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

[coleen.phillimore at oracle.com]

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
>