[Chicken-hackers] PATCH: more efficient scheduler

[Jörg F . Wittenberger]

Hi,

the attached patch does not fix any bugs.  It brings some performance
improvements under certain load conditions.  The cost: It adds the
overhead of another counter and a dispatch on a a fixnum modulo
operation - IMHO negotiable.

The change is helpful when sits watching on several file descriptors for
requests _and_ uses several threads, message passing etc. to produce
responses.

Under these circumstances I found a common call pattern to be few file
descriptors to become available resulting in many internal threads
forked.  Those threads have often very little actual work to do until
the block again.  At this point each ##sys#schedule will check those
file descriptors again and again - typically not returning any fd until
after all those threads completed eventually.  This results in a pretty
huge delay of the response.

With the change there is now a "pending-queue".  This holds all the
threads to be run once before we even check for new work from timeouts
or i/o.  Furthermore we only check for i/o at an adjustable rate or if
we would wait otherwise.

I have a test case where there are about 50 threads waiting on input and
observed up to 200 in state "ready" to pass messages among each other at
a time.  Those saved i/o times add up to a difference of about 30%
faster responses.

Best

/Jörg

0013-Modified-schedulerpolicy.patch
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