HotSpot VM GC 的种类
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.
类别serial collectorparallel collector单线程收集器
使用单线程去完成所有的gc工作，没有线程间的通信，这种方式会相对高效
并行收集器
使用多线程的方式，利用多CUP来提高GC的效率
主要以到达一定的吞吐量为目标
并发收集器
使用多线程的方式,利用多CUP来提高GC的效率
并发完成大部分工作，使得gc pause短
适用于科学技术和后台处理
有中规模/大规模数据集大小的应用且运行在多处理器上，关注吞吐量(throughput)
适合中规模/大规模数据集大小的应用，应用服务器,电信领域
关注response time，而不是throughput
Server模式下默认
--YGC:PS FGC:Parallel MSC
可用-XX:+UseParallelGC或-XX:+UseParallelOldGC强制指定
--ParallelGC代表FGC为Parallel MSC
--ParallelOldGC代表FGC为Parallel Compacting
优点:高效
缺点:当heap变大后,造成的暂停时间会变得比较长
可用-XX:+
UseConcMarkSweepGC强制指定
优点:
对old进行回收时,对应用造成的暂停时间非常端,适合对latency要求比较高的应用
缺点:
1.内存碎片和浮动垃圾
2.old去的内存分配效率低
3.回收的整个耗时比较长
4.和应用争抢CPU内存回收触发YGC-XX:UseAdaptiveSizePolicy 去掉YGC后动态调整eden from已经tenuringthreshold的动作
-XX:ParallelGCThreads 设置并行的线程数
-XX:CMSInitiatingOccupancyFraction 设置old gen使用到达多少比率时触发注：
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）????? SUN推荐的使用参数是：
-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
-XX:+UseConcMarkSweepGC \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。
-XX:+CMSIncrementalMode \
-XX:+CMSIncrementalPacing \
-XX:CMSIncrementalDutyCycleMin=0 \
-XX:CMSIncrementalDutyCycle=10 \
-XX:+PrintGC Details \
-XX:+PrintGCTimeStamps \
-XX:-TraceClassUnloading
如何选择collector:
UseSerialGC.UseParallelOldGC(optionally).重点考虑response time,pause time要小，UseConcMarkSweepGC.Garbage Collctor – Future
???? 还没尝试过使用…
Summary
import java.util.ArrayList;用以下两种参数执行,会执行几次YGC几次FGC?
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);
}
}
}
2.062: [GC(具体分析见http://rdc.taobao.com/team/jm/archives/440)
Desired survivor size 1310720 bytes, new threshold 7 (max 15)
6467K->6312K(29440K), 0.0038214 secs]
4.066: [GC
Desired survivor size 1310720 bytes, new threshold 7 (max 15)
12536K->12440K(29440K), 0.0036804 secs]
6.070: [GC
Desired 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]
2.047: [GC(具体分析见http://rdc.taobao.com/team/jm/archives/458)
Desired survivor size 524288 bytes, new threshold 15 (max 15)
- age 1: 142024 bytes, 142024 total
6472K->6282K(29696K), 0.0048686 secs]
4.053: [GC
Desired survivor size 524288 bytes, new threshold 15 (max 15)
- age 2: 141880 bytes, 141880 total
12512K->12426K(29696K), 0.0047334 secs]
6.058: [GC
Desired 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]
参考：