JEP 173: Remove Rarely-Used Combinations of Garbage Collectors

Srinivas Ramakrishna ysr1729 at
Mon Dec 17 20:05:19 UTC 2012

I suspect Andy (lordpixel) may be referring to CMS generally not giving old
gen heao memory back to the OS even when applicatio
footprint shrinks. I haven't ventured into CMS resizing code in a while,
but i assumed that it would not be difficult to make it do so at
the end of a sweep because of keeping a (hopefully) large contiguous free
block at the high end of the heap. I don't think though that
heap shrinking was ever a priority for the server environments in which CMS
mostly got used, so it never really got done.

-- ramki

On Mon, Dec 17, 2012 at 10:38 AM, Jon Masamitsu <jon.masamitsu at>wrote:

> -XX:+UseParNewGC  will give you a parallel young gen collector
> plus the serial old gen collector.   It does not have all the features
> that -XX:+UseParallelGC has (for example, does not have support
> for -XX:+UseNUMA).   It actually is one of the combinations that
> we would like to remove.
> Jon
> On 12/14/2012 9:24 PM, lordpixel at wrote:
>> For whatever reason I assumed this was only a collector for the young
>> generation... is that wrong? Even if it does give memory back, for one of
>> my apps at least its the old generation that needs to shrink.
>> On Dec 14, 2012, at 10:52 AM, Jon Masamitsu<jon.masamitsu@**<jon.masamitsu at>>
>>  wrote:
>>  -XX:+UseParNewGC
>> AndyT (lordpixel - the cat who walks through walls)
>> A little bigger on the inside
>>         (see you later space cowboy, you can't take the sky from me)
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