HotSpot VM GC 的种类（转）

collector种类

GC在 HotSpot VM 5.0里有四种：

incremental (sometimes called train) low pause collector已被废弃,不在介绍.

注：

• throughput
  collector与concurrent low pause collector的区别是throughput collector只在young
  area使用使用多线程，而concurrent low pause collector则在tenured generation也使用多线程。
• 根据官方文档，他们俩个需要在多CPU的情况下，才能发挥作用。在一个CPU的情况下，会不如默认的serial
  collector，因为线程管理需要耗费CPU资源。而在两个CPU的情况下，也提高不大。只是在更多CPU的情况下，才会有所提高。当然
  concurrent low pause collector有一种模式可以在CPU较少的机器上，提供尽可能少的停顿的模式，见CMS GC Incremental mode。
• 当
  要使用throughput collector时，在java opt里加上-XX:+UseParallelGC，启动throughput
  collector收集。也可加上-XX:ParallelGCThreads=<desired
  number>来改变线程数。还有两个参数 -XX:MaxGCPauseMillis=<nnn>和
  -XX:GCTimeRatio=<nnn>，MaxGCPauseMillis=<nnn>用来控制最大暂停时间，而-XX:
  GCTimeRatio可以提高GC说占CPU的比，以最大话的减小heap。

附注SUN的官方说明：

1.
The throughput collector: this collector uses a parallel version of the
young generation collector. It is used if the -XX:+UseParallelGC option
is passed on the command line. The tenured generation collector is the
same as the serial collector.

2. The concurrent low pause
collector: this collector is used if the -Xincgc™ or
-XX:+UseConcMarkSweepGC is passed on the command line. The concurrent
collector is used to collect the tenured generation and does most of the
collection concurrently with the execution of the application. The
application is paused for short periods during the collection. A
parallel version of the young generation copying collector is used with
the concurrent collector. The concurrent low pause collector is used if
the option -XX:+UseConcMarkSweepGC is passed on the command line.

3.
The incremental (sometimes called train) low pause collector: this
collector is used only if -XX:+UseTrainGC is passed on the command line.
This collector has not changed since the J2SE Platform version 1.4.2
and is currently not under active development. It will not be supported
in future releases. Please see the 1.4.2 GC Tuning Document for
information on this collector.

CMS GC Incremental mode

当要使用 concurrent low pause collector时，在java的opt里加上
-XX:+UseConcMarkSweepGC。concurrent low pause
collector还有一种为CPU少的机器准备的模式，叫Incremental
mode。这种模式使用一个CPU来在程序运行的过程中GC，只用很少的时间暂停程序，检查对象存活。

在Incremental mode里，每个收集过程中，会暂停两次，第二次略长。第一次用来，简单从root查询存活对象。第二次用来，详细检查存活对象。整个过程如下：

* stop all application threads; do the initial mark; resume all application threads（第一次暂停，初始话标记）* do the concurrent mark (uses one procesor for the concurrent work)（运行是标记）* do the concurrent pre-clean (uses one processor for the concurrent work)（准备清理）* stop all application threads; do the remark; resume all application threads（第二次暂停，标记，检查）* do the concurrent sweep (uses one processor for the concurrent work)(运行过程中清理)* do the concurrent reset (uses one processor for the concurrent work)(复原)

当要使用Incremental mode时，需要使用以下几个变量：

-XX:+CMSIncrementalMode default: disabled 启动i-CMS模式（must with -XX:+UseConcMarkSweepGC）-XX:+CMSIncrementalPacing default: disabled 提供自动校正功能-XX:CMSIncrementalDutyCycle=<N> default: 50 启动CMS的上线-XX:CMSIncrementalDutyCycleMin=<N> default: 10 启动CMS的下线-XX:CMSIncrementalSafetyFactor=<N> default: 10 用来计算循环次数-XX:CMSIncrementalOffset=<N> default: 0 最小循环次数（This is the percentage (0-100) by which the incremental mode duty cycle is shifted to the right within the period between minor collections.）-XX:CMSExpAvgFactor=<N> default: 25 提供一个指导收集数

