问题描述
最近我们开始使用YJP 11.0.9对我们的应用程序（基于XMPP的聊天服务器）进行压力测试。在我们的测试中，我们发现了以下奇怪的行为。
Recently we started stress testing our application (an XMPP based chat server) using YJP 11.0.9. During our test we noticed following strange behavior.
- 采样显示sun.misc.Unsafe.unpark（Object）占用了60％的CPU。
- 对于同一个应用程序跟踪显示LockSupport.park（Object）占用了52％的CPU。
我做了多次测试以确认结果，每次我得到类似的结果。
I did multiple tests to confirm results and every time I got similar results.
我无法理解为什么unpark应该花费60％的时间以及为什么跟踪显示正好相反结果。
I am unable to understand why unpark should take 60% time and why tracing shows exactly opposite results.
有人可以帮我理解这些结果。我在这里遗漏了什么吗？
Can someone help me understand these results. Am I missing something here?
环境：
java -version
java version "1.6.0_31"
Java(TM) SE Runtime Environment (build 1.6.0_31-b04)
Java HotSpot(TM) 64-Bit Server VM (build 20.6-b01, mixed mode)
推荐答案
高CPU时间 Unsafe.unpark 是过度的迹象上下文切换。这篇文章是为了了解上下文切换的代价是多少：
High CPU time of Unsafe.unpark is a sign of excessive Context Switching. Here is an article to get the idea of how expensive is a context switch:
最简单的选项找出Linux上的CS计数运行 vmstat< seconds> 。
The easiest option to find out CS count on Linux is run vmstat <seconds>.
一旦确认了高CS（例如，每个核心/每秒更多10K交换机），您就会接受违规线程（您可以按照其CPU时间对YJP中的线程进行排序）并运行 strace -p< pid> -c 以找出切换的原因，例如：线程正在从套接字中读取小块并关闭，在这种情况下，增加套接字缓冲区可能有所帮助。
Once high CS is confirmed (e.g. more 10K switches per core/per second) you take the offending thread (you can sort threads in YJP by their CPU time) and run strace -p <pid> -c to find out the cause of switching, e.g. thread is reading from socket in small pieces and get's switched off, in which case increasing socket buffer might help.
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