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    <blockquote> <p>Sometimes OOM pretty quick and got killed, sometime suffers long gc period (last time was over 10 hours).</p> </blockquote> <p>It sounds to me like a memory leak is at the root of your problems. </p> <p>A CMS failure won't (as I understand it) cause an OOM. Rather a CMS failure happens because the JVM needs to do too many collections too quickly, and CMS could not keep up. One situation where lots of collection cycles happen in a short period is when your heap is nearly full.</p> <p>The really long GC time sounds weird ... but is theoretically possible if your machine was thrashing horribly. However, a long period of repeated GCs is quite plausible if your heap is very nearly full. </p> <p>You can configure the GC to give up when the heap is 1) at max size and 2) still close to full after a full GC has completed. Try doing this if you haven't done so already. It won't cure your problems, but at least your JVM will get the OOM quickly, allowing a faster service restart and recovery.</p> <p><strong>EDIT</strong> - the option to do this is <code>-XX:GCHeapFreeLimit=nnn</code> where <code>nnn</code> is a number between 0 and 100 giving the minimum percentage of the heap that must be free after the GC. The default is 2. The option is listed in the aptly titled <a href="http://www.md.pp.ru/~eu/jdk6options.html" rel="nofollow noreferrer">"The most complete list of -XX options for Java 6 JVM"</a> page. (There are lots of -XX options listed there that don't appear in the Sun documentation. Unfortunately the page provides few details on what the options actually do.)</p> <p>You should probably start looking to see if your application / webapp has memory leaks. If it has, your problems won't go away unless those leaks are found and fixed. In the long term, fiddling with the Hotspot GC options won't fix memory leaks.</p>
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    1. POHow to reduce java concurrent mode failure and excessive gc
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    1. COUnderstood. I knew that there is slow leak in our program, we just yet be able to find it. :( At the meantime, we were trying to squeeze as much as we can to see if different gc policy can help to mitigate the issue. CMS wouldn't directly cause the OOM but its often with the full gc kicks in and that can cause major problems. We often see performance start degrading when CMS mode failure appears in our gc log. But maybe lack of experiences, we haven't yet been able to find the leak or find the suitable gc policy that can fit well.
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    2. COI feel that heap full does not necessarily mean there will be a severe gc issue but not being able to collect much of garbage after a full gc is a really bad sign. If I'd like to ask jvm kill the app early and quickly, what flags should I use? faster service restart and recovery does sound promising. At least we don't have to suffer long unresponsiveness. I'd rather choose die fast. Thanks.
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