hotspot-gc-use Digest, Vol 54, Issue 8

[Haim Yadid]

Thanks Jon,
Thats a pity since CMS full GC is unavoidable, and when this happen we will
experience unacceptable pause.
I was trying G1 as well and In theory G1GC should not have pauses longer
than the soft real time goal.
However in practice (as you can see from another question I have posted )
it seems that G1 do has pauses from time to time and in the application I
am tuning it is much worse than CMS.


On Mon, Aug 20, 2012 at 9:00 PM, <hotspot-gc-use-request at openjdk.java.net>wrote:

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>    1. Re: CMS Concurrent mode failure fallback to the serial old
>       collector? (Jon Masamitsu)
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> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 20 Aug 2012 09:16:59 -0700
> From: Jon Masamitsu <jon.masamitsu at oracle.com>
> Subject: Re: CMS Concurrent mode failure fallback to the serial old
>         collector?
> To: hotspot-gc-use at openjdk.java.net
> Message-ID: <503262FB.5020900 at oracle.com>
> Content-Type: text/plain; charset="iso-8859-1"
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>
>
> On 08/17/12 14:08, Haim Yadid wrote:
> > I am analysing a GC pause problem and I have noticed that when CMS is
> > used and a concurrent mode failure occurs or GC is triggered manually
> > (by System.gc()) the STW collector used does not seem to be parallel.
> > ( I am aware of the ExplicitGCInvokesConcurrent flag but it will not
> > solve concurrent failure ).
> > I tried to play with -XX:ParallelGCThreads=...
> > -XX:ParallelCMSThreads=... but they seem have no effect (only on the
> > ParNew GC).
> >
> > I am deducing it from the following GC log line
> >
> > 24.904: [Full GC (System) 24.904: [CMS: 302703K->303056K(2116864K),
> > 1.0847520 secs] 484492K->303056K(2423552K), [CMS Perm :
> > 7528K->7525K(21248K)], 1.0852780 secs] [Times: user=1.04 sys=0.02,
> > real=1.09 secs]
> > If it would have been parallel "user" would have been equal to
> > "nThreads" * "real".
> > In addition if I choose ParallelOld GC it will behave correctly.
> >
> > I really do not understand why the failover STW mechanism of CMS is
> > not parallel shouldn't it be finishing the work as soon as possible ?
> > I am not able to find anything useful on the internet.
>
> You are correct that the concurrent mode failure does a full GC serially.
> The parallel old collector used for UseParallelGC/UseParallelOldGC was
> never ported to CMS.  Because of differences between UseParallelGC and
> CMS, it is more work than we had expected.
> >
> > I think G1 behaves in the same manner BTW ( AFAIK the the fallback
> > collector of G1 is copied from CMS)
>
> Yes, G1 behaves the same.  G1 will not use the UseParallelGC implementation
> for a parallel full collection but will implement one in line with G1's
> design.
> Currently the G1 guys have been focusing on better policies for achieving
> pause goals and avoiding full collections.  Last I heard there was at
> least some
> work to be done for class unloading (JEP 156) before the parallel full
> collection.
>
> Jon
>
> >
> > Help will be appreciated.
> >
> >
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-- 
Haim Yadid | Performization Expert
Performize-IT | t +972-54-7777132
www.performize-it.com
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