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

[kjm at ut-emx.UUCP]

Melinda Shore writes:

> 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.

No argument so far...

> Remember also that these are
> word-oriented machines, and no instruction is smaller than 1 word (8
> bytes).

On the X-MP, at least, instructions are composed of one or two 16-bit
"parcels", thus there are two to four instructions per word.

> Swapping performance used to be pretty awful too;  I hope
> that's been fixed.

It depends on exactly how the system is configured.  It is possible to
get fairly substantial performance improvements.  There are also lots
of ways to hang oneself here.

> The whole question of the desirability of running Unix on a Cray is 
> generally brought up by batch OS people

Or by people who prefer another (interactive) operating system (I know of
one person who would like to run VMS on a Cray).

Or by people (like me) who just plain think it can be done better.
(Although not by the likes of VMS...)

> 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.

Actually, I have seen lots of code sequences in UNICOS 4.0 that look a
*whole lot* like the code I read in V6...

> -- 
> Melinda Shore                                     shore at mtxinu.com
> Mt Xinu                                  ..!uunet!mtxinu.com!shore

--
The above viewpoints are mine.  They are unrelated to those of
anyone else, including my wife, our cats, and my employer.

Kenneth J. Montgomery

kjm at hermes.chpc.utexas.edu          University of Texas System
kjm at cerberus.chpc.utexas.edu        Center for High Performance Computing