CMS does not want to run concurrently

Y Srinivas Ramakrishna Y.S.Ramakrishna at Sun.COM
Wed Dec 17 12:16:30 PST 2008


oh and, as Jon suggested before, it might  be a good
idea to tack on -XX:+DisableExplicitGC to ensure that
it's not RMI or other modules calling system.gc() here,
unbeknownst to you. (If nothing else but to get closer to the
root cause of the puzzling full gc's.)

-- ramki

----- Original Message -----
From: Y Srinivas Ramakrishna <Y.S.Ramakrishna at Sun.COM>
Date: Wednesday, December 17, 2008 12:12 pm
Subject: Re: CMS does not want to run concurrently
To: Jon Masamitsu <Jon.Masamitsu at Sun.COM>
Cc: hotspot-gc-use at openjdk.java.net


> Just a quick remark related to:-
> 
> ...
> > Here in the CMS gen (old generation) it is only 325384K full (out of
> > a total of 659140K) when the collection starts.  Actually, you might
> > have a severely fragmented heap.  In that case Full GC's would happen
> > and using the survivor spaces would help.
> 
> When you have a fragmented heap that causes a full collection
> the gc log message (in 5u8) would look a bit different as a
> scavenge is first attempted, but fails because of a fragmented
> heap, and then bails to a full collection. Because I do not see
> that abortive scavenge attempt logged here, I suspect this may not
> be a fragmentation-induced full gc, but perhaps something
> else. Nonetheless, Jon's suggestion of using survivor spaces
> to stem the tenuring of very young objects would be a good
> thing to reduce fragmentation and other stresses on CMS.
> 
> -- ramki
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