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RE: Free Memory vs. Total Memory vs. Max Memory Robinson, Eric Sat Feb 18 05:00:30 2012

> Robinson, Eric wrote:
> >> We have many servers that have been running 100-200 instances of 
> >> tomcat each for years without any performance problems.
> >> Most of our servers are Linux 8-core machines with 32GB 
> RAM, with the 
> >> tomcat instances configured with -Xms16M -Xmx192M.
> >> We also have some Windows servers with 100-150 instances of tomcat 
> >> each, most of which are configured with -Xms16M -Xmx64M.
> >>
> > 
> > To add to the above, we see zero swapping and very low 
> iowait (or disk
> > queue lengths in Windows).
> > 
> You would have saved everyone some time by being a little bit 
> more forward with the above.
> Your initial question sounded deceptively beginner-like, 
> which is why you got beginner 
> treatment.

Sorry about that. Regardless of the number of tomcats we run
(approximately 1000) I still think of myself as a tomcat beginner. :-)
Also, I was trying to avoid poisoning the well so I kept the facts to a
minumum.

> 
> Thanks for sharing the data above in any case.  It provides a 
> future reference point for 
> similar enquiries.
> 
> Anyway, it sounds like you have at least the possibility of 
> getting plenty of real-world 
> data to confirm or disprove you thesis.  Just enabling GC 
> logging at the minimum level on 
> that instance should be enough to provide a good impression 
> of what's going on at 
> different settings of Heap size.

I'll do that.

> Unless you also know the exact applications and usage of that 
> instance, you may still be 
> surprised.
> 
> I also administer a number of Tomcat instances, usually 
> running one single and in 
> principle smallish application, always the same, each Tomcat 
> being alone on one of a 
> similar set of (Linux) servers.  In most cases, 128 MB Heap 
> is enough and we never see a 
> problem.  In a couple of cases however, 512MB Heap is the 
> absolute minimum to provide 
> acceptable user-perceived performance.  

Understood. We also have a small percentage of instances that require
384M, and a couple that require 512M. But even with those, I set
-Xms16M. And that's really where I get to the core of my question. In my
view, it only takes a few microseconds for the instance to allocate more
RAM if it is required. I really don't see a downside. More imporetantly,
I don't see how setting a low initial pool would cause a thread to
freeze.

>The difference is not 
> so much in the absolute 
> number of users or requests, but in the pattern of usage 
> during the day.  Our favored 
> hypothesis is that several hundred employees always return 
> from their coffee breaks at 
> exactly the same time.
> Just to say that you never know until you measure.

Agreed. Anyway, in this case the thread is on a tomcat server that is
only used for scheduled java tasks. Users do not access it directly.
Very puzzling. What's I'd really like is for some well-known tomcat guru
to say that in our environment, -Xms16M is fine and that the assertion
that a low -Xms value could result in memory shortages at runtime or
thread freezes is silly. One can hope.

--Eric





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