RFR(m): 8214271: Fast primitive to wake many threads

[Robbin Ehn]

Forgot RFR in subject.

/Robbin

On 2018-11-23 17:51, Robbin Ehn wrote:
> Hi all, please review.
> 
> When a safepoint is ended we need a way to get back to 100% utilization as fast
> as possible. 100% utilization means no idle cpu in the system if there is a
> JavaThread that could be executed. The traditional ways to wake many, e.g.
> semaphore, pthread_cond, is not implemented with a single syscall instead they
> typical do one syscall per thread to wake.
> 
> This change-set contains that primitive, the WaitBarrier, and a gtest for it.
> No actual users, which is in coming patches.
> 
> The WaitBarrier solves by doing a cooperative semaphore posting, threads woken
> will also post. On Linux we can instead directly use a futex and with one
> syscall wake all. Depending on how many threads and cpus the performance vary,
> but a good utilization of the machine, just on the edge of saturated, the time 
> to reach 100% utilization is around 3 times faster with the WaitBarrier (where 
> futex is faster than semaphore).
> 
> Webrev:
> http://cr.openjdk.java.net/~rehn/8214271/webrev/
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8214271
> 
> Passes 100 iterations of gtest on our platforms, both fastdebug and release.
> And have been stable when used in safepoints (t1-8) (coming patches).
> 
> Thanks, Robbin