Major GC happening before suggested CMSInitiatingOccupancyFraction

[Jon Masamitsu]

Which  version of the JDK are you using?

It does seem curious that the CMS collection are starting so
early.  CMS uses different criteria for deciding when to start
a concurrent collection.  The occupancy of the CMS generation
(the tenured generation) is only one of the criteria.
You can add the flag -XX:+UseCMSInitiatingOccupancyOnly
which tells CMS to only use the occupancy of the CMS
generation in deciding when to start.

When you say you are seeing high cpu utilization, I assume
you mean during the CMS concurrent collection?  In
the example below between

8784.678: [GC [1 CMS-initial-mark: 1003680K(1998848K)]
1003698K(2097088K), 0.0052064 secs]

and

8798.633: [CMS-concurrent-sweep: 9.686/9.834 secs]

(about a 14 second period)?



prashant Kumar wrote On 01/12/09 00:59,:

> Hi There,
>
>  
>
> I am using below mentioned options for my server. The issue is that I
> have set CMSInitiatingOccupancyFraction=65 but major GC is happening
> at a lower value of heap used (approximately 50%) though it should
> happen at around 65%. Due to this CPU utilization increases to a very
> high value. I am not able to find out the reason of major GC.
>
>  
>
> P.S. Logs are attached with the mail.
>
>  
>
> *Options:*
>
> * *
>
> -Xdebug -Xnoagent -debug
> -Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=n,stdalloc=y
> -Xmx2048m -Xms2048m -XX:NewSize=96m -XX:MaxNewSize=96m -XX:+UseTLAB
> -XX:+UseParNewGC -XX:+UseConcMarkSweepGC
> -XX:CMSInitiatingOccupancyFraction=65 -XX:+DisableExplicitGC
> -verbose:gc -XX:+PrintGCTimeStamps -XX:+PrintGCDetails
> -XX:+PrintTenuringDistribution -XX:PrintCMSStatistics=1
> -XX:PrintFLSStatistics=1 -XX:+PrintHeapAtGC -XX:PermSize=64m
> -XX:MaxPermSize=64m
>
>  
>
>  
>
> *This is the output of the logs:*
>
>  
>
> Desired survivor size 32768 bytes, new threshold 0 (max 0)
>
> : 98176K->0K(98240K), 0.1769591 secs]
> 1093934K->1003680K(2097088K)Statistics for BinaryTreeDictionary:
>
> ------------------------------------
>
> Total Free Space: 252988590
>
> Max   Chunk Size: 252988590
>
> Number of Blocks: 1
>
> Av.  Block  Size: 252988590
>
> Tree      Height: 1
>
> Statistics for BinaryTreeDictionary:
>
> ------------------------------------
>
> Total Free Space: 0
>
> Max   Chunk Size: 0
>
> Number of Blocks: 0
>
> Tree      Height: 0
>
>  Heap after GC invocations=447:
>
> Heap
>
>  par new generation   total 98240K, used 0K [0x75800000, 0x7b800000,
> 0x7b800000)
>
>   eden space 98176K,   0% used [0x75800000, 0x75800000, 0x7b7e0000)
>
>   from space 64K,   0% used [0x7b7f0000, 0x7b7f0000, 0x7b800000)
>
>   to   space 64K,   0% used [0x7b7e0000, 0x7b7e0000, 0x7b7f0000)
>
>  concurrent mark-sweep generation total 1998848K, used 1003680K
> [0x7b800000, 0xf5800000, 0xf5800000)
>
>  concurrent-mark-sweep perm gen total 65536K, used 16879K [0xf5800000,
> 0xf9800000, 0xf9800000)
>
> } , 0.1779375 secs]
>
> After GC:
>
> After GC:
>
> 8784.678: [GC [1 CMS-initial-mark: 1003680K(1998848K)]
> 1003698K(2097088K), 0.0052064 secs]
>
> 8784.684: [CMS-concurrent-mark-start]
>
> 8787.045: [CMS-concurrent-mark: 2.361/2.361 secs]
>
>  (CMS-concurrent-mark yielded 0 times)
>
> 8787.046: [CMS-concurrent-preclean-start]
>
>  (cardTable: 2726 cards, re-scanned 2726 cards, 1 iterations)
>
> 8788.370: [CMS-concurrent-preclean: 1.320/1.324 secs]
>
>  (CMS-concurrent-preclean yielded 0 times)
>
> 8788.371: [GC8788.371: [Rescan (parallel) , 0.3424891 secs]8788.714:
> [weak refs processing, 0.0569442 secs] [1 CMS-remark:
> 1003680K(1998848K)] 1055351K(2097088K), 0.4066270 secs]
>
> 8788.799: [CMS-concurrent-sweep-start]
>
> 8791.891: [GC  {Heap before GC invocations=447:
>
> Heap
>
>  par new generation   total 98240K, used 98176K [0x75800000,
> 0x7b800000, 0x7b800000)
>
>   eden space 98176K, 100% used [0x75800000, 0x7b7e0000, 0x7b7e0000)
>
>   from space 64K,   0% used [0x7b7f0000, 0x7b7f0000, 0x7b800000)
>
>   to   space 64K,   0% used [0x7b7e0000, 0x7b7e0000, 0x7b7f0000)
>
>  concurrent mark-sweep generation total 1998848K, used 742483K
> [0x7b800000, 0xf5800000, 0xf5800000)
>
>  concurrent-mark-sweep perm gen total 65536K, used 16879K [0xf5800000,
> 0xf9800000, 0xf9800000)
>
> Statistics for BinaryTreeDictionary:
>
>------------------------------------------------------------------------
>
>
>  
>