CMS vs G1 - Scan RS very long

John Cuthbertson john.cuthbertson at
Thu Jan 31 17:56:49 UTC 2013

Hi Michal,

I haven't looked at the logs yet but from your description it sounds 
like a large number of the RSets have been coarsened and/or the fine 
grain tables have gotten very dense.

The RSet for a heap region tracks incoming references to objects in that 
region. There are three levels on granularity: sparse, fine, and coarse. 
The sparse and fine entries are encoded in open hash tables based upon 
the region containing the references. As the number of references from 
region A that point into region B increase, the number of cards in the 
hash table entry for region A increase (it's actually a bitmap with one 
bit per card) and as the number of regions that contain references that 
point into B increase, the number of region entries in fine grain table 
increase. Once we run out of space in the fine grain table (i.e. we 
can't add another bitmap for another region) we evict one of the densest 
region bitmaps and say "coarsen" that entry. When we have a coarsened 
entry we have to search the entire referencing region looking for 
incoming references compared to searching specific cards with the sparse 
and fine entries.

I'll take a look at your logs to see if I can confirm.


On 1/31/2013 6:12 AM, Michal Frajt wrote:
> Hi all,
> After the iCMS got officially deprecated we decided to compare the G1 collector with our best tuned (i)CMS setup. Unfortunately we are not able to make the G1 young collection running any closer to the ParNew. Actually we wanted to compare the G1 concurrent marking STW pauses with the CMS initial-mark and remark STW pauses but already incredibly long running G1 young collections are unacceptable for us.
> We were able to recognize that the very long G1 young collections are caused by the scanning remembered sets. There is not much documentation about G1 internals but we were able to understand that the size of the remembered sets is related to the amount of mutating references from old regions (cards) to young regions. Unfortunately all our applications mutate permanently thousands references from old objects to young objects.
> We are testing with the latest OpenJDK7u extended by the 7189971 patch and CMSTriggerInterval implementation. The attached GC log files represent two very equal applications processing very similar data sets, one running the G1, second running the CMS collector. The OpenJDK7u has an extra output of _pending_cards (when G1TraceConcRefinement activated) which somehow relates to the remembered sets size.
> Young Comparison (both 128m, survivor ratio 5, max tenuring 15)
> CMS - invoked every ~20 sec, avg. stop 60ms
> G1 - invoked every ~16 sec, avg. stop 410ms !!!
> It there anything what could help us to reduce the Scan RS time or the G1 is simply not targeted for applications mutating heavily old region objects?
> CMS parameters
> -Xmx8884m -Xms2048m -XX:NewSize=128m -XX:MaxNewSize=128m -XX:PermSize=128m -XX:SurvivorRatio=5 -XX:MaxTenuringThreshold=15 -XX:CMSMarkStackSize=8M -XX:CMSMarkStackSizeMax=32M -XX:+UseConcMarkSweepGC -XX:+CMSClassUnloadingEnabled -XX:CMSWaitDuration=60000 -XX:+CMSScavengeBeforeRemark -XX:CMSTriggerInterval=600000 -XX:+UseParNewGC -XX:ParallelGCThreads=8 -XX:ParallelCMSThreads=2
> G1 parameters (mind MaxNewSize not specified)
> -Xmx8884m -Xms2048m -XX:NewSize=128m -XX:PermSize=128m -XX:SurvivorRatio=5 -XX:MaxTenuringThreshold=15 -XX:+UseG1GC -XX:MaxGCPauseMillis=200 -XX:G1MixedGCCountTarget=16 -XX:ParallelGCThreads=8 -XX:ConcGCThreads=2
> G1 log file GC young pause
> [GC pause (young) [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 23697, predicted base time: 32.88 ms, remaining
> time: 167.12 ms, target pause time: 200.00 ms]
> [Parallel Time: 389.8 ms, GC Workers: 8]
>      >>>>
>      [Scan RS (ms): Min: 328.8, Avg: 330.4, Max: 332.6, Diff: 3.8, Sum: 2642.9]
>      <<<<
> [Eden: 119.0M(119.0M)->0.0B(118.0M) Survivors: 9216.0K->10.0M Heap: 1801.6M(2048.0M)->1685.7M(2048.0M)]
> Regards,
> Michal

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