Strange GC behaviour,

Srinivas Ramakrishna ysr1729 at
Thu Dec 8 01:44:02 UTC 2011

Hi Nhan --

Make sure you have the recent fix to Reference object processing when doing
MT/Parallel marking with CMS; i don't have the CR handy, but the fix was
pushed by
Stefan not long ago. (The workaround was to disable parallel concurrent
marking via
-XX:-CMSConcurrentMTEnabled -- but of course the performance impact of that,
depending on yr platform, might compromise yr benchmarking results.)

More inline below:-

On Wed, Dec 7, 2011 at 2:20 PM, Dang Nhan Nguyen <nhann at> wrote:

>  Hi guys,
>  I am benchmarking CMS collector and found it strange that there are many
> objects left in remark phase.
>  When I check the issue again by only running foreground CMS (I changed
> the source code a bit to force CMS running in foreground), so there is no
> promotion during collecting  (so no objects marked during Preclean and
> Abortable Preclean phase). Even in this case, I found that there are still
> many objects left in remark phase (in lusearch and tomcat in Dacapo
> benchmark, they are 10 times and 1/3 of number of objects marked in
> concmarking phase, while in suflow, they are just about 100 objects/refs).
> Most (maybe ALL) of these objects marked in remark phase were processed by
> refProcessingWork(). In turn, this refProcessingWork() run in single thread
> mode. The performance was reduced a lot because of this.

-XX:+ParallelRefProcEnabled uses multiple threads to do that phase.

>  My questions are:
> 1. What kinds of references were processed by refProcessingWork()? weak
> and soft ref? what are exactly they are?

all objects of type java.lang.ref.Reference (includes WeakReferencem=,
SoftReference, FinalReference, PhantomReference).

> 2. While I only run CMS in stop-the-world, why are there still many
> objects left to be processed by remark, in particular here is
> refProcessingWork()? Why aren't these references/objects processed in
> concurrent marking phase?

I am not sure I understand the question. What do you mean by "run CMS in
stop-the-world"? Note that Reference objects are processed
by all kinds of collectors, young gen (minor) or whole heap (major),
stop-world or concurrent. The volume of those objects may
vary a bit between them based on the heap size, NewRatio etc.

hope that helped.
-- ramki

>  Best,
> -Nhan Nguyen
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