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From Mark Thomas <ma...@apache.org>
Subject Re: Setting the Right Amount of Memory
Date Sun, 20 Jun 2010 11:06:58 GMT
On 20/06/2010 00:30, André Warnier wrote:
> Robinson, Eric wrote:
>> On 17/06/2010 08:59, Robinson, Eric wrote:
>>>> If your heap size is right on the edge of your minimum for a Tomcat
>>>> instance, you may be doing more GC work than is really needed.
>>>> However, if you're satisfied with the response time and CPU
>>>> utilization, you should be ok.
>>
>>> Time to hit the vendor around the head with the cluebat. If the app
>>> is happy with less heap space then increasing it is only going to
>>> cause problems - mainly that GC when it happens will take longer and
>>> trigger longer pauses. You can mitigate this with GC config (later
>>> VMs may make the right choices for you anyway) but all this is just
>>> adding unecessary complexity.
>>
>>> Mark
>>
>> With 160 instances of tomcat on the server, and most of them happy with
>> 64-96MB of RAM, could you take an educated guess at the negative impact
>> on the server of raising the RAM to 512MB for each instance? How much
>> extra CPU utilization do you think I could possibly see from all the
>> extra GC?
>>
> 
> Just a note here : 160 X 512 MB = 81 GB.
> If each Tomcat's JVM is allowed to use up to 512 MB of Heap, there might
> be moments where a lot of JVM's will be using close to that amount. 
> Unless your system can really support that amount of real RAM, you may
> be in for massive swapping.

And that could be a major problem. JVMs and swapping do not place nicely
together. To quote some folks at work that have been looking at this
recently performance "falls of a cliff".

Mark



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