Tuning the g1gc

Copyright 2018 Kirk Pepperdine
TUNING G1GC
A GUIDE TO

HOW TO TUNE THE G1GC
▸ Set -XX:+UseG1GC
▸ Set max heap size using -mx
▸default value is 1/4 of physical RAM
▸ Set pause time goal using -XX:MaxGCPauseMillis=200 (default value)
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Questions?
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I lift, you grab. … Was that
concept just a little too complex?

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If only it was
that easy!

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▸ Kirk Pepperdine
▸ Author of jPDM, a performance diagnostic model
▸ Author of the original Java Performance Tuning workshop
▸ Co-founded jClarity
▸ Building the smart generation of performance diagnostic tooling
▸ Bring predictability into the diagnostic process
▸ Co-founded JCrete
▸ The hottest unconference on the planet
▸ Java Champion(s)
OUR MARKETING SLIDE

▸Models help us to
▸ understand how things work
▸ develop a cost model
▸Memory management consists of
▸ allocators
▸ mutators
▸ Garbage Collector
IMPORTANCE OF MODELS
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▸Allocator/Mutator takes on all responsibility for managing memory
▸ unmanaged runtimes
▸Workload is shared between Allocator/Mutator and Garbage Collectors
▸ balance between allocation/mutation throughput and GC pause time
▸more work to GC equates to longer GC pause times
▸more work to allocator/mutators equates to reduces application throughput
▸ Lets develop an understanding so we can build a cost model
ALLOCATORS VS COLLECTORS
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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
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THE G1GC HEAP
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▸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

G1GC ALLOCATION
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All regions are put into a free region list

G1GC ALLOCATION
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Free Region List

ERGONOMICS
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Free Region List
Regions
allotted
for Eden

G1GC ALLOCATION
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Free Region List
MyDominObject mdo = new MyDomainObject
Take region from free list

G1GC ALLOCATION
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Free Region List
Label it as Eden

G1GC ALLOCATION
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Free Region List
allocate

G1GC ALLOCATION
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Free Region List
Continue until
allotted Regions
are full

G1GC ALLOCATION
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Free Region List
Allocation failure triggers Young Gen collection

FORMING A CSET
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Free Region List
Allocation failure triggers Young Gen collection
CSet

YOUNG COLLECTION
▸ Mark/Sweep (copy)
▸Mix of parallel and serial phases
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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

G1GC SIZING
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Free Region List
Head of List
New allocation limit
-XX:G1MaxNewSizePercentage=60, -XX:G1NewSizePercentage=5
Survivor regions

THE G1GC HEAP
Eden Regions
Unallocated Regions
Young
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Survivor Region
aka: to space

TENURING
Young
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Survivor Region
aka: to space
Tenured Region
Eden Regions
Unallocated Regions

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
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Time complexity!!!!

GC ROOTS
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GC ROOTS
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Metaspace

GC ROOTS
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Metaspace
Compressed
Classspace

GC ROOTS
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JNI
Metaspace
Compressed
Classspace

GC ROOTS
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Stack FramesJNI
Metaspace
Compressed
Classspace

GC ROOTS
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Code Cache
Stack FramesJNI
Metaspace
Compressed
Classspace

GC ROOTS
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Code Cache
Stack FramesJNI
Metaspace
Compressed
Classspace
C0
L1
L2
L3
Cn
L1
L2…
…
…
…
CPU
Registers

