CMS vs G1 - Scan RS very long

Michal Frajt michal at frajt.eu
Fri Feb 1 08:05:58 UTC 2013


Hi Monica / John,
 
Find the new archive which correctly contains both the CMS and G1 log files. Yesterday I somehow managed to overwrite the CMS log file when creating the tar archive. 
  
Node2 - running CMS
mmreutersservernode2_20130131105521_gc.log
mmreutersservernode2_jinfo.txt
 
Node3 - running G1
mmreutersservernode3_20130131105525_gc.log
mmreutersservernode3_jinfo.txt
 
Regards,
Michal 
 
Od: "Monica Beckwith" monica.beckwith at oracle.com
Komu: "John Cuthbertson" john.cuthbertson at oracle.com
Kopie: hotspot-gc-dev at openjdk.java.net,"Michal Frajt" michal at frajt.eu
Datum: Thu, 31 Jan 2013 12:44:46 -0600
Předmet: Re: CMS vs G1 - Scan RS very long


> Hi Michal (and John),
> 
> I did look at Michal's log files (BTW, both are the same G1 logs). And I can confirm that scan RS is the issue... here's the plot:
> 
> 
> The plot above only shows the max obj copy times (for all pauses), max RS Scan times and Parallel Time. So yes, scan RS is the culprit. 
> Also, looking at your logs, it seems that for mixedGCs the reclaimable bytes don't cross 6%. So can you please try increasing the G1HeapWastePercent to 10? (the latest builds will have 10 as the default value).
> 
> Please let me know if that improves your RT.
> 
> -Monica
> 
> 
> On 1/31/2013 11:56 AM, John Cuthbertson wrote: 
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. 
> 
> JohnC 
> 
> 
> 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 
> 
> 
> 

> 

> 
> -- 
> 
> Monica Beckwith | Java Performance Engineer
> VOIP: +1 512 401 1274 
> Texas 
>  Oracle is committed to developing practices and products that help protect the environment 
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