Cray I/O (was: Re: What kinds of things)

[Melinda Shore]

[]
In article <16618 at rpp386.Dallas.TX.US> jfh at rpp386.cactus.org (John F. Haugh II) writes:
>I have also heard that the overhead of sending single character I/O
>requests over the hyperchannel was extremely costly.  The discussion
>I've read claims that the amount of effort handling single character
>at a time I/O [ like with Vi ] is too expensive in terms of CPU
>cycles.

Yes, sending small packets over HyperChannel is very expensive, but
many sites are now using other media (notably Cray's FEI-3).  It's also
worth noting that HyperChannel is poorly suited to TCP/IP, but that's
another issue ...

Cray performance, at least on the Cray 1, the X-MP, and the Y-MP, is
far more hampered by limited main memory.  Until fairly recently you
couldn't get more than 8 megawords on an X and memory gets used up
pretty quickly.  Even the 32 MW available on the Y-MP is smallish when
you consider that you've probably got 8 processors and you've got
several hundred users running huge jobs.  Remember also that these are
word-oriented machines, and no instruction is smaller than 1 word (8
bytes).  Swapping performance used to be pretty awful too;  I hope
that's been fixed.

Back to I/O:  given the choice between editing the file on the Cray or
downloading it, doing the editing on a workstation, and uploading the
file back to the mainframe it would appear that it's cheaper to edit on
the Cray.  After all, the other way you're transferring the entire file
twice, which while probably not as expensive as typing the whole thing
in on the Cray in the first place is almost certainly more expensive
than making a typically limited number of edits.  At the same time, you
probably want to avoid the use of an editor like GNU Emacs, which has a
huge executable and apparently makes some obscene number of system
calls per keystroke (Unicos isn't multithreaded yet).  The best
solutions are probably a distributed editor like rvi or NSF mounting
your Cray directories on workstations and doing the editing there.

The whole question of the desirability of running Unix on a Cray is 
generally brought up by batch OS people who don't know Unix and by
Unix people who don't know supercomputing or the specifics of Cray's
Unix implementation, which doesn't look much like Unix anymore.
-- 
Melinda Shore                                     shore at mtxinu.com
Mt Xinu                                  ..!uunet!mtxinu.com!shore