Subject: Re: Identifying concurrent mode failures caused by
fragmentation



Jon,

I noticed that you are using the incremental mode of
CMS.  May I ask why you chose to turn that on?

Jon

On 10/31/2011 7:43 PM, Jon Masamitsu wrote: Jon,

I haven't looked at the longer log but in general I've found the
information in the GC logs inadequate to figure out if the
problem is fragmentation.  But more important, there has
been some good work in recent versions of hotspot so that
we're more successful at combating fragmentation.  Try
the latest release and see if it helps (u26 should be good
enough).

Jon

On 10/31/11 06:06, Jon Bright wrote:
Hi,

We have an application running with a 6GB heap (complete parameters below).  Mostly it has a fairly low turnover of memory use, but on occasion, it will come under some pressure as it reloads a large in-memory data set from a database.

Sometimes in this situation, we'll see a concurrent mode failure. Here's one failure:

20021.464: [GC 20021.465: [ParNew: 13093K->3939K(76672K), 0.0569240 secs]20021.522: [CMS20023.747: [CMS-concurrent-mark: 11.403/29.029 secs] [Times: user=41.11 sys=1.03, real=29.03 secs]
 (concurrent mode failure): 3873922K->2801744K(6206272K), 30.7900180 secs] 3886215K->2801744K(6282944K), [CMS Perm : 142884K->142834K(524288K)] icms_dc=33 , 30.8473830 secs] [Times: user=30.26 sys=0.71, real=30.85 secs]
Total time for which application threads were stopped: 30.8484460 seconds

(I've attached a lengthier log including the previous and subsequent CMS collection.)

Am I correct in thinking that this failure can basically only be caused by fragmentation?  Both young and old seem to have plenty of space. There doesn't seem to be any sign that the tenured generation would run out of space before CMS completes.  Fragmentation is the only remaining cause that occurs to me.

We're running with 1.6.0_11, although this will be upgraded to 1.6.0_26 tomorrow.  I realise our current version is ancient - I'm not really looking for help on the problem itself, just for advice on whether the log line above indicates fragmentation.

Thanks

Jon Bright



The parameters we have set are:

-server
-Xmx6144M
-Xms6144M
-XX:MaxPermSize=512m
-XX:PermSize=512m
-XX:+UseConcMarkSweepGC
-XX:+CMSIncrementalMode
-XX:+CMSIncrementalPacing
-XX:SoftRefLRUPolicyMSPerMB=3
-XX:CMSIncrementalSafetyFactor=30
-XX:+PrintGCDetails
-XX:+PrintGCApplicationStoppedTime
-XX:+PrintGCApplicationConcurrentTime
-XX:+PrintGCTimeStamps
-Xloggc:/home/tbmx/log/gc_`date +%Y%m%d%H%M`.log



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