Garbage Collection for Dummies

Copyright © 2012 Akira Koyasu. Some rights reserved.

Garbage Collection for DummiesOverview HotSpot GC

photo#1


AgendaJVM & GC heap & GC fundamentals

Collectors in HotSpot JVM HotSpot Collectors explained

Ergonomics & Tuning the start point of GC tuning

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JVM & GC

heap in JVM

Garbage Collection

stop the world

Generational GC

3


heap in JVM

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public static void main(String[] args) { int i = 1234; long[] longs = new long[16]; String str = "Hello!!"; Simple s = new Simple();}

Which objects are allocated in heap?

Copyright © 2012 Akira Koyasu. Some rights reserved.5

javap -c

Copyright © 2012 Akira Koyasu. Some rights reserved.5

$ javap -c study.gc.bootstrap.HeapCompiled from "Heap.java"public class study.gc.bootstrap.Heap { ... public static void main(java.lang.String[]); Code: 0: sipush 1234 3: istore_1 4: bipush 16 6: newarray long 8: astore_2 9: ldc #16 // String Hello!! 11: astore_3 12: new #18 // class study/gc/Simple 15: dup 16: invokespecial #20 // Method study/gc/Simple."<init>":()V 19: astore 4 21: return }

javap -c


Garbage Collection

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“Our resource is limited.So garbages should be

collected and recycled as soon as possible”


Dead or Alive

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void main() { sub();}void sub() { Greeter g = new Greeter(); Message msg; msg = new Message("Morning!"); g.doGreet(msg); msg = new Message("Hello!"); g.doGreet(msg);}

Greeter

Message

Message

at the end of each method, which objects are dead?


Dead or Alive

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Unreferenced objects are dead

reassignment

scope out


Fragment, Compaction, Allocation

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A CB ABB B



9

A CB A

fragment



9

A CB A



9

A CB A

Compaction



9

A CB A



9

A CB A

Allocation



9

A CB A C


Live object identification

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Reference counting

Traverse marking from root

Postman

Message

Message

Bag


stop the world

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Postman

Message

Message

Bag

In traversing, the Postman get a new message!!


stop the world

11

Postman

Message

Message

Bag

In traversing, the Postman get a new message!!

Message


Generational GC

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the weak generational hypothesis:“Most objects die young”

young -> frequently collected

old -> seldomly collected


Partitioned into generations

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C

B

AEden

Survivor

Old

Young{


Collectors in HotSpot JVM

throughput & low-latency

5 Collectors

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throughput & low-latency

throughput: more tasks in unit time

low-latency: fast response

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5 collectors

Serial Collector

Parallel Collector

Parallel Compacting Collector

Concurrent Mark Sweep Collector

Garbage First Collctor (1.7_u4)

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Serial Collector

1712

Eden

Survivor

Old

From To

for young generationcopying


Serial Collector

1812

for old generation

mark-sweep


Parallel Collector

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for young generation

use a parallel version of the serial collector algorithm

Serial Collector

Parallel Collector



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for old generation

R1 R2 R3 R4 R5



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for old generation

R1 R2 R3 R4 R5

T1 T2



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for old generation

non-compactingPhases

initial mark (STW)concurrent markremark (STW)concurrent sweep



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for old generationPostman

Message

Message

Bag

new Message is marked in remark phase

Bag is updated in concurrent marking phase



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for old generationPostman

Message

Message

Bag

Message

new Message is marked in remark phase

Bag is updated in concurrent marking phase


Garbage First Collector

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for young & old generation

Y

Y O

Y O

O O

heap is devided into regions

some regions are young, some other regions are old


Garbage First Collector

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for young & old generation

collection regions chosen based on time prediction

(some garbages remain)


select collector

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collector optionSerial -XX:+UseSerialGC

Parallel -XX:+UseParallelGC

Parallel Compacting -XX:+UseParallelOldGC

CMS -XX:+UseConcMarkSweepGC

G1 -XX:+UseG1GC


Ergonomics & Tuning

Ergonomics

JVM options

command line tools

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GC tuning

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the first step to tune GC is to do nothing


Ergonomics

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latency: -XX:MaxGCPauseMillis=n

throughput: -XX:GCTimeRatio=99

Behavior-based options


JVM options

-Xmxn

-Xmsn

-XX:ParallelGCThreads=n

-XX:+PrintFlagsFinal

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command line tools

jps

jstat

jmap

jhat

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jstat

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jstat

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$ jps -l23437 study.gc.bootstrap.Heap23489 sun.tools.jps.Jps

$ jstat -gcutil -h20 23437 1s S0 S1 E O P YGC YGCT FGC FGCT GCT 50.00 0.00 94.64 0.56 12.63 32 0.054 0 0.000 0.054 0.00 0.00 12.08 0.56 12.63 35 0.054 0 0.000 0.054 50.00 0.00 0.00 0.56 12.63 38 0.090 0 0.000 0.090 0.00 0.00 0.00 0.56 12.63 40 0.092 0 0.000 0.092 0.00 25.00 74.50 0.56 12.63 41 0.092 0 0.000 0.092 0.00 0.00 22.15 0.56 12.63 43 0.093 0 0.000 0.093 0.00 25.00 10.07 0.56 12.63 45 0.093 0 0.000 0.093 0.00 25.00 0.00 0.56 12.63 47 0.094 0 0.000 0.094 0.00 25.00 0.00 0.56 12.63 49 0.096 0 0.000 0.096 0.00 0.00 24.16 0.56 12.63 51 0.098 0 0.000 0.098 0.00 0.00 42.28 0.56 12.63 53 0.099 0 0.000 0.099 0.00 0.00 44.29 0.56 12.63 55 0.100 0 0.000 0.100...


jmap

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jmap

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$ jmap -histo:live 23437

num #instances #bytes class name---------------------------------------------- 1: 5822 797008 <methodKlass> 2: 5822 699712 <constMethodKlass> 3: 393 456488 <constantPoolKlass> 4: 360 286400 <constantPoolCacheKlass> 5: 393 283512 <instanceKlassKlass> 6: 1094 98656 [C 7: 511 97864 [B 8: 452 55128 java.lang.Class 9: 632 40536 [[I 10: 589 37440 [S...

$ jmap -dump:live,format=b,file=heap.dump 23437Dumping heap to heap.dump ...Heap dump file created


jhat

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jhat

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$ jhat heap.dump Reading from heap.dump...Dump file created Thu Dec 27 15:44:54Snapshot read, resolving...Resolving 5895 objects...Chasing references, expect 1 dots.Eliminating duplicate references.Snapshot resolved.Started HTTP server on port 7000Server is ready.


References

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Memory Management in the Java HotSpot Virtual Machine (Sun Microsystems, April 2006)

Garbage-First Garbage Collection (Sun Microsystems, October 2004)

A Generational Mostly-concurrent Garbage Collector (Sun Microsystems, June 2000)

Garbage-First Collector http://docs.oracle.com/javase/7/docs/technotes/guides/vm/G1.html

Parallel Compaction http://docs.oracle.com/javase/7/docs/technotes/guides/vm/par-compaction-6.html

Concurrent Mark Sweep Collector Enhancements http://docs.oracle.com/javase/7/docs/technotes/guides/vm/cms-6.html

Storage Management http://openjdk.java.net/groups/hotspot/docs/StorageManagement.html


Notes

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This work is licensed under the Creative Commons Attribution-NonCommercial 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc/3.0/.

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Garbage Collection for Dummies

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