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tomcat进程消失(被杀)问题

缘由

最近，一个机器上的tomcat进程莫名消失了，为了暂时解决问题，设置提升了内存设置。但是，之后过了没多久tomcat又消失了。为了排查问题，我仔细分析了机器上的log。
一开始去查看运行目录是否有dump文件，发现并没有，看似并不是OutOfMemoryError的问题, 也没有JVM crash之后会生成的hs_err_pidxxx.log, JVM也没有出错。
再去查看tomcat的日志，发现也没有异常，突然就结束了，看起来也并不是tomcat崩溃了。
不是JVM崩溃，也不是tomcat崩溃，这个让我们很困惑。通过搜索，得知可能是被os杀掉了。
通过demsg命令，得到如下的输出:

Java进程被杀

google之后，发现有可能是linux的奇怪特性。

分析

memory overcommit

linux有一个比较奇怪的特性，叫memory overcommit。详细内容可以参考这里。
主要表现就是malloc永远会返回一个合法的指针。举个例子，假设我们的机器有4G内存，有两个JVM，每个JVM都说我要4G，linux仍然会成功的分配给他们。

这个特性对于大多数程序来说没有影响，因为一般程序不会占满自己申请的内存，他们可能只用申请的30%内存，于是相安无事。

JVM and GC

但是对于JVM，尤其是长时间运行的JVM，memory overcommit会有很大影响。
对于大多数GC算法来说，只有在堆分配不出内存的时候才会进行GC活动，释放到达不了的object。
回到上面的例子，假设两个JVM长时间运行，内存都逼近1.5G，对于JVM来说，自己才使用了1.5G内存，可能新生代还很空，没有必要进行GC。但是linux不一样，他知道实际的内存(3G+内核+其他软件，比如vim)可能已经没有多少了，于是启动oom-killer杀死某一个进程（根据运行时间，内存占用等决定），而长时间运行，占用大量内存的Java进程很容易被选择。

总结

综上，如果tomcat真的是每次被内核杀死，要做的不是提高Xmx，Xms，反而是降低。
不过即使不是每次都被内核杀死，我也建议在beta机器上降低内存设置，试着让tomcat长时间的跑，看会不会OutOfMemoryError，并且看看那时JVM产生的dump，更加利于定位内存问题。

另外：

IBM Heap Analyzer其实挺坑爹的（附连接）：

他的root object根本不只是gc root，还包含所有可以被gc但是还没有被gc的object。所以他显示的leak suspect根本大多是可回收的对象，容易造成误解。

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