JDK6u25里添加的按线程统计分配内存量: JMX

红太狼 2022-04-17 05:06 46阅读 0赞

转载自 [http://rednaxelafx.iteye.com/blog/1021619][http_rednaxelafx.iteye.com_blog_1021619]

Oracle几天前发布的JDK 6 update 25里添加的一个新功能非常有趣，可以按照线程来跟踪（GC堆）内存的分配量。这个功能在VM核心、解释器、C1编译器、C2编译器以及GC中都有代码支持，并且通过JMX API暴露出来。   
不过新加的这功能不是加在java.开头的包里，而是加在com.sun.management.ThreadMXBean这个接口上，要用的话还得cast一下。   
  
当然，这么底层的功能不可能没有代价。添加这个功能后，在GC堆上分配空间的slow-path会比以前稍微慢一些。但希望对整体性能的影响并不大吧。   
Fast-path是在TLAB上分配空间的，而TLAB的分配数据是在TLAB refill的时候才批量更新，所以这个功能对fast-path的执行效率基本上没影响，以稍微放宽数据精准性为代价。   
  
相关链接：   
[Bug ID 7003271: Hotspot should track cumulative Java heap bytes allocated on a per-thread basis][Bug ID 7003271_ Hotspot should track cumulative Java heap bytes allocated on a per-thread basis]   
[changeset][]   
  
JMX中，该功能由ThreadMXBean上新增的几个方法提供。详情可见下面例子。   
ThreadMXBean.getThreadAllocatedBytes(long threadId)的用法基本上可以看成跟System.currentTimeMillis()用于计时的用法一样，在两点上记录并且求差即可。   
  
不知道为什么JVMTI没得到这个更新，或许是因为更新JVMTI spec涉及committee stuff？   
  
==============================================================   
  
直接拿一段Groovy脚本来演示吧：   
  
先看JDK 6 update 24的情况

Groovysh代码

![收藏代码][icon_star.png]

1.  D:\\sdk\\groovy-1.7.2\\bin>groovysh  
2.  Groovy Shell (1.7.2, JVM: 1.6.0\_24)  
3.  Type 'help' or '\\h' for help.  
4.  \-------------------------------------------------------------------------------  
5.  groovy:000> import java.lang.management.\*  
6.  ===> \[import java.lang.management.\*\]  
7.  groovy:000> tb = ManagementFactory.threadMXBean  
8.  ===> sun.management.ThreadImpl@8b677f  
9.  groovy:000> tb.class.methods.name.unique().sort()  
10. ===> \[dumpAllThreads, equals, findDeadlockedThreads, findMonitorDeadlockedThreads, getAllThreadIds, getClass, getCurrentThreadCpuTime, getCurrentThreadUserTime, getDaemonThreadCount, getPeakThreadCount, getThreadCount, getThreadCpuTime, getThreadInfo, getThreadUserTime, getTotalStartedThreadCount, hashCode, isCurrentThreadCpuTimeSupported, isObjectMonitorUsageSupported, isSynchronizerUsageSupported, isThreadContentionMonitoringEnabled, isThreadContentionMonitoringSupported, isThreadCpuTimeEnabled, isThreadCpuTimeSupported, notify, notifyAll, resetPeakThreadCount, setThreadContentionMonitoringEnabled, setThreadCpuTimeEnabled, toString, wait\]  
11. groovy:000> tb.class.methods.findAll \{ it.name =~ /Alloc/\}.each \{ println it \}; null  
12. ===> null

这个时候ThreadMXBean上还没有跟alloc相关的方法。

Groovysh代码

![收藏代码][icon_star.png]

1.  D:\\sdk\\groovy-1.7.2\\bin>groovysh  
2.  Groovy Shell (1.7.2, JVM: 1.6.0\_25)  
3.  Type 'help' or '\\h' for help.  
4.  \-------------------------------------------------------------------------------  
5.  groovy:000> import java.lang.management.\*  
6.  ===> \[import java.lang.management.\*\]  
7.  groovy:000> tb = ManagementFactory.threadMXBean  
8.  ===> sun.management.ThreadImpl@9b1670  
9.  groovy:000> tb.class.methods.name.unique().sort()  
10. ===> \[dumpAllThreads, equals, findDeadlockedThreads, findMonitorDeadlockedThreads, getAllThreadIds, getClass, getCurrentThreadCpuTime, getCurrentThreadUserTime, getDaemonThreadCount, getPeakThreadCount, getThreadAllocatedBytes, getThreadCount, getThreadCpuTime, getThreadInfo, getThreadUserTime, getTotalStartedThreadCount, hashCode, isCurrentThreadCpuTimeSupported, isObjectMonitorUsageSupported, isSynchronizerUsageSupported, isThreadAllocatedMemoryEnabled, isThreadAllocatedMemorySupported, isThreadContentionMonitoringEnabled, isThreadContentionMonitoringSupported, isThreadCpuTimeEnabled, isThreadCpuTimeSupported, notify, notifyAll, resetPeakThreadCount, setThreadAllocatedMemoryEnabled, setThreadContentionMonitoringEnabled, setThreadCpuTimeEnabled, toString, wait\]  
11. groovy:000> tb.class.methods.findAll \{ it.name =~ /Alloc/\}.each \{ println it \};  
12. null  
13. public boolean sun.management.ThreadImpl.isThreadAllocatedMemoryEnabled()  
14. public boolean sun.management.ThreadImpl.isThreadAllocatedMemorySupported()  
15. public long\[\] sun.management.ThreadImpl.getThreadAllocatedBytes(long\[\])  
16. public long sun.management.ThreadImpl.getThreadAllocatedBytes(long)  
17. public void sun.management.ThreadImpl.setThreadAllocatedMemoryEnabled(boolean)  
18. ===> null  
19. groovy:000> tb.threadAllocatedMemoryEnabled  
20. ===> true  
21. groovy:000> tid = Thread.currentThread().id  
22. ===> 1  
23. groovy:000> tb.getThreadAllocatedBytes(tid)  
24. ===> 48106672  
25. groovy:000> tb.getThreadAllocatedBytes(tid)  
26. ===> 48751520  
27. groovy:000> tb.getThreadAllocatedBytes(tid)  
28. ===> 49384752  
29. groovy:000> tb.getThreadAllocatedBytes(tid)  
30. ===> 50086240  
31. groovy:000> quit

[http_rednaxelafx.iteye.com_blog_1021619]: http://rednaxelafx.iteye.com/blog/1021619
[Bug ID 7003271_ Hotspot should track cumulative Java heap bytes allocated on a per-thread basis]: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7003271
[changeset]: http://hg.openjdk.java.net/jdk7/hotspot/hotspot/rev/b1a2afa37ec4
[icon_star.png]: /images/20220417/b50947c153504a4c9a7d1ff4dfbf1786.png