[ruby-core:21302] [PATCH] drastically improve rb_waitpid for short-lived children

This evening I wrote this patch while helping a friend diagnose why,  
when Threads are off, `system("date")` takes 2.2ms, and when Threads  
are on, 60ms.

The radical difference is a symptom of MRI's green threads and how you  
wait for a child in unix. The current implementation of system  
immediately calls rb_syswait(), which calls rb_waitpid() right after  
forking the child. rb_waitpid() calls waitpid(..., WNOHANG), which  
almost always returns nothing, since the child probably hasn't even  
actually started yet.

My patch improves upon this by adding a 5ms 'quicksleep' zone. At  
intervals of 0.5ms for the first 5ms after rb_waitpid is called,  
waitpid() is called in a loop. This allows short-lived, fast exiting  
children to be caught and processed almost right away, without  
incurring the 60ms minimum time we currently have.

The patch also adds a poll time backoff for use after the first 5ms.  
Rather than jumping right to backing off for 60ms, it backs off  
increments up to 60ms. This allows for children children that take  
just a little bit longer to be processed a bit quicker too.

For long running children, the behavior is the same.

This improves behavior a lot for users who run a lot of small, fast  
programs as children, in true unix style.

  - Evan Phoenix // [e-mail]