SUN推荐的使用参数是：

-XX:+UseConcMarkSweepGC \-XX:+CMSIncrementalMode \-XX:+CMSIncrementalPacing \-XX:CMSIncrementalDutyCycleMin=0 \-XX:CMSIncrementalDutyCycle=10 \-XX:+PrintGC Details \-XX:+PrintGCTimeStamps \-XX:-TraceClassUnloading

注：如果使用throughput collector和concurrent low pause collector，这两种垃圾收集器，需要适当的挺高内存大小，以为多线程做准备。

如何选择collector:

• app运行在单处理器机器上且没有pause time的要求，让vm选择或者UseSerialGC.
• 重点考虑peak application performance(高性能)，没有pause time太严格要求，让vm选择或者UseParallelGC+UseParallelOldGC(optionally).
• 重点考虑response time,pause time要小，UseConcMarkSweepGC.

Garbage Collctor – Future

• Garbage First(G1)
  jdk1.6 update 14 or jdk7
• few flags need to set
  -XX:MaxGCPauseMillis=100
  -XX:GCPauseIntervalMillis=6000

还没尝试过使用…

Summary

import java.util.ArrayList;import java.util.List;public class SummaryCase {    public static void main(String[] args) throws InterruptedException {        List<Object> caches = new ArrayList<Object>();        for (int i = 0; i < 7; i++) {            caches.add(new byte[1024 * 1024 * 3]);            Thread.sleep(1000);        }        caches.clear();        for (int i = 0; i < 2; i++) {            caches.add(new byte[1024 * 1024 * 3]);            Thread.sleep(1000);        }    }}

用以下两种参数执行,会执行几次YGC几次FGC?

• -Xms30M -Xmx30M -Xmn10M  -Xloggc:gc.log -XX:+PrintTenuringDistribution -XX:+UseParallelGC

  2.062: [GCDesired survivor size 1310720 bytes, new threshold 7 (max 15) 6467K->6312K(29440K), 0.0038214 secs]4.066: [GCDesired survivor size 1310720 bytes, new threshold 7 (max 15) 12536K->12440K(29440K), 0.0036804 secs]6.070: [GCDesired survivor size 1310720 bytes, new threshold 7 (max 15) 18637K->18584K(29440K), 0.0040175 secs]6.074: [Full GC 18584K->18570K(29440K), 0.0031329 secs]8.078: [Full GC 24749K->3210K(29440K), 0.0045590 secs]

  (具体分析见http://rdc.taobao.com/team/jm/archives/440)

• -Xms30M -Xmx30M -Xmn10M  -Xloggc:gc.log -XX:+PrintTenuringDistribution -XX:+UseSerialGC

  2.047: [GCDesired survivor size 524288 bytes, new threshold 15 (max 15)- age   1:     142024 bytes,     142024 total 6472K->6282K(29696K), 0.0048686 secs]4.053: [GCDesired survivor size 524288 bytes, new threshold 15 (max 15)- age   2:     141880 bytes,     141880 total 12512K->12426K(29696K), 0.0047334 secs]6.058: [GCDesired survivor size 524288 bytes, new threshold 15 (max 15)- age   3:     141880 bytes,     141880 total 18627K->18570K(29696K), 0.0049135 secs]8.063: [Full GC 24752K->3210K(29696K), 0.0077895 secs]

  (具体分析见http://rdc.taobao.com/team/jm/archives/458)

参考：

1. http://jiangyongyuan.iteye.com/blog/356502
2. http://www.helloying.com/blog/archives/164
3. http://hi.baidu.com/sdausea/blog/item/c599ef13fcd3a7dbf6039e12.html
4. Tuning Garbage Collection with the 1.4.2 JavaTM Virtual Machine .
5. http://blog.bluedavy.com/?p=200
6. http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html