CMS vs G1 - Scan RS very long

Wed Feb 6 20:05:11 UTC 2013

Hi Michal,

Apologies for the delay.

You can confirm whether RSet coarsening is the cause of your long RSet 
scan times using the flags:

-XX:+G1SummarizeRSetStats -XX:G1SummarizeRSetStatsPeriod=n

Basically every n-th GC you'll get a summary of the RSet activity - 
including total size, region with the largest RSet, concurrent activity, 
and the number of coarsenings. These are diagnostic flags so you'll need 
-XX:+UnlockDiagnosticVMOptions.

If the number of coarsenings is high then you increase the number of 
entries in the fine-grain RSet table using by setting 
G1RSetRegionEntries. By default this gets set ergonomically using a base 
number (256) per Mb of region size. So for the region size in your log 
it will be set to 256.  So this means that we can hold references from 
256 distinct regions in each fine grain table before we start coarsening 
references from regions. This equates to 12.5% of your heap.  I would 
suggest bumping that value up to 512.

Alternatively you can also try changing the heap region size by 
explicitly setting -XX:G1HeapRegionSize=2m. In addition to increasing 
the capacity of the fine grain tables, this should reduce the number of 
entries in the RSets since there should be fewer cross-region references.

Regards,

JohnC

On 02/01/13 09:15, Michal Frajt wrote:
>
> Monica,
>
>  
>
> Today morning I sent an email with the correct log files. The 
> attachment size was over the limit, it is still waiting for an 
> approval to be added to the posting list.
>
>  
>
> Some points
>
> -          we compare G1 with CMS-ParNew (not iCMS, even we are using 
> iCMS everywhere else)
>
> -          currently we evaluate the young gen processing only
>
> -          now we set the G1RSetRegionEntries to 1024, it reduced Scan 
> RS time but it is still 3 times slower than ParNew
>
> -          we do set NewSize to 128 for both the G1 and ParNew to keep 
> young collection invoked on same amount of garbage
>
> -          setting G1 NewSize to 16GB and the target time to 60ms is 
> able to keep the track with the ParNew but the G1 young gen is invoked 
> 3 times more frequently as adapted Eden size is around 30MB (ParNew 
> has 128MB-survivor spaces)
>
> -          I will be able to provide you the new log files and more 
> details on Tuesday
>
>  
>
> Thanks,
>
> Michal
>
>  
>
> *From:* Monica Beckwith [mailto:monica.beckwith at oracle.com]
> *Sent:* Friday, February 01, 2013 4:39 PM
> *To:* Michal Frajt
> *Cc:* hotspot-gc-dev at openjdk.java.net; John Cuthbertson
> *Subject:* Re: CMS vs G1 - Scan RS very long
>
>  
>
> Thanks, Michal for providing us with the logs. John (Cuthbertson) and 
> I had a great discussion yesterday pertaining to your G1 log and we 
> think we understand what's going on with your workload and have a 
> experiment(s) plan for you. John is going to send in more details 
> (with explanation). Your workload's pattern is very interesting to 
> me/us and I have started a "work folder" for it, so that I can 
> document our experiment plan and the results and keep track of the 
> progress.
>
> Thanks for sharing your logs!
>
> -Monica
>
> On 2/1/2013 2:12 AM, Michal Frajt wrote:
>
> Sorry, forgot to attach the archive in the last message. 
>
>  
>
> Od: hotspot-gc-dev-bounces at openjdk.java.net 
> <mailto:hotspot-gc-dev-bounces at openjdk.java.net>
>
> Komu: monica.beckwith at oracle.com <mailto:monica.beckwith at oracle.com>
>
> Kopie: hotspot-gc-dev at openjdk.java.net 
> <mailto:hotspot-gc-dev at openjdk.java.net>
>
> Datum: Fri, 1 Feb 2013 09:05:58 +0100
>
> Předmet: Re: CMS vs G1 - Scan RS very long
>
>  
>
> > 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 
> <mailto:monica.beckwith at oracle.com>
>
> > Komu: "John Cuthbertson" john.cuthbertson at oracle.com 
> <mailto:john.cuthbertson at oracle.com>
>
> > Kopie: hotspot-gc-dev at openjdk.java.net 
> <mailto:hotspot-gc-dev at openjdk.java.net>,"Michal Frajt" 
> michal at frajt.eu <mailto: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
> > >
> > >
> > >
>
>
> > >
>
>
> > >
>
> > > --
> > > Oracle <http://www.oracle.com/>
> > > Monica Beckwith | Java Performance Engineer
> > > VOIP: +1 512 401 1274 <tel:+1%20512%20401%201274>
> > > Texas
> > > Green Oracle <http://www.oracle.com/commitment>Oracle is committed 
> to developing practices and products that help protect the environment
>
>  
>
> -- 
> Oracle <http://www.oracle.com>
> Monica Beckwith | Java Performance Engineer
> VOIP: +1 512 401 1274 <tel:+1%20512%20401%201274>
> Texas
> Green Oracle <http://www.oracle.com/commitment>Oracle is committed to 
> developing practices and products that help protect the environment
>

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