8214271: Fast primitive to wake many threads

Robbin Ehn robbin.ehn at oracle.com
Fri Nov 23 16:51:07 UTC 2018

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).



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

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