[jboss-user] [Performance Tuning] - Re: Cpu 100% and GC is running

[Peter Johnson]

Peter Johnson [http://community.jboss.org/people/peterj] created the discussion

"Re: Cpu 100% and GC is running"

To view the discussion, visit: http://community.jboss.org/message/644878#644878

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Looking at the GC data you posted, here is what I see:

a) A minor collection is taking place about every 3 second

b) The minor collection is taking anywhere from 1/10 to 1/20 of a second. Even with 0.1 sec per collection, out of 3 seconds, that amounts to only about 3% of the wall-clock time is spent in GC, which is very good.

c) The minor collection cleans up about 1.2GB of objects, leaving about 20MB in the survivor space, and promoting about 5MB to the tenured generation. That also looks very good. 

d) Based on (a) and (c), your app is generating 400MB of garbage per second. That is a lot of garbage. You might want to look at why that is an if there is anything you can do about it. 

e) Your heap increased from 400MB to 650MB in about 3.5 minutes. That is an increase of about 70MB per minute. Based on the acitivity of this app, that doesn't look that bad either.

In all, this looks fairly good to me. It will be interesing, though, to see what happens when you hit a full GC. Cleaning up a 2.5GB tenured generation will be very painful (possibly very long pause time). There are two things you could do about that. First, reduce your overall heap size because based on the data you posted, I don't think that you really need a large heap. Though based on the amount of garbage being generated per second, you might really need a large heap. Second, switch to using the CMS collector. And when you do so, reset the tenuring threshold and survivor space sizes so that you can continue to clean up the 400MB of garbage per second with the minro collector rather than putting the burden for that on the CMS collector.
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