JEP 173: Remove Rarely-Used Combinations of Garbage Collectors

[Jon Masamitsu]

Ramki,

There is an unintended consequence :-) of having defined a 
compute_new_size()
for the CMS gen.  That compute_new_size() doesn't do shrinking and
gets called in the do_collection() when a full collection is done.  The 
result
is that even if the full collection results in lots of free space in the CMS
gen, there still isn't any shrinking.  I have a fix that hasn't been 
polished
yet.  Can't say it's coming soon but hopefully it's coming.

Jon

On 12/17/2012 12:05 PM, Srinivas Ramakrishna wrote:
> 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 oracle.com>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 me.com 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@**oracle.com<jon.masamitsu at oracle.com>>
>>>   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)
>>>
>>>
>>>
>>>