Request for review (s) - 7198873

[Jon Masamitsu]

Mikael,

Do you want a default value (less than max uint) for MaxMetaspaceSize before
this change goes back?

Jon

On 9/25/2012 11:09 AM, Mikael Gerdin wrote:
> On 2012-09-25 18:23, Jon Masamitsu wrote:
>> Mikael,
>>
>> Thanks for the review.
>>
>> The expand_and_allocate() does not do a GC.  It expands
>> the Metaspace and does an allocation from the expanded
>> space.  Only if that fails does the CMS case fall through to
>> the full GC.
>
> Yes.
>
>>
>> The policy for CMS is
>>
>> 1) Hitting the HWM should start a concurrent collection if
>> CMS is doing class unloading.
>> 2) Always  expand the Metaspace and allocate from
>> the expanded space.
>> 3) If expanding the Metaspace does not provide any free
>> space, do a full GC to reclaim classloaders and class metadata
>> and then retry the allocation.
>
> But at what point does expanding the Metaspace not provide any free 
> space? Is there some sort of back-off so that we don't just go ahead 
> and allocate all available memory and then try to do a full gc when 
> we've filled up the address space?
> I'm kind of concerned about the case with CMS without 
> CMSClassUnloadingEnabled. What I'm mainly worried about is slow leaks 
> and cases where an application loads a bunch of classes and then 
> releases the java level references to them but does not unload them 
> since we don't get to the expand_and_allocate returning null.
>
> /Mikael
>
>>
>> Jon
>>
>>
>> On 09/25/12 07:23, Mikael Gerdin wrote:
>>> Jon,
>>>
>>> On 2012-09-24 23:46, Jon Masamitsu wrote:
>>>> NPG: VM Does not unload classes with UseConcMarkSweepGC
>>>>
>>>> If CMS is not doing class unloading, don't start a concurrent
>>>> collection for classloader (and metadata) collection (since
>>>> it won't happen without class unloading).
>>>
>>> It looks like you still unconditionally call expand_and_allocate when
>>> running with CMS, no matter the value of CMSClassUnloadingEnabled.
>>> I think that the code:
>>>
>>>  213       // For CMS expand since the collection is going to be
>>> concurrent.
>>>  214       _result =
>>>  215 _loader_data->metaspace_non_null()->expand_and_allocate(_size,
>>> _mdtype);
>>>
>>> Should be inside the "if (CMSClassUnloadingEnabled)" and if running
>>> without it set then CMS users will have to take the hit of a stw full
>>> gc when running into the metadata threshold.
>>>
>>> /Mikael
>>>
>>>>
>>>> http://cr.openjdk.java.net/~jmasa/7198873/webrev.00/
>>>>
>>>> Also, refactored the code for readability and guarded extra
>>>> output with Verbose.
>>>>
>>>> Thanks.
>>>>
>>>> Jon
>>>
>