Intermittent issue with long concurrent marking phase

[Srini Padman]

Hi Ramki,
 
Thank you very much for your reply. 
 
It is not *always* that the concurrent marking phase takes this long, although it happens often enough. For example, in the full GC log corresponding to the snippet I pasted in my posting (attached, zipped) there is only that one instance.
 
I think I know why you are asking - based on my understanding of Bug # 6692906 (more accurately, based on discussions around it on this list), I was under the impression that such long CM phases will happen all the time (if they happen at all). Does the fact that it is intermittent raise the possibility that this is a different issue? I realize that you might not be able to answer this based on the bits of information you have, but perhaps the full GC log will tell you something that you don't already know.
 
Regards,
Srini.

--- On Thu, 9/29/11, Ramki Ramakrishna <y.s.ramakrishna at oracle.com> wrote:


From: Ramki Ramakrishna <y.s.ramakrishna at oracle.com>
Subject: Re: Intermittent issue with long concurrent marking phase
To: "Srini Padman" <srini_was at yahoo.com>
Cc: hotspot-gc-dev at openjdk.java.net
Date: Thursday, September 29, 2011, 4:24 AM


Hi Srini -- (inline below)

On 9/28/2011 4:50 AM, Srini Padman wrote: 





Questions:
 
1\ is it clear based on the description above that the issue is identical to 6692906 (http://bugs.sun.com/view_bug.do?bug_id=6692906)?

Very likely the same bug.






2\ will we benefit by upgrading to a more recent JRE [1.6.0_26 being the one under consideration]?

Definitely worth trying.






3\ I have seen recommendations to use "-XX:-CMSConcurrentMTEnabled" on some web forums - but I have concerns about this; if we don't allow for concurrent marking to use multiple threads, then isnt there a danger of marking proceeding so slowly that we might end up running out of memory i.e., garbage created much faster than it is collected]?

Your concerns are very legitimate (especially given the length of the concurrent mark phase) and the number of cores you have.






 
Any help is greatly appreciated. Please let me know if any additional information is needed at all. I haven't attached the full GC log (it caused problems with posting) but will gladly send it directly to anybody who would like.

The long initial mark pause is definitely concerning -- Does it show up regularly
in the GC logs or is the snippet above an anomaly? Curisously, as the process time
shows, the user and system time are both low but the elapsed time is very large.
That looks like a total stall of the process, and I have no conjectures
based on available data. 

I suggest talking with your Java support folk if you reproduce this after upgrading to
6u28 (or whatever).

best regards.
-- ramki







Regards,
Srini.
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