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

[Harsha Wardhana B]

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/>

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.

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.

-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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