Request for review (S): 8005396: Use ParNew with only one thread instead of DefNew as default for CMS on single CPU machines
John.Coomes at oracle.com
Fri Dec 21 16:12:19 PST 2012
Bengt Rutisson (bengt.rutisson at oracle.com) wrote:
> Hi All,
> Can I have a couple of reviews for this change?
> Currently we use ParNew as default for the young generation when CMS is
> selected. But if the machine only has a single CPU we set the
> ParallelGCThreads to 0 and and select DefNew instead of ParNew.
Looks good to me.
> As part of another change, 8003820, we will deprecate the DefNew + CMS
> combination. Thus, it does not make sense anymore to have this selected
> by default. This fix is to make CMS always pick ParNew by default.
> The change also has the side effect that the, in my opinion, rather
> strange behavior that setting ParallelGCThreads=0 on the command line
> overrides the GC choice. I would expect this command line to give me
> ParNew, but it actually gives me DefNew:
> -XX:+UseParNewGC -XX:ParallelGCThreads=0
> After my proposed change you get ParNew with the above command line.
> I have done some performance testing to verify that ParNew with one
> thread is not slower than DefNew. The details are in the bug report:
> but as a summary it can be said that there is no noticeable difference.
> I am also running some more SPECjbb2005 runs and will analyze the gc times.
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