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Thread: heap size vs heap used

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    Default heap size vs heap used

    I'm getting a java.lang.outofmemoryerror: java heap space exception.

    I downloaded VisualVM to try and monitor.

    It seems the heap size increases but never decreases, however the heap that's used stays reasonable flat, with the odd spike.

    So the heap size doesn't seem to adjust down, only up, and I eventually get the exception. The system I'm using has 12GB of RAM so, no problem there.

    it's the first time I have used VisualVM, so unsure what to look for


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    Default Re: heap size vs heap used

    I found these two bits of text, I think i'll try System.gc() at each iteration of the bigger loops for starters

    "In addition to freeing unreferenced objects, a garbage collector may also combat heap fragmentation. Heap fragmentation occurs through the course of normal program execution. New objects are allocated, and unreferenced objects are freed such that free blocks of heap memory are left in between blocks occupied by live objects. Requests to allocate new objects may have to be filled by extending the size of the heap even though there is enough total unused space in the existing heap. This will happen if there is not enough contiguous free heap space available into which the new object will fit. On a virtual memory system, the extra paging required to service an ever growing heap can degrade the performance of the executing program. "

    "Fortunately, very good garbage collection algorithms have been developed, and adequate performance can be achieved for all but the most demanding of applications. Because Java's garbage collector runs in its own thread, it will, in most cases, run transparently alongside the execution of the program. Plus, if a programmer really wants to explicitly request a garbage collection at some point, System.gc() or Runtime.gc() can be invoked, which will fire off a garbage collection at that time. "

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    Default Re: heap size vs heap used

    Make sure you are not holding onto references to objects that are not needed - if you are they cannot be garbage collected. If you need more memory, increase the max heap size using -Xmx VM argument.

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    Default Re: heap size vs heap used

    would GC lower the heap size or the amount of heap used?

    my program is running atm

    heap size: 3.8GB
    heap max: 8GB
    used heap: 350MB
    Last edited by aueddonline; February 10th, 2012 at 09:42 AM.

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    Default Re: heap size vs heap used

    Quote Originally Posted by aueddonline View Post
    would GC lower the heap size or the amount of heap used?
    It depends. If memory can be garbage collected then the garbage collector will do its best to free memory as it sees the need.

    my program is running atm
    You need to be more explicit if you intend to use acronyms - I have no idea what you mean by 'atm' (and GC can mean a lot when taken out of context)

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    Default Re: heap size vs heap used

    does the 'used heap' only include referenced objects? or is this where non-referenced objects are before a garbage collection?

    Noted.

    atm - at the moment

    Thanks for the information. My program seems to be running better now after I added the System.gc() statements. However this particular class is running rather slow in comparison to the rest of my program so I think I'll start again with it.

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