Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!ll-xn!mit-eddie!killer!elg
From: elg@killer.UUCP (Eric Green)
Newsgroups: comp.arch
Subject: Re: Cray 2 has 2GW address
Message-ID: <3534@killer.UUCP>
Date: 29 Feb 88 04:49:41 GMT
References: <416@micropen>
Organization: Bayou Telecommunications
Lines: 49

in article <416@micropen>, dave@micropen (David F. Carlson) says:
[NASA dude discusses running GNU Emacs on his Cray:]

>> By keeping around 20 active jobs, we can still give good interactive
>> response, and "batch" throughput an order of magnitude better than
>> most mainframes.  You haven't lived until you've run on a machine
>> where GNU emacs is considered a small process. ;-)
> 
> I know most of these CRAYs are used in DoD research on important things like
> bombs and SDI, but my running an EDITOR (ie. slow interactive process)
> on a CRAY--presumably payed for with my tax dollars.  Ouch!  Can't you
> find any good emacs for a VT100 on a VAX11/780 to run twenty editor jobs?

The reason GNU Emacs is slow on a Vax11/780 is simple: thrashing. A
well-engendered Vaxen has maybe 8 megabytes of main memory. The GNU Emacs core
image can get up to 2 megabytes large with no problem. Run 20 of those on a
Vax 780... well, you can see that you better have a hefty swap space, because
that baby's gonna be swappin' her heart out :-}.

But of course you don't have that thrashing on a Cray. Not with 2 Gigawords of
RAM! Not to mention that the lack of paging eliminates the task-switching task
of flushing (and eventually reloading) the MMU TLB's (and replaces it with the
chore of flushing the vector registers, alas -- although I wonder if processes
that do not use the vector registers do, in fact, flush them). I assume that
the "Ouch" that you're talking about is the character-at-a-time task switch
overhead. It appears that task switch overhead on a Cray would be no worse
than on a Vax 780 -- on a machine that's, well, slightly faster :-). Somehow,
I think the task switch time for 20 interactive Emacsen would be in the noise,
insofar as percentage of CPU time is concerned. "Slow interactive process"?
Surely you jest. You've been using an IBM 370 too long... 2 second interrupt
latencies went out with the 60's! (though IBM doesn't seem to have 
noticed :-). 

Somehow, I don't think that 20 Emacs processes are about to bring a Cray ][ to
it's knees... I really truly doubt that the CPU time saved by doing editing
elsewhere, could be justified by paying all those programmers and scientists
for the time involved in moving that text over to the Cray (and it can be a
hassle... at the very least, you have to invoke FTP over a network, and at the
worst, we're talking major troubles).

Hmm. An architectural issue here, maybe. Does the lack of MMU refills etc.
REALLY speed up process switching time? Are Unix processes "lightweight" as
far as a Cray 2 is concerned? 

--
Eric Lee Green  elg@usl.CSNET     Snail Mail P.O. Box 92191      
{cbosgd,ihnp4}!killer!elg         Lafayette, LA 70509            

Come on, girl, I ain't looking for no fight/You ain't no beauty, but hey
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