CMS collection keep working during holiday

Y Srinivas Ramakrishna Y.S.Ramakrishna at Sun.COM
Thu Oct 9 16:09:46 UTC 2008

> Y Srinivas Ramakrishna wrote:
> > If you use jmap -histo:live <pid> from the command-line you will
> > get a full gc before the histogram is computed. This full gc will
> > be done despite having +DisableExplicitGC.
> >   
> In fact, given that we want to find out what's growing in the heap 
> during the CMS cycles (which seems to be GCed during Full GCs), it's 
> better to use jmap -histo (i.e., no :live) to avoid the Full GC. But 
> I'm 

Right, both would be useful. (My suggestion was merely in response to
Ken's remark that he wanted the full gc which he was not getting
from jconsole because of hist +DisableExplicitGC.
Indeed, the first one that Ken posted (which might
have been from a different VM and therefore not comparable
apples-to-apples with this dump) was indeed without :live and did show the
large number of hash tables / entries, as you pointed out,
which this one, following a full gc, does not. And so the mystery
deepens as to why CMS was apparently keeping these objects which
mark-compact is apparently able to reclaim.)

> forgetting: which VM Ken using? IIRC only jmap in 6 has the :live / no 
> :live functionality (before that, it always did a Full GC, which is 
> not 
> what you want some of the time).

To the parenthetic remark: AFAIR, the pre-6.0 jvm's just dumped, no full gc --
thus jmap -histo did not change across 6.0; 6.0 added the new :live
feature; the latter feature could be simulated in pre-6.0 JVM's
by means of +PrintClassHistogram+SIGQUIT(ctl-break). AFAICT, Ken is using 6u7

-- ramki

> Tony
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