Growing GC Young Gen Times

Jon Masamitsu jon.masamitsu at oracle.com
Thu May 13 16:23:18 UTC 2010


Matt,

As Ramki indicated fragmentation might be an issue.  As the fragmentation
in the old generation increases, it takes longer to find space in the 
old generation
into which to promote objects from the young generation.  This is 
apparently not
the problem that Wayne is having but you still might be hitting it.  If 
you can
connect jconsole to the VM and force a full GC, that would tell us if it's
fragmentation.

There might be a scaling issue with the UseParNewGC.  If you can use
-XX:-UseParNewGC (turning off the parallel young
generation collection) with  -XX:+UseConcMarkSweepGC the pauses
will be longer but may be more stable.  That's not the solution but just 
part
of the investigation.

You could try just -XX:+UseParNewGC without -XX:+UseConcMarkSweepGC
and if you don't see the growing young generation pause, that would indicate
something specific about promotion into the CMS generation.

UseParallelGC is different from UseParNewGC in a number of ways
and if you try UseParallelGC and still see the growing young generation
pauses, I'd suspect something special about your application.

If you can run these experiments hopefully they will tell
us where to look next.

Jon


On 05/12/10 15:19, Matt Fowles wrote:
> All~
>
> I have a large app that produces ~4g of garbage every 30 seconds and
> am trying to reduce the size of gc outliers.  About 99% of this data
> is garbage, but almost anything that survives one collection survives
> for an indeterminately long amount of time.  We are currently using
> the following VM and options:
>
> java version "1.6.0_20"
> Java(TM) SE Runtime Environment (build 1.6.0_20-b02)
> Java HotSpot(TM) 64-Bit Server VM (build 16.3-b01, mixed mode)
>
>                -verbose:gc
>                -XX:+PrintGCTimeStamps
>                -XX:+PrintGCDetails
>                -XX:+PrintGCTaskTimeStamps
>                -XX:+PrintTenuringDistribution
>                -XX:+PrintCommandLineFlags
>                -XX:+PrintReferenceGC
>                -Xms32g -Xmx32g -Xmn4g
>                -XX:+UseParNewGC
>                -XX:ParallelGCThreads=4
>                -XX:+UseConcMarkSweepGC
>                -XX:ParallelCMSThreads=4
>                -XX:CMSInitiatingOccupancyFraction=60
>                -XX:+UseCMSInitiatingOccupancyOnly
>                -XX:+CMSParallelRemarkEnabled
>                -XX:MaxGCPauseMillis=50
>                -Xloggc:gc.log
>
>
> As you can see from the GC log, we never actually reach the point
> where the CMS kicks in (after app startup).  But our young gens seem
> to take increasingly long to collect as time goes by.
>
> The steady state of the app is reached around 956.392 into the log
> with a collection that takes 0.106 seconds.  Thereafter the survivor
> space remains roughly constantly as filled and the amount promoted to
> old gen also remains constant, but the collection times increase to
> 2.855 seconds by the end of the 3.5 hour run.
>
> Has anyone seen this sort of behavior before?  Are there more switches
> that I should try running with?
>
> Obviously, I am working to profile the app and reduce the garbage load
> in parallel.  But if I still see this sort of problem, it is only a
> question of how long must the app run before I see unacceptable
> latency spikes.
>
> Matt
> ------------------------------------------------------------------------
>
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