Commits on Sep 29, 2017

1. Fix excess work stealing under low loads …

   Fixes #1787
   
   Interestingly, all the info needed to solve this issue a while ago was
   already in the issue but it wasn't until @slfritchie put his additional
   comments in
   #1787 (comment)
   that it all clicked for me.
   
   The excess CPU time is from us doing too much work stealing. In a normal
   scenario, with nothing to do, we'd not doing anything for a long time
   and we'd end up sleeping for quite a while.
   
   With the timer that goes off every few seconds as seen in the issue,
   that isn't what happens. We regularly get woken and end up in a work
   stealing cycle.
   
   Then, due to the lack of an `else` block for yielding, on OSX, we'd
   nanosleep for 0 which is the same as an immediate return. To see what
   the impact of that would be on any platform change the:
   
   ```c
     // 10m cycles is about 3ms
       if((tsc2 - tsc) < 10000000)
           return;
   ```
   
   to
   
   ```c
     // 10m cycles is about 3ms
       if((tsc2 - tsc) < 1000000000)
           return;
   ```
   
   This is effectively what we were running. That's a lot more
   work-stealing. And, not the increased CPU usage. The reason this was
   happening more on OSX is that on Linux, nanosleep 0 will sleep for at
   least a bit. Here we remove the variability and do a small nanosleep
   that will be the same across all platforms.

   

   SeanTAllen committed Sep 29, 2017
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