Subject: COMMS1/2 discussions

From: <[email-masked]> (Edgar Kalns)
Date: Fri, 17 Jan 1997 18:01:09 -0500 (EST)

Regarding the COMMS1/2 issues and the linear fitting of message-passing
data, I concur with Charles Grassl and Ron Sercely that the most
important aspect of the COMMs kernels is to report exact measurements
for a wide range of message sizes (including 0-byte messages; more on this
later). Of the data reported in COMMS1/2, this is the data of most utility
to multi-processor/multi-computer programmers.  The fitted vs. measured
startup values on the IBM RS/6000 SP {Tstart vs. t(0 Byte) using Roger
Hockney's nomenclature} differ by a factor of 2.8, similar to the Cray
T3E's variance of a factor of 3.4.  I believe these variances are far
too large to be of any value. Roger's suggestion to move to a square
of the relative error improves the situation somewhat by not having such
large absolute errors; however, stated errors in the range of 50 to 100%
("small" to "medium"-sized messages) are still unacceptably large.
That is not to say that the fitting method ought to be jettisoned entirely;
I'm intrigued by Roger's investigations of 3-parameter fits and I
encourage the committee to pursue this avenue in earnest as it appears
promising.

0-byte messages: Charles asserts that "... zero length messages are not
of interest because this length message might be trapped by the programmer
or the message passing library or the OS."  True, it might be, but if so,
then there should be a non-trivial difference between the latency measured
at 0 bytes and 1 byte. For the SP, a 0-byte latency measurement is the
best-known method to obtain the cost of message-passing protocol
processing.  I believe that it can be safely retained for COMMS1/2.

To summarize, tightening up on the error of the fitted values is of
paramount importance for COMMS1/2 so that potential consumers of this
data can use it effectively. Perhaps 3-parameter fits will solve the problems
associated with the current method.

Regards,
Edgar
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From: Ron Sercely <[email-masked]>
Date: Mon, 20 Jan 1997 08:11:27 -0600

I would like say thati I agree with Edgar point that 0 length
messages should be reported.  I think some users will use these for
synchronization of processes, and will be interested in the reported
values.
-- 
Ron Sercely

HP/CXTC Toolsmith

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From: Charles Grassl <[email-masked]>
Date: Tue, 21 Jan 1997 15:50:28 -0600

Ron and Edgar propose retaining tests of zero length messages.

We must be careful that we make our test programs consistent, useful and
extendable for more than one message passing implementation.  In
particular, we
should leave the door open for testing one-side message passing.   The
use of zero length messages for this kind message passing might not have
an unambiguos interpretation with respect to synchronization.

Here is a point for discussion:
  Do the various message passing definitions treat the case of zero
length messages?  That some users might use zero length messages for
synchronization is not reason to test it.  This practice might not be
portable and "safe".

Charles Grassl
Cray Research