CMS GC tuning under JVM 5.0

Y Srinivas Ramakrishna Y.S.Ramakrishna at Sun.COM
Mon Apr 21 23:29:59 UTC 2008

Hi Doug --
> 3) As suggested in the second bug mentioned by Ramki, we may also set 
> +CMSScavengeBeforeRemark. This flag is I believe not on by default in 
> any current JDK version, but in our case with a relatively small 
> NewSize, a ParNew collection takes max something like 0.02sec (and 
> that's when at its full 24Mb) so should add virtually nothing to the 
> time the app is stopped.

Unfortunately, it turns out that because of an engineering process
error the suggested fix of turning CMSScavengeBeforeRemark into a "product"
flag was not accomplished as part of the CR I mentioned, a fact that I discovered a few
hours ago when assisting a customer running on 5uXX. This is being corrected
in the next update of 5uXX and the bug report will be fixed to reflect that
unfortunate and inadvertent omission. (Note that this problem is specific to 5uXX;
with 6uXX and 7, there is no problem -- the flag is indeed available for setting in
6uXX and 7.)

-- ramki

> Actually from looking at other CMS Collections in the same log, just 
> 1) and 3) on their own may fix the problem because we see several 
> examples where the CMS GC has kicked in with eden containing less than 
> 1Mb and the abortable-preclean phase is omitted altogether. However 
> this is specific to the characteristics of our application. I have no 
> doubt that there are other situations where setting 
> CMSScavengeBeforeRemark is not desirable.
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