Tomcat GC threads

[Chetan Chheda <ch...@yahoo.com>]

All, 

    We are using -XX:+UseParNewGC option in our Tomcat JVMs with -XX:ParallelGCThreads=16 on a 6 CPU system. What is the real impact of having a higher number of GC threads than the number of CPUs? With the 2 tomcat JVMs running on the box, would you recommend changing it to 6 for each tomcat?

Thanks,
Chetan


      

Re: Tomcat GC threads

[Rainer Jung <ra...@kippdata.de>]

On 14.05.2009 15:29, Chetan Chheda wrote:
> We are using -XX:+UseParNewGC option in our Tomcat JVMs with
> -XX:ParallelGCThreads=16 on a 6 CPU system. What is the real impact
> of having a higher number of GC threads than the number of CPUs? With
> the 2 tomcat JVMs running on the box, would you recommend changing it
> to 6 for each tomcat?

Nowadays it is more relevant how many cores or even hardware threads
your hardware effectively supports.

GC threads are only used for GC. So they don't have an influence of the
performance of your webapp apart from the times where the GC is running
(yes, they use a bit of memory etc. but we are not talking about 100
threads here).

Depending on your JVM version and start parameters, it is very likely,
that you are only sing parallel (mutli-threaded) GC for the young
generation. In most common situations this takes a 2 digit number of
milliseconds, so for most applications not really something to worry. If
you machine doesn't carry a lot of other load apart from your JVM, then
you can try to make efficient use out of the availabel CPU resource
during these pauses by setting the GC thread number to something like
the number of available parallelity in hardware (e.g. cores). The
defaults are not tha bad, except for huge machines, were you should
think about lowering from the default. In your case you could go with
the default or set the number to 6 (assuming 6 cores).

Using to many GC threads will be bad because it will increase GC pauses
instead of decreasing them.

The number is more important if you are using a huge heap (several
gigabytes) and a recent JVM version, so that you can collect the
tenured/old generation with multiple threads in parallel.

Regards,

Rainer

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