RFR 6987597: ManagementFactory.getGarbageCollectorMXBeans() returns empty list with CMS

Mandy Chung mandy.chung at oracle.com
Wed Nov 27 13:30:35 PST 2013

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> On Nov 27, 2013, at 1:15 PM, Jaroslav Bachorik <jaroslav.bachorik at oracle.com> wrote:
> Thanks for looking at this, Mandy.
> Here is the patch with added explanatory comments - http://cr.openjdk.java.net/~jbachorik/6987597/webrev.05
> -JB-
>> On 27.11.2013 21:33, Mandy Chung wrote:
>>> On 11/27/2013 10:41 AM, Jaroslav Bachorik wrote:
>>> Hi,
>>> I've uploaded the patch with the minimal changes that should resolve
>>> this particular problem.
>>> http://cr.openjdk.java.net/~jbachorik/6987597/webrev.04
>> Looks good.  I'm happy to see this simple change resolves the issue. It
>> would be useful to add a comment before you push to explain why
>> -XX:+ExplicitGCInvokesConcurrent flag is used for simplicity even though
>> this flag is ignored by non-concurrent GC.
>> Mandy
>>> -JB-
>>>> On 22.11.2013 14:57, Jaroslav Bachorik wrote:
>>>>> On 21.11.2013 17:51, Mandy Chung wrote:
>>>>> Hi Jaroslav,
>>>>>> On 11/19/2013 6:23 AM, Jaroslav Bachorik wrote:
>>>>>> Please, review this test fix.
>>>>>> Issue : https://bugs.openjdk.java.net/browse/JDK-6987597
>>>>>> Webrev: http://cr.openjdk.java.net/~jbachorik/6987597/webrev.03
>>>>>> The fix moves the execution of the test routine to the
>>>>>> GarbageCollectorMXBean notification handler - at that moment it's
>>>>>> certain that GC has already happened and it is safe to assert the data
>>>>>> provided by the MBean.
>>>>> This patch may have a potential issue when GC happens during the
>>>>> process
>>>>> of adding the notification handlers such as the number of increments is
>>>>> greater than the number of registered handlers.   By the time the main
>>>>> thread reaches p.arriveAndAwaitAdvance, the phaser has advanced to the
>>>>> next phase with few arrived parties and will wait forever (until the
>>>>> next GC happens).   This should rarely happen though.  Note that
>>>>> p.arriveAndAwaitAdvance continue to run even if the thread is
>>>>> interrupted and what happens if jtreg attempts to time out the test?
>>>> Thanks for catching this. It seems that using a Semaphore with adding
>>>> permissions when a GC notification arrives would be more stable.
>>>>> Does this issue only happen to CMS with background GC thread?   The
>>>>> proposed patch seems a little overkill.   I wonder if the test should
>>>>> skip if running in CMS background mode.  Does
>>>>> -XX:+ExplicitGCInvokesConcurrent flag will get System.gc() to run in
>>>>> foreground mode in CMS (I think that may get the GC to complete before
>>>>> System.gc() returns?)
>>>> Yes, currently only CMS. It seems that the semantics of System.gc()
>>>> ("When control returns from the method call, the Java Virtual Machine
>>>> has made a best effort to reclaim space from all discarded objects.")
>>>> doesn't apply to the CMS collector.
>>>> Adding "-XX:-ExplicitGCInvokesConcurrent" should force System.gc() to
>>>> wait till the GC has been performed. This could help resolve this
>>>> particular problem.
>>>> -JB-
>>>>> Mandy

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