Garbage Collection for Dummies

Garbage Collection
for Dummies
Overview HotSpot GC

photo#1

Copyright © 2012 Akira Koyasu. Some rights reserved.

Agenda
JVM & GC
heap & GC fundamentals

Collectors in HotSpot JVM
HotSpot Collectors explained

Ergonomics & Tuning
the start point of GC tuning

2 Copyright © 2012 Akira Koyasu. Some rights reserved.

JVM & GC

heap in JVM

Garbage Collection

stop the world

Generational GC


heap in JVM
Which objects are allocated in heap?

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


javap -c


javap -c
$ javap -c study.gc.bootstrap.Heap
Compiled 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
}


Garbage Collection

“Our resource is limited.
So garbages should be
collected and recycled as
soon as possible”


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

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


Dead or Alive

Unreferenced objects are dead

reassignment

scope out


Fragment, Compaction,
Allocation

A B B B B C A


Allocation

fragment

A B C A


Allocation

A B C A


Allocation

Compaction

A B C A


Allocation

Allocation

A B C A


Allocation

A B C A C


Live object
identification

Postman
Reference counting
Bag
Traverse marking
from root
Message

Message


stop the world

In traversing, Postman
the Postman get
a new message!! Bag

Message

Message


stop the world

In traversing, Postman
the Postman get
a new message!! Bag

Message

Message Message


Generational GC

the weak generational hypothesis:
“Most objects die young”

young -> frequently collected

old -> seldomly collected


Partitioned into
generations
Eden A
Young {
Survivor B

C
Old


Collectors in
HotSpot JVM

throughput & low-latency

5 Collectors


throughput &
low-latency

throughput:
more tasks in unit time

low-latency:
fast response


5 collectors

Serial Collector

Parallel Collector

Parallel Compacting Collector

Concurrent Mark Sweep Collector

Garbage First Collctor (1.7_u4)


Serial Collector
for young generation
copying
Eden
From To

Survivor

Old

12

Serial Collector
for old generation

mark-sweep

12

Parallel Collector
for young generation

use a parallel version of
the serial collector algorithm

Parallel
Serial Collector
Collector


Parallel Compacting
Collector
for old generation

R1 R2 R3 R4 R5


Parallel Compacting
Collector
for old generation

R1 R2 R3 R4 R5

T1 T2


Concurrent Mark
Sweep Collector
for old generation

non-compacting
Phases
initial mark (STW)
concurrent mark
remark (STW)
concurrent sweep


Concurrent Mark
Sweep Collector
for old generation
Postman
Bag is updated in
concurrent marking phase
Bag

new Message is marked
Message
in remark phase
Message


Concurrent Mark
Sweep Collector
for old generation
Postman
Bag is updated in
concurrent marking phase
Bag

new Message is marked
Message
in remark phase
Message Message


Garbage First
Collector
for young & old generation

heap is devided into Y
regions
Y O
some regions are
young,
Y O
some other regions
are old
O O


Garbage First
Collector
for young & old generation

collection regions
chosen based on
time prediction
(some garbages remain)


select collector

collector option
Serial -XX:+UseSerialGC

Parallel -XX:+UseParallelGC

Parallel Compacting -XX:+UseParallelOldGC

CMS -XX:+UseConcMarkSweepGC

G1 -XX:+UseG1GC


Ergonomics & Tuning

Ergonomics

JVM options

command line tools


GC tuning

the first step to tune GC is
to do nothing


Ergonomics

Behavior-based options

latency: -XX:MaxGCPauseMillis=n

throughput: -XX:GCTimeRatio=99


JVM options

-Xmxn

-Xmsn

-XX:ParallelGCThreads=n

-XX:+PrintFlagsFinal


command line tools

jps
jstat
jmap
jhat


jstat


jstat
$ jps -l
23437 study.gc.bootstrap.Heap
23489 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


jmap
$ 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 23437
Dumping heap to heap.dump ...
Heap dump file created


jhat


jhat

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


References
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

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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