RFR (XS) 8215329: Modify ZGC requirement for HeapMonitorThreadTest.java

[Per Liden]

Hi, me again ;)

I think I've found the root cause of this. There's a tiny race in the 
ZGC allocation path, which can lead to pre-mature OOME being thrown. 
It's not trivial to fix, so I suggest you go ahead with your original 
patch (Looks good btw), and I'll file a separate bug to fix the ZGC 
issue (and update this test to run with ZGC again).

cheers,
Per

On 12/13/2018 12:21 PM, Per Liden wrote:
> Hi again,
> 
> I ran this test some more and managed to get an OOME even with a 768M 
> heap. I'm getting a bit suspicious that something else is wrong here. 
> Let me dig into this some more and see if I can understand what the real 
> issue is.
> 
> cheers,
> Per
> 
> On 12/13/2018 10:31 AM, Per Liden wrote:
>> Hi JC,
>>
>> What's the reason to exclude ZGC from this test to begin with? From 
>> what I can tell, it's because the test is using a slightly too small 
>> heap, or are there some other reason? I ran it a few times using 
>> various heap sizes and the test passes with ZGC when using anything 
>> above 612M. So if we instead just dump the heap size a bit, then we 
>> get test coverage with ZGC too. I picked 768M here to have some 
>> headroom in case the exact limit is run-to-run dependent.
>>
>> diff --git 
>> a/test/hotspot/jtreg/serviceability/jvmti/HeapMonitor/MyPackage/HeapMonitorThreadTest.java 
>> b/test/hotspot/jtreg/serviceability/jvmti/HeapMonitor/MyPackage/HeapMonitorThreadTest.java 
>>
>> --- 
>> a/test/hotspot/jtreg/serviceability/jvmti/HeapMonitor/MyPackage/HeapMonitorThreadTest.java 
>>
>> +++ 
>> b/test/hotspot/jtreg/serviceability/jvmti/HeapMonitor/MyPackage/HeapMonitorThreadTest.java 
>>
>> @@ -29,8 +29,7 @@
>>    * @build Frame HeapMonitor ThreadInformation
>>    * @summary Verifies the JVMTI Heap Monitor Thread information sanity.
>>    * @compile HeapMonitorThreadTest.java
>> - * @run main/othervm/native -Xmx512m -agentlib:HeapMonitorTest 
>> MyPackage.HeapMonitorThreadTest
>> - * @requires !vm.gc.Z
>> + * @run main/othervm/native -Xmx768m -agentlib:HeapMonitorTest 
>> MyPackage.HeapMonitorThreadTest
>>    */
>>
>>   import java.util.List;
>>
>> cheers,
>> Per
>>
>> On 12/13/2018 05:44 AM, JC Beyler wrote:
>>> Hi all,
>>>
>>> When working on another webrev, I saw this problem:
>>>
>>> Webrev: http://cr.openjdk.java.net/~jcbeyler/8215329/webrev.00/
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8215329
>>>
>>> (Basically, from what I understood from an email from Per Liden:
>>>     - @requires !vm.gc.Z -> ZGC is built in the JDK
>>>     - @requires vm.gc != "Z" -> ZGC is being used for the runtime
>>> )
>>>
>>> Thanks,
>>> Jc