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List:       openjdk-hotspot-gc-use
Subject:    value of dense prefix address used?
From:       jon.masamitsu () oracle ! com (Jon Masamitsu)
Date:       2012-02-20 17:34:09
Message-ID: 4F428411.80504 () oracle ! com
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Ramki,

This is speculation on my part but we don't do anything
at the start of the compaction phase to explicitly
create more parallelism.  What I mean is that we
compact into a chunk when it becomes available
and it becomes available when it is empty or
when we can start compacting it into itself.  This
conceivably could result in near serial compaction
of the former dense prefix.  The extreme example
is 1 small dead object at the beginning of the heap
could allow only 1 chunk to be compacted at a time
in the dense prefix.

If not allowing maximal compactions does not work,
you might try more frequent maximal compactions.
If there is some adverse situation building up in the
dense prefix, more frequent maximal compactions
might avoid the more extreme degradation.

Jon


On 02/19/12 02:13, Srinivas Ramakrishna wrote:
> Hi Jon --
> 
> After looking at the code and the pattern we observed, we are pretty 
> confident now that the maximal
> compaction was the root of the problem. We are going to effectively 
> turn off the maximal compaction
> and see if it does any harm (don't think it will), and use that to 
> work around the problem of extreme degradation
> when doing parallel compaction. It;s interesting why maximal 
> compaction would degrade parallel compaction
> by so much... some experiments would be useful and perhaps help 
> correct a specific issue the lack of initial parallelism
> may be causing to make the whole collection so much more inefficient. 
> Hopefully we'll be able to collect some
> numbers that might help you folks address the issue.
> 
> later.
> -- ramki
> 
> On Fri, Feb 17, 2012 at 12:48 PM, Srinivas Ramakrishna 
> <ysr1729 at gmail.com <mailto:ysr1729 at gmail.com>> wrote:
> 
> Hi John, thanks for those suggestions...
> 
> So far the pattern has not repeated, but occurred on two different
> servers (in each case it was the
> same full gc ordinal too, albeit at different time). There didn't
> seem anything external that would
> explain the difference observed. Yes, we'll play around a bit with
> the compaction related parameters and look at the phase times
> as well. I am also looking at how the dense prefix address is
> computed to see if it sheds a bit of
> light may be, but it could also be something happening early in
> the life of the process that doesn't happen
> later that causes this... it's all a bit of a mystery at the
> moment. Thanks!
> 
> -- ramki
> 
> 
> On Fri, Feb 17, 2012 at 12:10 PM, Jon Masamitsu
> <jon.masamitsu at oracle.com <mailto:jon.masamitsu at oracle.com>> wrote:
> 
> Ramki,
> 
> I didn't find a product flag that would print the end of the
> dense prefix.
> Don't know about jmx.
> 
> The phase accounting (PrintParallelOldGCPhaseTimes)
> as you say is a good place to start.  The summary phase is
> serial so look for an increase in that phase.   Does this pattern
> repeat?
> 
> You could also try changing HeapMaximumCompactionInterval
> and see if it affects the pattern.
> 
> Jon
> 
> 
> On 2/17/2012 9:46 AM, Srinivas Ramakrishna wrote:
> > Hi Jo{h}n, all --
> > 
> > Is there some way i can get at the dense prefix value used for ParOld in
> > each (major) collection? I couldn't find an obvious product flag for
> > eliciting that info, but wondered if you knew/remembered.
> > JMX would be fine too -- as long as the info can be obtained in a product
> > build.
> > 
> > I am seeing a curious looking log where one specific major gc seems to have
> > greater user and real time, lower "parallelism" [=(user+sys)/real] and
> > takes much longer than the rest of the ParOld's. It
> > also lowers the footprint a tad more (but definitely not proportionately
> > more vis-a-vis the user time ratios) than the gc's preceding (but not
> > succeeding) that one, so one conjecture was that perhaps
> > something happens with the dense prefix computation at that time and we
> > somehow end up copying more. We'll see if we can get some data with
> > printing the ParOld phase times, but i wondered if
> > we might also be able to elicit the dense prefix address/size. I'll
> > continue to dig around meanwhile.
> > 
> > thanks for any info!
> > -- ramki
> > 
> > 
> > 
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