PermGen, Garbage Collection and CPU utilization

Discussion in 'Java' started by Holmbrew, Feb 2, 2009.

  1. Holmbrew

    Holmbrew Guest

    I am struggling with some performance issues that seem to be related
    to garbage collection. We have a sort of homegrown scheduling
    application the can have a pretty good memory foot print so we have
    given it 3000m of heap space and recently upped it's PermGen
    allocation to 256m. Overall the PermGen, which would fill up on us
    over time as we loaded and unload object, has been really good, but it
    seems strangely coincidental that not long after increasing the
    PermGen allocation we started to see random period of high CPU
    utilization that looks like "stop the world" garbage collection
    because it does not coincide with any other actions or job running in
    the affected JVM.

    Can increasing PermGen change the way GC is handles by the JVM?

    General Java & System Specs:
    JVM version is 1.5.0_03-b07
    java.vm.vendor = Sun Microsystems Inc. = Java HotSpot(TM) Server VM
    OS: SunOS v5.9

    Holmbrew, Feb 2, 2009
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  2. Holmbrew

    Dave Miller Guest

    If you've not seen it, this article, while old, lays it out pretty well:
    Dave Miller, Feb 2, 2009
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  3. PermGen is a Sun VM specificity and there
    are --albeit rare-- known and documented
    very weird bugs related to the PermGen.

    Hibernate + Sun VM + Tomcat can exhibit, under
    special circumstances, this bug and has proved
    to be a real headaches to diagnose. No easy
    solution besides changing one of the components
    because "it's nobody's fault" of course.

    (the link should work)

    My point is that maybe you're not doing anything
    wrong and it's really a bug out of your control,
    depending on the VM and 3rd party APIs you're using.

    Have you tried with a non-Sun VM to see if it
    could fix your problem?
    alexandre_paterson, Feb 3, 2009
  4. Holmbrew

    Holmbrew Guest

    Thank you for the response. That is very interesting and actually
    related more to a problem I had in a prior project and I have passed
    that info along to folks doing the care and feeding these days.

    I learned something this morning that should have beeen obvious, but
    went overlooked and that is the distinction between Java Process
    Memory and Java Heap Space. In my situation, Solaris 32-bit running on
    Sparc hardware, any single process cannot take and or use more that
    2Gb of physical memory. So therefore I cannot actually allocate the
    Xmx3000m and that really becomes 2000m (max memory per process). So,
    that would seem fine, but the problems occur when Java fills all the
    allocated 'heap' space with objects and leaves nothing for any of the
    other tasks that need to happen to keep the JVM happy and the process
    running like creating threads and garbage collection and mundane
    things like that.

    When you read the Java garbage collection tuning material (http:// and it tells you:

    "Unless you have problems with pauses, try granting as much memory as
    possible to the virtual machine."

    Make sure you know that there are caveats to that statement. "As much
    memory as possible" does not mean as much memory as the OS will allow
    you to grant. It really means do your homework and profile your
    application to learn how it is using the memory it has been given.
    Learn what its upper and lower limit regions of usage are and allocate
    accordingly, if necessary. Java 1.5 or greater running on a server-
    class machine will automatically calculate the memory it can allocate
    to the Java heap, but it is capped by the HotSpot JVM to 1Gb no matter
    the amount of physical memory available. This seems to make sense as
    Java probably need the other gig available to the process to do the
    other fun things it needs to do to stay happy and healthy.

    The moral of the story is:

    Java likes memory, but there are really two "heaps" to consider.


    It pays to know your hardware as well as you know your software.

    Lost in the heap,
    Holmbrew, Feb 5, 2009
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