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.
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.
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. 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.
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