Newsgroups: comp.unix.sysv386
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!ispd-newsserver!ism.isc.com!ico!rcd
From: rcd@ico.isc.com (Dick Dunn)
Subject: Re: Son of FAS?
Message-ID: <1991Apr25.173627.2642@ico.isc.com>
Summary: polling would reduce overhead, but it's tricky
Organization: Interactive Systems Corporation, Boulder, CO
References: <1991Apr25.010758.1522@pegasus.com>
Date: Thu, 25 Apr 1991 17:36:27 GMT

richard@pegasus.com (Richard Foulk) writes:
> I recall hearing that Equinox (or someone like that) equips their
> smart serial card with a driver that doesn't use interrupts.  They
> do some kind of polling from within one of the kernels inner loops.

Hmmm...the most often you're guaranteed to get into the kernel is once per
clock tick, or 10 ms, which isn't often enough to poll a serial device.
Once you get into the device code on an interrupt, you can hang around and
pick up any additional characters that arrive while you're processing the
first one...but I think most serial drivers nowadays do that anyway.

> Doing away with the interrupt overhead supposedly results in a marked
> performance gain.  (Seems reasonable.)

Quite likely--the interrupt overhead really is most of the time you spend
processing an incoming character on a serial port.

The only catch is that you've got to be careful not to hang around polling
the device for very long, or you'll miss other interrupts.  (This is a
DOSism that just won't work in UNIXland.)
-- 
Dick Dunn     rcd@ico.isc.com -or- ico!rcd       Boulder, CO   (303)449-2870
   ...While you were reading this, Motif grew by another kilobyte.