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From Rollo du Pre <>
Subject Re: Websphere and Dark Matter
Date Thu, 18 Jan 2007 10:49:12 GMT
Thanks for this information, it was very useful.


Nadav Har'El wrote:
> On Tue, Jan 16, 2007, Rollo du Pre wrote about "Re: Websphere and Dark Matter":
>> I was hoping it would, yes. Does websphere not release memory back to 
>> the OS when it not longer needs it? I'm concerned that if the memory 
>> spiked for some reason (indexing a large document) then that would 
>> hamper the rest of the OS because it'd hold on to far more memory than 
>> is needed.
> This is a well known, may I say infamous, Java issue. Java could, in theory,
> easily shrink its heap as soon as it needs less memory, because in Java's GC
> model, memory can be moved around so fragmentation is not a problem. But
> unfortunately, the JVM's heap rarely does shrink by default: once the JVM's
> heap grows, it rarely ever shrinks back.
> It turns out that in Sun's JVM (at least in the 1.4 version which I tried),
> you can actually control and improve the shrinking behavior. There's an option
> "-XX:MaxHeapFreeRatio=" (see [1]) which basically means how much of the Heap
> should be free before it is physically shrunk. Another option
> "-XX:MinHeapFreeRatio=" specifies how much of the heap to leave free after
> the shrinking.
> The default values are 70 and 40 respectively, which means that the JVM only
> shrinks its memory use after 70% (!!!) of the heap is free, and it only shrinks
> to the point that 40% of the heap is still free. A few numbers to put this
> waste into perspective: If your JVM's heap grew to 300 MB, and now only
> requires 100 MB, it will NOT shrink (because only 66% is free). If later
> you need just 90 MB, the heap will finally shrink, but only to 150 MB (so
> that 40% still remains free).
> One important thing you should note if you decide to lower these percentages,
> is that much of the JVM's excellent GC performance relies on its "generation"
> approach and the availability of a lot of free heap (so that by the time the
> heap is exausted, most of the new stuff in it is already garbage). So to
> keep performance - not just minimal memory use - you might want to tune other
> GC parameters as well. A good combination I once used is this:
> 	-XX:NewRatio=2 -XX:MinHeapFreeRatio=20 -XX:MaxHeapFreeRatio=30
> But your milage may vary.
> [1]

Rollo du Pre
Software Architect

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