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From: alan@mn-at1.UUCP (Alan Klietz)
Newsgroups: comp.arch,comp.misc
Subject: Re: Cray 2 has 2GW address (really comm. overhead for supercomputers)
Message-ID: <411@mn-at1.UUCP>
Date: 5 Mar 88 06:13:30 GMT
References: <416@micropen> <3534@killer.UUCP> <305@amelia.nas.nasa.gov> <308@amelia.nas.nasa.gov>
Reply-To: alan@mn-at1.UUCP (0000-Alan Klietz)
Organization: Roseville, MN
Lines: 43

In article <308@amelia.nas.nasa.gov> fouts@orville.nas.nasa.gov (Marty Fouts) writes:
<In article <305@amelia.nas.nasa.gov> msf@amelia.nas.nasa.gov (Michael S. Fischbein) writes:
<>If I remember correctly (I've got the reference here somewhere, but I'm
<>sure I'll get flamed if I'm far off -- or even just a little off), tests
<>on the first Ames Cray-2 showed twenty SIMULATED hot-and-heavy interactive
<>edits used about 8% of one cpu.  These were all running as canned scripts.
<>Unfortunately, they also simulated using more than half of the available
<>"outside world" i/o bandwidth.
<
<Actually we never ran those tests.  I believe that numbers from a test
<like this were derived at the University of Minnesota, although I
<don't know their results. 

I ran those tests in 1985.  The important results were,

1) Cray CPU time is not a significant overhead factor when performing
   simple single-keystroke operations by a reasonable number (10-20) of
   users.

2) The NSC Hyperchannel is not designed for transferring small packets
   of data.  An A130 will saturate at 300 keystrokes/sec, due to the
   large overhead of setting up and tearing down a virtual circuit for
   each message.  The Hyperchannel also does not perform well with
   asynchronous full duplex transmissions (e.g. TCP/IP).  This is due to
   reservation deadlocks between pairs of adapters that attempt to send
   to each other simultaneously over what is really a half duplex trunk.

Hence the development of "rvi" - remote vi that runs "ed" on the Cray-2.

The general solution is to multiplex large numbers of smaller packets
into larger Hyperchannel messages and send them in synchronous alternating
trains.  The problem is a general one, and applies to HSX, HSC, ULTRA,
CNT, VMEbus, as well as Hyperchannel.  See the paper, "An Investigation
and Analysis of High Performance Data-links for Supercomputers", MSCTR112
(MSC Tech Report 112) for a more detailed discussion.

--
Alan Klietz
Minnesota Supercomputer Center (*)
1200 Washington Avenue South
Minneapolis, MN  55415    UUCP:  alan@mn-at1.k.mn.org
Ph: +1 612 626 1836       ARPA:  alan@uc.msc.umn.edu  (was umn-rei-uc.arpa)

(*) An affiliate of the University of Minnesota