Commits on Mar 7, 2019

1. Optimize Windows contention performance …

   The dispatch_cascade test is very slow on Windows right now (often taking 4-5
   minutes to execute on my machine and sometimes locking it up entirely) because
   contention is not being handled correctly.
   
   `_dispatch_contention_usleep()` is expected to put the thread to sleep, but on
   Windows it spins on `QueryPerformanceCounter()`. This is causing a huge amount
   of starvation in the dispatch_cascade test. Implement it using `Sleep()`, and
   accordingly adjust `DISPATCH_CONTENTION_USLEEP_START` to be 1ms on Windows.
   
   Additionally, `_dispatch_contention_spins()` is currently implemented using the
   `rand_s()` function. This is slow (and experimentally seems to be slower than
   not randomizing the spin count at all!) because `rand_s()` guarantees
   cryptographic security, which is unnecessary for dispatch's use case. Replace it
   with a basic linear congruential generator (from K&R) since there isn't any
   other `rand_r()` equivalent. Based on the average wall clock times reported by
   bsdtestharness, this is around 35% faster on my PC (i7-8700K).
   
   These changes bring the runtime of the dispatch_cascade test down to around 1-2s
   at most for me. (It's even faster than this if stdout isn't a console window
   because that slows down the histogram display.)

   

   adierking committed Mar 7, 2019
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