Very long young gc pause (ParNew with CMS)

[Srinivas Ramakrishna]

On Mon, Jan 9, 2012 at 3:08 AM, Florian Binder <java at java4.info> wrote:

> ...
> I have seen that this problem occurs only after about one week of
> uptime. Even thought we make a full (compacting) gc every night.
> Since real-time > user-time I assume it might be a synchronization
> problem. Can this be true?
>
>
Together with your and Chi-Ho's conclusion that this is possibly related to
paging,
a question to ponder is why this happens only after a week. Since your
process'
heap size is presumably fixed and you have seen multiple full GC's (from
which
i assume that your heap's pages have all been touched), have you checked to
see if the size of either this process (i.e. its native size) or of another
process
on the machine has grown during the week so that you start swapping?

I also find it interesting that you state that whenever you see this problem
there's always a single block in the old gen, and that the problem seems to
go
away when there are more than one block in the old gen. That would seem
to throw out the paging theory, and point the finger of suspicion to some
kind
of bottleneck in the allocation out of a large block. You also state that
you
do a compacting collection every night, but the bad behaviour sets in only
after a week.

So let me ask you if you see that the slow scavenge happens to be the first
scavenge after a full gc, or does the condition persist for a long time and
is independent if whether a full gc has happened recently?

Try turning on -XX:+PrintOldPLAB to see if it sheds any light...

-- ramki
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