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

Bengt Rutisson bengt.rutisson at
Sun Dec 30 08:11:57 UTC 2012

Hi John and Jon,

Thanks for the reviews!

I discovered a bug in my fix. ParNew actually does not support 
ParallelGCThreads=0. I fixed this by making sure that we don't set 
ParallelGCThreads to 0 on single CPU machines. Instead we keep it at 1.

And if someone explicitly set -XX:ParallelGCThreads=0 on the command 
line while trying to use ParNew I print an error message and exit.

I assume that this is the reason that we previously picked DefNew if 
ParallelGCThreads was set to 0, but I think now that we want to 
deprecate DefNew for CMS it makes more sense to require users to 
explicitly turn ParNew off with -XX:-UseParNewGC if this is what they 
really want.

Updated webrev:

The only change to the previous version is in arguments.cpp. Here is the 
small diff compared to the previous webrev:

I have tested the fix on a single CPU virtual box instance.


On 12/22/12 1:12 AM, John Coomes wrote:
> Bengt Rutisson (bengt.rutisson at 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.
> -John
>> 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

More information about the hotspot-gc-dev mailing list