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Date: Sat, 3 Sep 2011 10:35:46 -0700 (PDT)
From: "Darius D." <darius.ski@gmail.com>
To: users@tomcat.apache.org
Subject: Re: Performance for many small requests
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David Kerber wrote:
> 
> On 9/3/2011 1:15 PM, Darius D. wrote:
>>
>>
>> n828cl wrote:
>>>> From: Darius D. [mailto:darius.ski@gmail.com]
>>>> Subject: Re: Performance for many small requests
>>>> in my opinion using 64bit JVM with such a small
>>>> heap is only needed if performance testing shows
>>>> gains versus 32bit JVM.
>>> The main advantage of using a 64-bit JVM is the increased number of
>>> registers available in the x86-64 architecture, which can result in
>>> vastly
>>> reduced memory references.  Whether or not that's important to overall
>>> performance is highly dependent on the application, of course.
>>>
>>>
>> Yeah, but I'd err on 32bit JVM side :) Gains from more registers are very
>> specific, but penalty from increased cache/TLB misses is big, and if you
>> start hitting hard page faults ( that would have been avoidable due to
>> lower
>> heap size with 32bit JVM ) - even one of those will erase all gains :)
> 
> Then why shouldn't I just double my heap size?  Wouldn't that eliminate 
> the risk of increased cache misses?  I was just using the settings from 
> my 32-bit installation, but have plenty of RAM to allow me to increase 
> the memory settings if that would help.
> 
> Dave
> 


Umm, sorry, it seems that 64 vs 32bit discussion has thrown this thread of
track. In your case it probably makes little difference. I'd definately
start by profiling and looking at gc logs.

As a side note - (CPU)cache/TLB misses have nothing to do with heap size.
Too big heap size can be as bad as too low ( by stealing memory from OS that
could have been used for file caches and other apps and increasing GC pauses
).

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