Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!amdcad!rpw3
From: rpw3@amdcad.AMD.COM (Rob Warnock)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: interrupt-driven vs. polled I/O performance
Message-ID: <25394@amdcad.AMD.COM>
Date: 25 Apr 89 08:05:59 GMT
References: <25231@amdcad.AMD.COM> <421@logicon.arpa>
Reply-To: rpw3@amdcad.UUCP (Rob Warnock)
Organization: [Consultant] San Mateo, CA
Lines: 84

In article <421@logicon.arpa> Makey@LOGICON.ARPA (Jeff Makey) writes:
+---------------
| In article <25231@amdcad.AMD.COM> rpw3@amdcad.UUCP (Rob Warnock) writes:
| >But the echo delay doesn't need to be kept to a *minimum*, but only
| >well below the objectionable level to the human user.
| >I claim that 100ms for echo is not noticable to a user of a video display
| Your point about only needing to keep echo delay below the level of
| perception is well taken.  However, 100ms is 1/10 second, which I am
| sure I would find quite noticeable (my terminal runs at 19,200 baud).
+---------------

But you're only typing about 5 chars/sec. What I'm saying is that the
hand-eye echo delay can be a large fraction (even >1) of the inter-keystroke
time before it becomes noticeable. The fact that you are using 9600, 19200,
or 56k baud is irrelevant.  [O.k. you're a touch-typist, and are doing
10 c/s... but touch-typists depend even *less* on visual feedback.]

+---------------
| Taking a cue from motion pictures, I would assume that 1/30 second is
| the order of magnitude of the level of perception, but I would want to
| perform some experiments with different delays and lots of people to
| make sure.  :: Jeff Makey
+---------------

Well, the analogy doesn't really apply.

1. We're not talking about merging a succession of frames into a seamless
   whole here ["eye-eye coordination", if you will], but [in the worst case]
   about maintaining hand-eye coordination between cursor keys and cursor
   location.

2. There is actually very little feedback from visual input back to the
   keyboarding process; the major feedbacks are (a) auditory -- you *hear*
   the keys being struck, and (b) tactile -- you *feel* the keys hitting
   bottom (or clicking or whatever). Compared to these two, the visual
   feedback (for most people) is quite minor, and is almost ignored.

3. Most error detection while typing is unrelated to the visual perception;
   it has instead to do with typing the wrong thing, and then *realizing*
   it without having to see it. [You may not believe this at first, but
   watch someone else type for a while...]

Of course, it was quite true that when the echo from the computer *did*
have a large auditory component, as with the Teletype [*klunk* *klunk*],
then the 1/10 second delay [110 baud] between keyboard and typehead was
quite noticable [*ker*-*chunk* *ker-chunk*]. And when you used a Teletype in
full-duplex with host echo, that gave you 2/10 sec delay [*ker*-gap-*chunk*
*ker*-gap-*chunk*] that was initially annoying [the "rubber band" touch],
but users got used to it. [They had to.]

With a video display, the irritation begins when the delay is such that
there is a noticable mismatch in action and result. In my experience,
this starts about 0.2-0.3 sec, but gets *really* bad above about 1 second,
when it becomes difficult to match cursor key strikes to cursor location.
[Even worse is the sort of variation you get with program-"echo" on a loaded
system or with Telnet on a slow connection, where you think you're typing
ahead the right number, only to stop and see the cursor "coast" far beyond
where you wanted it.]

But you can try the experiments you want fairly easily by modifying
a copy of Telnet [doing it in the client's easiest] to delay sending
keystrokes in to the host by some amount of time.

I have no really hard data to hand, but based comment from users when I was
building async mux networks back at Dig. Comm. Assoc., I'd still say that a
hand-eye echo delay [with *no* auditory component of echo from the "display"!]
of 100ms is not noticable, and up to 200-300ms is quite acceptable.

[But note that that has to encompass *all* components of the echo path,
including possibly several layers of terminal servers and intermediate
Telnet hosts, if that's what a "typical" user does. Design goals of 20-50ms
each for terminal input and output dally/startup timers are probably o.k.]

This should probably go in comp.ergonomics, except that is *is* germane
to designing networking systems that balance efficiency and latency
considerations...


Rob Warnock
Systems Architecture Consultant

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