Request for review (S): 8005396: Use ParNew with only one thread instead of DefNew as default for CMS on single CPU machines

Jon Masamitsu jon.masamitsu at
Fri Dec 21 22:43:20 UTC 2012

1098       FLAG_SET_DEFAULT(ParallelGCThreads, par_gc_threads);

I'd suggest using FLAG_SET_ERGO instead of FLAG_SET_DEFAULT.

1627 bool ParNewGeneration::in_use() {
1628   return UseParNewGC;
1629 }

Is "in_use()" useful anymore?  It was invented to avoid a direct
use of ParallelGCThreads.   Maybe just replace it with UseParNewGC?

The rests looks good.


On 12/21/2012 5:42 AM, Bengt Rutisson 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.
> 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.
> Thanks,
> Bengt

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