Request for review (s) - 7198873

Srinivas Ramakrishna ysr1729 at gmail.com
Thu Sep 27 07:49:55 UTC 2012


I agree that if the user repeatedly does deploy-undeploy and is running
with class unloading disabled,
it would be good if he were to run out of native memory and realized his
mistake :-) and turned on class unloading.

As regards yr policy suggestion of forcing a perm gen collection every so
many cycles, i believe that is possible to do
so in a concurrent cycle rather than doing a stop-world collection for that
purpose  --
there was talk at one time of making the cms class unloading flag a
"manageable" flag (so it could be turned on or off
on-the-fly) -- there's probably even a bug about adaptively collecting the
perm gen with CMS.
 If it was found that the cost of class unloading at each major CMS cycle
was too expensive, one could,
as you suggest, do it ever so often.

How about a quick survey on the "-dev" and "-use" lists to see how many
users who use CMS explicitly switch on class
unloading vs not?

In any case, as regards Jon's question, I would say that the changes should
go back in woithout waiting for a decision
on MaxMetaSpaceSize or CMSClassUnloadingEnabled. If they run out of native
space they will probably quickly figure out what
happened (i am assuming the GC logs that inform one about one's usage of
metadata space would clue one in fairly soon if one
discovered how much space that was taking... I am assuming +PrintHeapAtGC
would produce some information, even if
+PrintGCDetails didn't re metatdataspace use?).

my $0.02.
-- ramki

On Thu, Sep 27, 2012 at 12:27 AM, Mikael Gerdin <mikael.gerdin at oracle.com>wrote:

> Jon,
>
>
> On 2012-09-26 18:44, Jon Masamitsu wrote:
>
>> Mikael,
>>
>> Do you want a default value (less than max uint) for MaxMetaspaceSize
>> before
>> this change goes back?
>>
>
> While I see a value in setting some sort of "sensible" value for
> MaxMetaspaceSize this is not really the exact issue I was worried about.
>
> Consider this scenario:
>
> * Person runs a Java app server/container with +UseConcMarkSweepGC (but
> without +CMSClassUnloadingEnabled).
> * Some app is repeatedly deployed and un-deployed from the instance, every
> time an app is un-deployed its class loader and classes are made
> unreachable.
>
> In this case CMS would not opt to unload classes until MaxMetaspaceSize is
> hit or the process/system runs out of memory.
>
> Maybe there could be some sort of threshold for when to trigger a full
> collection (which includes class unloading) without that threshold value
> being a limit on the amount of memory available for meta data?
>
> I know this is a complicated policy issue, and I agree that it would be
> good if we could enable +CMSClassUnloadingEnabled and get rid of the issue
> altogether. The question is whether or not performance testing resources
> are available to evaluate that.
>
> /Mikael
>
>
>
>> 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/<http://cr.openjdk.java.net/%7Ejmasa/7198873/webrev.00/>
>>>>>>
>>>>>> Also, refactored the code for readability and guarded extra
>>>>>> output with Verbose.
>>>>>>
>>>>>> Thanks.
>>>>>>
>>>>>> Jon
>>>>>>
>>>>>
>>>>>
>>>
> --
> Mikael Gerdin
> Java SE VM SQE Stockholm
>
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