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Make use of sched_yield optional in runtime

Authored by tlwilmar on Feb 12 2019, 2:11 PM.



This patch cleans up the yielding code and makes it optional. An environment variable, KMP_USE_YIELD, was added. Yielding is still on by default (KMP_USE_YIELD=1), but can be turned off completely (KMP_USE_YIELD=0), or turned on only when oversubscription is detected (KMP_USE_YIELD=2). Note that oversubscription cannot always be detected by the runtime (for example, when the runtime is initialized and the process forks, oversubscription cannot be detected currently over multiple instances of the runtime).

Because yielding can be controlled by user now, the library mode settings (from KMP_LIBRARY) for throughput and turnaround have been adjusted by altering blocktime, unless that was also explicitly set.

In the original code, there were a number of places where a double yield might have been done under oversubscription. This version checks oversubscription and if that's not going to yield, then it does the spin check.

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tlwilmar created this revision.Feb 12 2019, 2:11 PM
This revision is now accepted and ready to land.Feb 28 2019, 11:05 AM
This revision was automatically updated to reflect the committed changes.