jmx-dev RFR 8020467: Inconsistency between usage.getUsed() and isUsageThresholdExceeded() with CMS Old Gen pool

[Jaroslav Bachorik]

On 23.10.2013 10:08, Staffan Larsen wrote:
> I think you can simplify the logic for forcing a GC to just a simple call to "System.gc();". AFAIK System.gc() will cause a full collection to happen for all collectors.

Hm, will it now? I had the impression that it was just hinting the GC 
system to perform GC but it might decide to ignore it. I need to be sure 
that the GC was performed before continuing - otherwise I might get 
inconsistent data again.

-JB-

>
> /Staffan
>
> On 23 okt 2013, at 10:02, Jaroslav Bachorik <jaroslav.bachorik at oracle.com> wrote:
>
>> On 22.10.2013 22:04, Mandy Chung wrote:
>>> Hi Jaroslav,
>>>
>>> On 10/22/13 6:47 AM, Jaroslav Bachorik wrote:
>>>> Please, review the following test fix:
>>>>
>>>> Issue:  https://bugs.openjdk.java.net/browse/JDK-8020467
>>>> Webrev: http://cr.openjdk.java.net/~jbachorik/8020467/webrev.01
>>>>
>>>
>>> Have you considered to force GC when getUsed() == 0 regardless of which
>>> memory pool it is?  This will avoid special casing for CMS old gen in
>>> the test and will handle similar issue in the future for a different
>>> collector implementation.  To make the test reliable, the test should
>>> still pass if the memory pool has no object in it (G1 survivor space
>>> case?).
>>
>> Hi Mandy,
>>
>> I don't know whether GC will help for other pools - but I can enable it for all pools - it should not hurt.
>>
>> The test should pass even with on object in the monitored pool since the pool should not report an exceeded threshold.
>>
>> -JB-
>>
>>>
>>> Mandy
>>>
>>>> The test tries to make sure that the "pool usage threshold" trigger
>>>> and the reported pool memory usage are not contradicting each other.
>>>> The problem is that it is not possible to get the "pool usage
>>>> threshold exceeded" flag and the pool memory usage atomicly in regard
>>>> to the GC. Specifically, when "CMS Old Gen" pool is examined and the
>>>> usage is retrieved before a GC promotes some objects to the old gen
>>>> but the usage threshold is checked after the GC has promoted some
>>>> instance into the old gen the test will fail.
>>>>
>>>> The patch makes sure that there are some instances promoted in "CMS
>>>> Old Gen" before checking the "pool usage threshold" to get
>>>> semi-consistent view.
>>>>
>>>> Thanks,
>>>>
>>>> -JB-
>>>
>>
>