Add notes about CPU and RAM limits #900
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openjdk/content.md
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| ## Make JVM respect CPU and RAM limits | ||
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| On starup JVM tries to detect the number of available CPU cores and the amount of RAM to adjust its internal parameters (like the number of garbage collector threads to spawn) accordingly. When container is run with limited CPU/RAM, standard system API, used by JVM for probing, will return host-wide values. This can cause excessive CPU usage and memory allocation errors with older versions of JVM. |
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@TimWolla , thanks for the correction! Fixed. |
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| On startup JVM tries to detect the number of available CPU cores and the amount of RAM to adjust its internal parameters (like the number of garbage collector threads to spawn) accordingly. When container is run with limited CPU/RAM, standard system API, used by JVM for probing, will return host-wide values. This can cause excessive CPU usage and memory allocation errors with older versions of JVM. | ||
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| Inside Linux containers, recent versions of OpenJDK 8 can correctly detect container-limited number of CPU cores by default. To enable the detection of container-limited amount of RAM the following options can be used: |
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Does the version we have actually include this new functionality? (I thought we were still a release or two behind due to Debian's backport process? 8u121 in jessie-backports vs 8u131 in stretch)
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not yet, openjdk:8-jdk is at 1.8.0_121
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Hi, it looks like all docker-library jdks 8 are now at least on 8u131 that has cgroups flag, is it possible to have this doc change merged?
| $ start /b /wait /affinity 0x3 path/to/java.exe ... | ||
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| In this example CPU affinity hex mask `0x3` will limit JVM to 2 CPU cores. |
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Does this limit to 2 CPU cores generally, or does this limit to 2 specific cores? (ie, docker run --cpuset-cpus vs docker run --cpus)
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Mask 0x3 - 0011 limits CPU to 2 specific cores - core0 and core1, its a windows feature, not jdk-specific.
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| In this example CPU affinity hex mask `0x3` will limit JVM to 2 CPU cores. | ||
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| RAM limit is supported by Windows Server containers, but currently JVM cannot detect it. To prevent excessive memory allocations, `-XX:MaxRAM=...` option must be specified with the value that is not bigger than a containers RAM limit. |
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Ooh, nice -- I had no idea there was a JVM parameter for controlling MaxRAM rather than trying to set -Xmx and friends intelligently! (related to docker-library/openjdk#57 and docker-library/openjdk#71)
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Yep, MaxRAM may be more convenient. This one-liner can be used to check what Xmx (max heap) value will be used when MaxRAM is specified:
jrunscript -J-XX:MaxRAM=256M -e "print(Packages.java.lang.Runtime.getRuntime().maxMemory()/(1<<20) + 'M')"
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Thanks @akashche! |
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@akashche Thank you for this!!! |
Relevant links:
https://developers.redhat.com/blog/2017/04/04/openjdk-and-containers/
https://bugs.openjdk.java.net/browse/JDK-6515172
https://bugs.openjdk.java.net/browse/JDK-8170888
https://bugs.openjdk.java.net/browse/JDK-8157478
moby/moby#27892
moby/moby#30594
https://superuser.com/a/908870
https://technet.microsoft.com/en-us/library/cc770297%28v=ws.11%29.aspx