SCAN FOR ROOTS
▸ Multi-threaded but….
▸Each individual space can be managed by a single thread
▸Large collections can only be managed by a single thread
▸ Scan for root time will be the longest time for scanning any single space
▸Often scan for roots is linear to the size of tenured
▸ G1 avoids this by introducing a remembered set (RSet)
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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
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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
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[15.316s][info ][gc,start ] GC(0) Pause Young (G1 Evacuation Pause)
[15.316s][info ][gc,start ] GC(0) Pause Young (G1 Evacuation Pause)
[15.316s][info ][gc,task ] GC(0) Using 8 workers of 8 for evacuation
[15.322s][info ][gc,phases ] GC(0) Pre Evacuate Collection Set: 0.0ms
[15.322s][info ][gc,phases ] GC(0) Evacuate Collection Set: 4.9ms
[15.322s][info ][gc,phases ] GC(0) Post Evacuate Collection Set: 1.0ms
[15.322s][info ][gc,phases ] GC(0) Other: 0.2ms
[15.322s][info ][gc,heap ] GC(0) Eden regions: 24->0(150)
[15.322s][info ][gc,heap ] GC(0) Survivor regions: 0->3(3)
[15.322s][info ][gc,heap ] GC(0) Old regions: 0->2
[15.322s][info ][gc,heap ] GC(0) Humongous regions: 0->0
[15.322s][info ][gc,metaspace ] GC(0) Metaspace: 16151K->16151K(1064960K)
[15.322s][info ][gc ] GC(0) Pause Young (G1 Evacuation Pause) 24M->4M(256M) 6.183ms
[15.322s][info ][gc,cpu ] GC(0) User=0.04s Sys=0.00s Real=0.00s
YOUNG COLLECTION
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[15.322s][info ][gc,phases ] GC(0) Pre Evacuate Collection Set: 0ms
[15.322s][debug][gc,phases ] GC(0) Choose Collection Set: 0.0ms
[15.322s][debug][gc,phases ] GC(0) Humongous Register: 0.0ms
YOUNG COLLECTION
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[15.322s][info ][gc,phases ] GC(0) Evacuate Collection Set: 4.9ms
[15.322s][debug][gc,phases ] GC(0) Ext Root Scanning (ms): Min: 0.3, Avg: 0.8, Max: 2.5, Diff: 2.2, Sum: 6.3, Workers: 8
[15.322s][debug][gc,phases ] GC(0) Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 8
[15.322s][debug][gc,phases ] GC(0) Processed Buffers: Min: 0, Avg: 0.0, Max: 0, Diff: 0, Sum: 0, Workers: 8
[15.322s][debug][gc,phases ] GC(0) Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 8
[15.322s][debug][gc,phases ] GC(0) Code Root Scanning (ms): Min: 0.0, Avg: 0.2, Max: 0.4, Diff: 0.4, Sum: 1.7, Workers: 8
[15.322s][debug][gc,phases ] GC(0) Object Copy (ms): Min: 1.8, Avg: 3.4, Max: 4.3, Diff: 2.5, Sum: 27.0, Workers: 8
[15.322s][debug][gc,phases ] GC(0) Termination (ms): Min: 0.0, Avg: 0.4, Max: 0.6, Diff: 0.6, Sum: 3.4, Workers: 8
[15.322s][debug][gc,phases ] GC(0) Termination Attempts: Min: 1, Avg: 17.4, Max: 34, Diff: 33, Sum: 139, Workers: 8
[15.322s][debug][gc,phases ] GC(0) GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1, Workers: 8
[15.322s][debug][gc,phases ] GC(0) GC Worker Total (ms): Min: 4.8, Avg: 4.8, Max: 4.8, Diff: 0.0, Sum: 38.6, Workers: 8
YOUNG COLLECTION
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[15.322s][info ][gc,phases ] GC(0) Post Evacuate Collection Set: 1.0ms
[15.322s][debug][gc,phases ] GC(0) Code Roots Fixup: 0.1ms
[15.322s][debug][gc,phases ] GC(0) Preserve CM Refs: 0.0ms
[15.322s][debug][gc,phases ] GC(0) Reference Processing: 0.7ms
[15.322s][debug][gc,phases ] GC(0) Clear Card Table: 0.0ms
[15.322s][debug][gc,phases ] GC(0) Reference Enqueuing: 0.0ms
[15.322s][debug][gc,phases ] GC(0) Merge Per-Thread State: 0.0ms
[15.322s][debug][gc,phases ] GC(0) Code Roots Purge: 0.0ms
[15.322s][debug][gc,phases ] GC(0) Redirty Cards: 0.0ms
[15.322s][debug][gc,phases ] GC(0) Free Collection Set: 0.2ms
[15.322s][debug][gc,phases ] GC(0) Humongous Reclaim: 0.0ms
[15.322s][debug][gc,phases ] GC(0) Expand Heap After Collection: 0.0ms
YOUNG COLLECTION
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[73.082s][info ][gc ] GC(263) Concurrent Cycle
[73.082s][info ][gc,marking ] GC(263) Concurrent Clear Claimed Marks
[73.082s][info ][gc,marking ] GC(263) Concurrent Clear Claimed Marks 0.018ms
[73.082s][info ][gc,marking ] GC(263) Concurrent Scan Root Regions
[73.084s][info ][gc,marking ] GC(263) Concurrent Scan Root Regions 2.325ms
[73.084s][info ][gc,marking ] GC(263) Concurrent Mark (73.084s)
[73.084s][info ][gc,marking ] GC(263) Concurrent Mark From Roots
[73.084s][info ][gc,task ] GC(263) Using 2 workers of 2 for marking
[73.138s][info ][gc,marking ] GC(263) Concurrent Mark From Roots 53.902ms
[73.138s][info ][gc,marking ] GC(263) Concurrent Mark (73.084s, 73.138s) 53.954ms
[73.139s][info ][gc,start ] GC(263) Pause Remark
[73.160s][info ][gc,stringtable] GC(263) Cleaned string and symbol table, strings: 7924 processed, 21 removed, symbols: 55530
processed, 17 removed
[73.160s][info ][gc ] GC(263) Pause Remark 211M->211M(256M) 21.685ms
[73.160s][info ][gc,marking ] GC(263) Concurrent Create Live Data
[73.168s][info ][gc,marking ] GC(263) Concurrent Create Live Data 8.089ms
[73.169s][info ][gc,start ] GC(263) Pause Cleanup
[73.169s][info ][gc ] GC(263) Pause Cleanup 223M->213M(256M) 0.271ms
[73.169s][info ][gc,marking ] GC(263) Concurrent Complete Cleanup
[73.169s][info ][gc,marking ] GC(263) Concurrent Complete Cleanup 0.013ms
[73.169s][info ][gc,marking ] GC(263) Concurrent Cleanup for Next Mark
[73.171s][info ][gc,marking ] GC(263) Concurrent Cleanup for Next Mark 1.646ms
[73.171s][info ][gc ] GC(263) Concurrent Cycle 89.437ms
TENURED COLLECTION
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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]
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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
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MARK AND CALCULATE LIVELINESS
Tenured regions
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SORT BY LIVELINESS
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SORT BY LIVELINESS
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free eligible for collection not eligible
-XX:G1MixedGCLiveThresholdPercent=85

