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

= 5m

Number 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 small

that will be ﬁxed in a moment

2015-07-13T02:17:35.909-0700: 47.896: [GC pause (G1 Evacuation Pause) (young), 0.0711540 secs]


Captured using:

-Xloggc:gc.log

2015-07-13T02:17:35.909-0700: 47.896: [GC pause (G1 Evacuation Pause) (young), 0.0711540 secs]
















[Clear CT: 1.0 ms]

[Other: 25.6 ms]


[Ref Proc: 22.7 ms]

[Ref Enq: 0.5 ms]


[Free CSet: 0.4 ms]


Captured using:

-Xloggc:gc.log

—XX:+PrintGCDetails



- age 1: 54191968 bytes, 54191968 total

- age 2: 40340312 bytes, 94532280 total

- age 3: 41274112 bytes, 135806392 total

, 0.0711540 secs]
















[Clear CT: 1.0 ms]

[Other: 25.6 ms]


[Ref Proc: 22.7 ms]

[Ref Enq: 0.5 ms]


[Free CSet: 0.4 ms]


Captured using:

-Xloggc:gc.log


-XX:+PrintTenuringDistribution



- age 1: 54191968 bytes, 54191968 total

- age 2: 40340312 bytes, 94532280 total

- age 3: 41274112 bytes, 135806392 total

















[Clear CT: 1.0 ms]

[Other: 25.6 ms]


[Ref Proc: 22.7 ms]

[Ref Enq: 0.5 ms]


[Free CSet: 0.4 ms]


Captured using:

-Xloggc:gc.log



-XX:+PrintReferenceGC



- age 1: 54191968 bytes, 54191968 total

- age 2: 40340312 bytes, 94532280 total

- age 3: 41274112 bytes, 135806392 total




















[Clear CT: 1.0 ms]

[Other: 25.6 ms]


[Ref Proc: 22.7 ms]

[Ref Enq: 0.5 ms]


[Free CSet: 0.4 ms]


Captured using:

-Xloggc:gc.log



-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


……..

, 0.0711540 secs]

……..

……..



Parallel Phases













Serial Phases




[Clear CT: 1.0 ms]

[Other: 25.6 ms]


[Ref Proc: 22.7 ms]

[Ref Enq: 0.5 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)


- age 1: 1585104 bytes, 1585104 total




, 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

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]

CSet construction

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]

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]


2.187: [GC cleanup 35M->15M(256M), 0.0004233 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:+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:G1MixedGCLiveThresholdPercent=85/65

Flags (you should never use)


-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
1 2 3 4 5 6 7 8 9 10 11

More results
0
17500
35000
52500
70000
1 2 3 4 5 6 7 8 9 10 11 12

Performance Seminar
www.kodewerk.com
Java
Performance Tuning,
May 26-29, Chania
Greece

Moving to G1GC

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