RFR: 8220795: Introduce TimedYield utility to improve time-to-safepoint on Windows

[Robin Westberg]

Hi David,

Thanks for taking a look!

> On 5 Apr 2019, at 10:49, David Holmes <david.holmes at oracle.com> wrote:
> 
> Hi Robin,
> 
> On 5/04/2019 6:05 pm, Robin Westberg wrote:
>> Hi all,
>> Please review the following change that modifies the way safepointing waits for other threads to stop. As part of JDK-8203469, os::naked_short_nanosleep was used for all platforms. However, on Windows, this function has millisecond granularity which is too coarse and caused performance regressions. Instead, we can take advantage of the fact that Windows can tell us if anyone else is waiting to run on the current cpu. For other platforms the original waiting is used.
> 
> Can't you just make the new code the implementation of os::naked_short_nanosleep on Windows and avoid adding yet another sleep/yield abstraction? If Windows nanosleep only has millisecond granularity then it's broken.

Right, I considered that approach, but it's not obvious to me what semantics would be appropriate for that. Depending on whether you want an accurate sleep or want other threads to make progress, it could conceivably be either implemented as pure spinning, or spinning combined with yielding (either cpu-local or Sleep(0)-style).

That said, perhaps os_naked_short_nanosleep should be removed, the only remaining usage is in spinYield, which turns to sleeping after giving up on yielding. For windows, it may be more appropriate to switch to Sleep(0) in that case.

Best regards,
Robin

> 
> Thanks,
> David
> -----
> 
>> Various benchmarks (specjbb2015, specjm2008) show better numbers for Windows and no regressions for Linux.
>> Issue: https://bugs.openjdk.java.net/browse/JDK-8220795
>> Webrev: http://cr.openjdk.java.net/~rwestberg/8220795/webrev.00/
>> Testing: tier1
>> Best regards,
>> Robin