CMS parallel initial mark

[Hiroshi Yamauchi]

>  - I think CMSCollector::_eden_chunk_**sampling_active should be volatile
>>>> to avoid any clever compilers skipping the read in sample_eden_chunk().
>>>> (Although I don't think it's possible here). See also below for avoiding
>>>> that variable completely.
>>>>
>>> Could you explain you thinking for making it volatile.  As you have, I
>>> didn't think
>>> it was necessary and thought ,"If not necessary, don't do it".
>>>
>> It's not necessary I think, just as a reminder to the compiler to not
>> optimize that load away. I think the Atomic instruction contain the
>> necessary instructions though.
>>
>
> Ok.  I might be a bit of a distraction to include volatile when
> not necessary.


I haven't thought about this volatile thing too much. But it seems
unnecessary, even if we keep this _eden_chunk_**sampling_active, as opposed
to using try_lock() which I haven't thought about yet.
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