G1GC success examples (compared to CMS) ?

[Sekiya Nobuhiko]

(resendig to mailing list)
Hi Charlie,

Thanks for your response. (I've read your javaOne presentations and you
Java performance book. I liked it very much. )

I guess fragmentation is the key word here.
I have done a test that aimed on getting fragmentations.
But as a result, after some time running, using CMS,  it seemed not to
increase in heap usage, and no Full GCs.
May be my way of allocating objects was not good. ( or, maybe not running
it long enough)

What I did is, I ran a Web Application, one jmeter thread doing 3 requests.
First request adding byte[] of 60kbyte * 10 (so 600kbyte)  to HttpSession
(so it will last till session timeout),
second request adding byte[] of 80kbyte * 10, third 100 kbyte * 10.
Done that for about 3000 user sessions (=3000 jmeter threads) . Eventually
increasing threads.
There is interval of like 3 seconds for each requests of the three.
So, when the session reaches timeout, I hoped getting fragments of
600kbyte, 800kbyte, 1000kbytes.

In this case, compared to G1, CMS was slightly better in resonse time
(compared by 90% line). CMS:46msec to G1: 56msec . This is jmeter response
time, not gc time. No Full GC happening on both.
This was on Windows with -Xmx5g -Xms5g. Jdk7u7.
CMS: -Xmx5g -Xms5g -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -XX:NewRatio=4
G1: -Xmx5g -Xms5g -XX:+UseG1GC -XX:MaxGCPauseMillis=50

May be I should first test with a more simple application to create
fragmentations.
Any sugestions on that?

Also I would love to know a good way to monitor fragmentations. I'm not
sure if I'm really creating fragmentations with my application.

Regards,
Nobuhiko

2012/11/15 Charlie Hunt <chunt at salesforce.com>

> Hi Nobuhiko,
>
> I had done some comparisons with CMS (and with Parallel[Old]GC).
>
> What I had done was induced the undesirable behavior of heap fragmentation
> and object allocation spikes into application running with CMS via
> -javaagent.
> * Fyi, -javaagent essentially allows you to run another Java program in
> parallel with your Java app.  (Thanks Vladimir Ivanov, cc'd, for suggesting
> the idea to use -javaagent.)
> I also did the same experiment against G1.
>
> What those experiments illustrated was G1 in the presence of either heap
> fragmentation or wide swings in object allocation rates offered much more
> predictable behavior than CMS, in particular, avoiding (the painful) full
> GCs.
>
> To date, the focus with G1 was been more so on larger Java heaps, than on
> smaller Java heaps with lower than say 100ms - 150ms pause times.  The
> folks at Oracle may be able to offer additional comments.
>
> If I were to summarize where I see G1 wrt CMS today, I would say that if
> you are experiencing full GCs with CMS, due to fragmentation, or "losing
> the race" [1], or you have a desire to be able to run your Java application
> with a smaller Java heap, then G1 will likely help.  If you are able to run
> with CMS today, and experience no full GCs and the amount of memory your
> Java heap is using, then you're probably (at this point in time) will have
> better results with CMS.  I would additionally add that since G1 is
> expected to one day be the replacement for CMS, (which won't happen until
> it's pretty well shown that G1 can do better than CMS on those apps running
> CMS today), it's advisable that you if you're using CMS today, you probably
> should periodically take a look at G1 and offer feedback to this mailing
> list of your observations.  And, most importantly, if you can, offer both
> the CMS and G1 logs (please include -XX:+PrintGC[Date|Time]Stamps and
> -XX:+PrintGCDetails) along with the JDK/JRE version (i.e. java -version)
> and the set of command line options you are using for both G1 and CMS.
>
> If you happen to have those GC logs today, I'd be happy to take a look.
>
> hths,
>
> charlie ...
>
> [1] "losing the race" -- a situation where objects are getting promoted to
> old generation faster than the rate at which than the CMS collector can
> collect them from old generation and leads to a compaction of old
> generation via a full GC ... other words, the old generation concurrent
> collector lost the race with the promotion rate and the JVM needed to do a
> full GC as a corrective action.
>
> On Nov 13, 2012, at 2:20 PM, Sekiya Nobuhiko wrote:
>
> > Hi,
> >
> > Has anybody found successful test case in G1GC compared to CMS?
> > I search this mailing list but couldn't find anybody commenting such one.
> >
> > If you know any good cases, I would like to know that test case.
> >
> > If you know that there are really no good examples at current, I would
> appreciate that information too.
> >
> > I'm testing G1 with 7u7, but really could not get good results.
> >
> > Best Regards,
> > Nobuhiko
> >
> > _______________________________________________
> > hotspot-gc-use mailing list
> > hotspot-gc-use at openjdk.java.net
> > http://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use
>
>
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