ParallelGCThreads

[Y Srinivas Ramakrishna]

By the way, i believe that the default #gc workers you would have 
gotten (with UseParallelGC) for process A would have been 16 and
for process B eight, if you had launched them bound.
(Re)Binding after launching does not readjust from the
default, something we need to fix; so, in that case,
you would need to explicitly set ParallelGCThreads.
(With CMS, the numbers are similar, if linearly scaled down by
a suitable factor.)

-- ramki

----- Original Message -----
From: Y Srinivas Ramakrishna <Y.S.Ramakrishna at Sun.COM>
Date: Tuesday, October 28, 2008 11:06 am
Subject: Re: ParallelGCThreads
To: Sujit.Das at cognizant.com
Cc: hotspot-gc-use at openjdk.java.net


> Hi Sujit --
> 
> > We have a distributed trading application. I have a processor set of 
> 
> > 16 CPU bound to process A and 8 CPU to process B. Please consider 
> > following scenarios:
> > 
> > Scenario1: ParallelGCThreads set to 16 for both Process A and B.
> 
> 16 for Process B is definitely the wrong setting. You should use
> 8 or less for Process B, since it has only an 8 processor pset to run 
> on.
> 
> > 
> > Scenario2: ParallelGCThreads set to 8 for both Process A and B.
> > 
> > Now for scenario2, I see that threadstop time has increased (got 
> > worse) for process A and reduced (improved) for process B. Is this 
> an 
> > expected behaviour? Also will this (# of ParallelGCThreads vs # of 
> > CPUs) impact rate of minor collections?
> 
> Yes, exactly as expected. In Scenario 2, you have more than 8 cpu's
> available for GC of A, so using more than 8 GC threads is paying off.
> If you exceed the # cpu's available, however, gc times will increase.
> That's what you saw in the case of B going from Scenario 2 to Scenario 
> 1.
> 
> #parallel gc threads does not affect the time between parallel collections,
> but only the duration of the collections. The time between gcs is affected
> (mainly) by the size of Eden and speed of the mutators.
> 
> -- ramki
> 
> > 
> > Thanks and Regards,
> > Sujit
> > 
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