RFR (XS) 8174734: Safepoint sync time did not increase

[coleen.phillimore at oracle.com]

On 2/7/18 4:56 AM, David Holmes wrote:
> Hi Coleen,
>
> I've just updated the bug report with a patch to test if you are able 
> to (I don't have any access to a mac unfortunately :( ). It's possible 
> the underlying problem on OS X is an intermediate overflow in 
> calculating the elapsed time via (a*b)/c

I doubt this is the problem since I can reproduce this problem on 
Linux.   Maybe this is a different problem and you should file a bug for it.

Coleen
>
> Thanks,
> David
>
> On 7/02/2018 9:29 AM, coleen.phillimore at oracle.com wrote:
>>
>>
>> On 2/6/18 4:06 PM, coleen.phillimore at oracle.com wrote:
>>>
>>>
>>> On 2/6/18 12:13 AM, David Holmes wrote:
>>>> Hi Coleen,
>>>>
>>>> On 6/02/2018 7:37 AM, coleen.phillimore at oracle.com wrote:
>>>>> Summary: allow safepoint time to be zero in the test
>>>>>
>>>>> See bug for more details.
>>>>>
>>>>> open webrev at http://cr.openjdk.java.net/~coleenp/8174734.01/webrev
>>>>> bug link https://bugs.openjdk.java.net/browse/JDK-8174734
>>>>
>>>> I guess I'm still surprised that 300 thread dumps can take less 
>>>> than a millisecond! There's always more than one thread running. I 
>>>> did some basic benchmarking and dumpAllStacks() from main takes at 
>>>> least 150us on the Linux box I tested on. I just can't see 300 
>>>> dumps taking less than 1ms ... though I can see them taking < 10ms 
>>>> if we're measuring time using a coarse clock - where do these times 
>>>> come from?
>>>>
>>>
>>> I think the thread dumps only the actual JavaThread which is not 
>>> "hidden_from_view".  There are lots of threads but they're all GC 
>>> and compiler threads when I ran this test.
>>>
>>>> That aside this change seem unnecessary:
>>>>
>>>>       // Careful with these values.
>>>> !     private static final long MIN_VALUE_FOR_PASS = 0;
>>>>       private static final long MAX_VALUE_FOR_PASS = Long.MAX_VALUE;
>>>
>>> This was another one of the failures modes, so we need this change 
>>> to make this test more reliable.
>>>>
>>>> this is for the minimum number of safepoints that need to be seen, 
>>>> which I think should still be 1. By allowing 0 here (and for the 
>>>> elapsed time), the test could actually fail to do anything related 
>>>> to safepoints and still pass - and that seems wrong. Or the 
>>>> safepoint stat code could be completely broken and we'd never 
>>>> notice. Basically the test just wants to check that we get 
>>>> reasonable looking statistics from the MBean
>>>>
>>>> Maybe we need to be measuring the time at a higher resolution than 
>>>> milliseconds - though that would be a non-trivial RFE I expect. ?
>>>>
>>>
>>> So, looking at and debugging the runtimeService.cpp code, it appears 
>>> to be doing the thing that it's supposed to be doing. I agree that 
>>> it's not a particularly useful test when changing the times to zero, 
>>> although I traced through and it does exercise the code, and logging 
>>> makes it non-zero.
>>>
>>> What you're suggesting would be a lot more work.  I guess my work 
>>> was to get the test off the ProblemList.txt but if you'd prefer 
>>> doing more work, I'll reassign it and withdraw this RFR.  I thought 
>>> getting it running without failure is more worth doing than writing 
>>> a new test for this feature honestly.
>>
>> Just rereading this.  It might be more useful to add the check that 
>> the safepoint count is non-zero.
>>
>> thanks,
>> Coleen
>>>
>>> thanks,
>>> Coleen
>>>
>>>
>>>> Thanks,
>>>> David
>>>>
>>>>> Thanks,
>>>>> Coleen
>>>
>>