Sun jdk 1.6内存管理 -实现篇 -毕玄

Sun JDK 1.6 内存管理实现篇

毕玄

2010-10、2010-11

目标

• 掌握内存管理的通常实现方法

• 了解Sun JDK是如何实现内存管理的，以及做了哪些优化

• 了解JRockit做了哪些优化

OS内存管理

图片来源于JavaOne 2010 《where does all native memory go》 Session

内存分配和回收

• allocate

A B C D


• allocate

A B C B

freelist


• fragmenation

A B C B

freelist

E

i need allocate

sorry,no space


• Compaction

– need update pointer

A BC B

free space pointer

Discussion

• how to find garbage and claim it..

– let’s talk about this question first

GC Algorithm

• Reference Counting

• Tracing

– Mark-Sweep

– Mark-Compact

– Copying

GC Algorithm Details


图片来源于JavaOne 2010 《The Garbage Collection Mythbusters》 Session by tony,john



– extra space,time overhead

– non-moving

– cyclic garbage


• Advance Reference Counting

– two-bit reference counts

• when max count(3) is reached,object becomes “sticky”

– buffer reference updates

– use a backup gc algorithm to handle cyclic garbage and “sticky” objects

– complex,but still non-moving


• Tracing – Mark-Sweep



• Tracing – Mark-Sweep

– need scan all objects,so if java heap becomes larger,then gc slower

– 可清除所有的Garbage

– 内存碎片，分配低效

– 需要在一定的时机触发


• Tracing – Mark-Compact



• Tracing – Mark-Compact

– 没有内存碎片，分配高效

– 增加了回收需要耗费的时间

– 需要更新所有移动过的object的ref pointer

– 需要在一定时机触发


• Tracing – Copying(a special case)



• Tracing – Copying(a special case)

– only need scan live objects,so gc speed only decided by lds

– no fragmentation

– need update object pointer

– need keep an empty memory area

GC Algorithm Summary

• Why gc need stop the world?

– Mark-Sweep,Mark-Compact

• if ref changes or new object created when marking;

• compact need update pointer,so...

– Copying

• copying need update pointer,so...

目标




GC in Hotspot

• rootset

– runtime stack

– static or global variables

– jni handles

– jvm handles

GC in Hotspot

• Generational GC

– most objects are temp-lived

• so JDK decide split heap into two generations to use gcalgorithm properly

New Old

Copying Mark-Sweep or Mark-Compact

GC in Hotspot

• how to stop the world

– safepoint at ref change

• safepoint is a memory page check

• compile

– when gc need execute,it submit “stop thread” request to jvm core,then jvm core set the memory page not readable

– when code execute to safepoint, check the memory page if readable,if not then fail and stop

GC in Hotspot

• YGC

– allocate is slow,because need lock

– how to make allocate faster?

• do u have any other ideas?

TLAB Use CAS replace lock

GC in Hotspot

• YGC

– how to find live objects faster?


Live Maps Scan Algorithm

GC in Hotspot

• YGC

– how to handle object in old gen ref new gen object


Write Barrier & Card table

GC in Hotspot

• YGC

– how to make scan and copying faster?


ParallelAdaptive based on runtime feedback

GC in Hotspot

• FGC

– optimize for throughput

• parallel

– optimize for low latency

• concurrent

GC in Hotspot

• FGC – parallel

– based on Mark-Compact

– allocate use the bump pointer

GC in Hotspot


– how to make mark phase faster?

• do u have any other ideas?Parallel

GC in Hotspot


– how to make compact phase faster?


Partial Compact when use ParallelOldGC

GC in Hotspot

• FGC – concurrent

– based on mark-sweep

– when concurrent gc executes,ref may be changed and new objects may be created,how to solve the question?

GC in Hotspot


– when concurrent gc executes,ref may be changed and new objects may be created,how to solve the question?

Write Barrier & Card Table

GC in Hotspot


– how to mark remark phase faster?


add a preclean phase

Futhermore – G1

• Region-based GC

• how to behave better?

garbage first

hot regions

目标




Futhermore – Open eyes

• JRockit GC

– YGC

• pinned object for semi-long lived object

• keep area

– Mark

• Two-Color for prefetching

– Compaction

• partial,for example some objects not be compacted because many objects ref them.

– Adjust gens,size adaptive based on runtime feedback

目标




how to research jvm

• JDK Source codes

• References

References

• JVM References

http://blog.bluedavy.com/?p=187

Sun jdk 1.6内存管理 -实现篇 -毕玄

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