Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!bu-cs!encore!xylogics!gws
From: gws@Xylogics.COM (Geoff Steckel)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: interrupt-driven vs. polled I/O performance
Summary: very short echo delays are noticeable
Keywords: echo polling
Message-ID: <1410@xenna.Xylogics.COM>
Date: 25 Apr 89 21:51:44 GMT
References: <25231@amdcad.AMD.COM> <421@logicon.arpa>
Reply-To: gws@Xylogics.COM (Geoff Steckel)
Organization: Omnivore Technology, Newton, MA
Lines: 32

There is some knowledge about human sensitivity to delays in
tactile/aural/visual feedback.  Some of the old experiments did things
like have people try to write while watching their hand in a delayed
video monitor.  Some of this was published in general science magazines
like Scientific American as much as 30 years ago.
There are three regimes of interest:

0 to about 50 milliseconds:
   Subject doesn't notice much.  At the upper end, performance slows slightly,
but error rate doesn't increase much.

50 to about 500 milliseconds:
   The **REALLY** annoying range.  Performance slows by x10 or more, even
to a complete stop.  Error rate climbs to virtually 100% at the high end.
Subjects become fatigued and irritable.

500 milliseconds and up:
   If people are forced to use feedback with this much delay, oscillations,
gross errors (like hitting yourself with a pen, etc.), and great annoyance
are the result.  Performing with any accuracy is very slow and very fatiguing.
Most subjects ignore everything except tactile and internal muscle/joint
feedback and `fly blind' -- and feel much better.  They then stop occasionally
and wait for the returned information to catch up with reality.

I worked on a cross-country proprietary network about 15 years ago.
We worked VERY hard to get user feedback time < 100 ms; we wanted 50 or less.
Users started complaining at about 80 ms or so.

Moral: ergonomics are important even (or especially) when designing comm
systems, if humans are in the loop.

	geoff steckel (steckel@alliant.COM, steckel@xenna.COM)