RFR : JDK-8154166 - java/lang/management/MemoryMXBean/ResetPeakMemoryUsage.java fails with RuntimeException

[Harsha Wardhana B]

Hi Jaroslav,

I am not sure how @required tag works. I searched code base and it is 
not used in any file. Also, the documentation on Jtreg page is sparse.

Could you paste an example as to how to use it?

Also, I would still think that repeated gc via weak-reference is right 
and defensive approach. So I would like to leave that in place unless it 
is causing any side-effects.

Thanks
Harsha

On Tuesday 26 April 2016 04:05 PM, Jaroslav Bachorik wrote:
>
>
> On Mon, Apr 25, 2016 at 9:27 AM, Harsha Wardhana B 
> <harsha.wardhana.b at oracle.com <mailto:harsha.wardhana.b at oracle.com>> 
> wrote:
>
>     Hi,
>
>     Please review below patch to disable concurrent GC option.
>     http://cr.openjdk.java.net/~hb/8154166/webrev.01/
>     <http://cr.openjdk.java.net/%7Ehb/8154166/webrev.01/>
>
>
> I'm sorry to be a PITA, but why it is not possible to use the @require 
> tag?
>
>
>
>     Jaroslav,
>
>     According to Javadoc of Runtime.gc(),
>
>     https://docs.oracle.com/javase/8/docs/api/java/lang/Runtime.html#gc--
>
>     The call will only make it's best effort to do a GC and provides
>     no guarantee that a given object can be collected even if GC runs.
>     It does not say that Runtime.gc() call will block till entire GC
>     cycle is finished and hence we cannot be making that assumption.
>
>
> I know, I had the same discussion a while ago when fixing some other 
> tests failing when run with allowed concurrent explicit GC and I was 
> pointed to the fact that all the known implementation actually do wait 
> until the complete GC cycle is over before returning. Otherwise all 
> those tests relying on some memory having been reclaimed or some 
> counters having been increased would have to be considered random.
>
>
>     Hence it is required that we encapsulate the target object in
>     WeakReference and repeatedly call GC till weakRef returns null.
>     Granted that we will have a small window when weakRef returns null
>     and the target object is not removed from memory. But I see no way
>     how to fix that problem.
>
>
> Exactly. The only guarantee for all the GC related metrics having been 
> updated before proceeding with the test is being able to run the 
> explicit GC in blocking manner. Otherwise the tests are not really 
> deterministic and can intermittently fail.
>
> -JB-
>
>
>     -Harsha
>
>
>     On Sunday 24 April 2016 03:17 PM, Jaroslav Bachorik wrote:
>>     The reproducer would be very time sensitive as with the provided
>>     'ExplicitGCInvokesConcurrent' it will run GC concurrently with
>>     the invoker. Otherwise, in the current implementation, calling
>>     Runtime.gc() would guarantee the GC cycle has finished before
>>     that method returns.
>>
>>     The WeakReference javadoc
>>     (https://docs.oracle.com/javase/7/docs/api/java/lang/ref/WeakReference.html)
>>     is only stating that the referenced object will be made
>>     finalizable at the same time as the reference is cleared. As a
>>     consequence a cleared reference might not always mean that the
>>     heap usage has been changed (unless a particular GC
>>     implementation makes some additional guarantees).
>>
>>     I know we were stabilizing a bunch of related tests relying on GC
>>     doing its work before checking for some post-conditions and the
>>     only way to make the tests reliable was to forbid running those
>>     tests with '-XX:+ExplicitGCInvokesConcurrent'.
>>
>>     -JB-
>>
>>     On Sat, Apr 23, 2016 at 12:15 PM, Harsha Wardhana B
>>     <harsha.wardhana.b at oracle.com
>>     <mailto:harsha.wardhana.b at oracle.com>> wrote:
>>
>>         Hello,
>>
>>         The issue was not reproducible with or without,
>>
>>         "-XX:+ExplicitGCInvokesConcurrent"
>>
>>         Flag. The patch ensures that GC happens before we start
>>         measuring memory. Without the patch, GC might or might not
>>         happen.
>>
>>         -Harsha
>>
>>
>>         On Friday 22 April 2016 07:58 PM, Jaroslav Bachorik wrote:
>>>         Hi,
>>>
>>>         On Fri, Apr 22, 2016 at 9:10 AM, Harsha Wardhana B
>>>         <harsha.wardhana.b at oracle.com
>>>         <mailto:harsha.wardhana.b at oracle.com>> wrote:
>>>
>>>             Hi,
>>>
>>>             Please review the below simple fix for issue,
>>>
>>>             issue : https://bugs.openjdk.java.net/browse/JDK-8154166
>>>             webrev :
>>>             http://cr.openjdk.java.net/~hb/8154166/webrev.00/
>>>             <http://cr.openjdk.java.net/%7Ehb/8154166/webrev.00/>
>>>
>>>
>>>         Shouldn't this test rather declare the conditions when it is
>>>         supposed to work? According to the issue this was caused by
>>>         introducing the "-XX:+ExplicitGCInvokesConcurrent" which
>>>         makes it very tricky to make any assumptions about the GC
>>>         process.
>>>
>>>         See eg.
>>>         jdk/tests/java/lang/management/MemoryMXBean/LowMemoryTest.java
>>>         for enabling the test only for allowed configurations.
>>>
>>>         Cheers,
>>>
>>>         -JB-
>>>
>>>
>>>
>>>             -Harsha
>>>
>>>
>>
>>
>
>

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