Number of Parallel GC Threads

Y Srinivas Ramakrishna Y.S.Ramakrishna at Sun.COM
Fri Jan 23 20:00:56 UTC 2009

Depends on how heavy the load is and whether there are
convoying/synchronizing effects between the JVM's.
Typically this will manifest as large variance in the
scavenge times (or if you plot pause distributions
they will be at least bimodal and possibly combinatorially
multi-modal -- you will see a "ringing" tail).
I have seen this when customers do not pbind to psets
and load is heavy. If you use PrintGCStats/CompareGCStats
it'll show up as a larger "std-dev" reading.
When you are concerned about pause times (as Nick is)
the longer, fatter, ringing tail can hurt.

-- ramki

----- Original Message -----
From: Clemens Eisserer <linuxhippy at>
Date: Friday, January 23, 2009 11:42 am
Subject: Re: Number of Parallel GC Threads
To: kirk <kirk at>, hotspot-gc-dev at

> > I should add that I think that the bigger problem is that GC doesn't 
> play nice
> > when you have many VMs running on the same hardware.
> > (point taken about partitioning and all that).
> > Some how it has to figure out if it has free reign or if it need to 
> share.
> In theory there are slowdowns when three or more JVMs run their GC at
> exactly the same time, but how often does that happen.
> Do you have any evidence that this is a problem in real-world useage?
> - Clemens

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