixedGCLiveThresholdPercent=85/65


Flags (you should never use)

-XX:+UnlockExperimentalVMOptions"-XX:G1OldCSetRegionThresholdPercent=10-XX:G1MaxNewSizePercent=60-XX:G1HeapWastePercent=10-XX:G1RSetUpdatingPauseTimePercent=10


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


Some results

00:12:35

00:14:40

00:16:45

00:18:50

00:20:55

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