ELIGIBLE REGIONS
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MIXED COLLECTION COUNT
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-XX:G1OldCSetRegionThresholdPercent=10
-XX:G1HeapWastePercent=10
mixed 1 mixed 2 mixed 3 mixed 8…

HUMONGOUS ALLOCATIONS
Tenured
Young
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Tenured Region
Humungous Region (start)
Humungous Region (cont)
Survivor Region
aka: to space
Eden Region
Unallocated Region

HUMONGOUS ALLOCATIONS
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▸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

RESERVED SPACE
Tenured Region
Survivor Region
aka: to space
Eden Region
Unallocated Region
Tenured
Young
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Reserved

SOMETHINGS GONE OFF
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HOW TO GET A GC LOG
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-Xloggc:gc.log
-XX:+PrintGCDetails
-XX:+PrintTenuringDistribution
-XX:+PrintGCDateStamps
-XX:+PrintGCApplicationStoppedTime
-XX:+PrintGCApplicationConcurrentTime
-XX:+PrintReferenceGC
-Xlog:gc*,gc+ref,gc+phases=debug,gc+age=debug,safepoint:ﬁle=gc.log

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DON’T LIKE READING GC LOGS?
I’ve got
your back

▸ JDK 9
▸ G1GC is default collector
▸ improve ability to determine important memory setting
▸ migrate GC logging to Uniﬁed logging
▸ JDK 10
▸ Parallel full GC
▸ JDK 11
▸ new perf counters for STW during concurrent collection
▸ adaptive parallel reference processing
▸ Adaptive scaling for threads
RELEASE NOTES
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WHY IS IT TAKING SO LONG???
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Lets look at a GC Log using Censum

CONTROLLING OBJECT COPY COSTS
volume of reachable objects
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Occupancy
Age

TEXT
TITLE TEXT
▸ Body Level One
▸ Body Level Two
▸ Body Level Three
▸ Body Level Four
▸ Body Level Five
Java Performance Tuning Workshop
Send us a Java 11 GC log or
tweet about @jclarity
and #censum
and
receive a free Censum License

Tuning the g1gc

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