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From: koontz@CAPSRV.JHUAPL.EDU ("Kenneth W. Koontz, JHU/APL ", koontz@capsrv.jhuapl.edu)
Newsgroups: comp.sys.transputer
Subject: Re:C004 crossbar communication delays
Message-ID: <9101221515.AA11853@theory.TN.CORNELL.EDU>
Date: 22 Jan 91 14:09:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 34

Norman Goldstein writes:

>On page 55 of "Transputer Technical Notes", INMOS Limited, 1989, it is
>stated that using a crossbar connection "... introduces a 1.6 to 2 bit
>time delay on the signal."  Is this per byte? Are there results of
>testing to compare hardwired connections with C004-implemented connections?

     Way back in '89 (or so), I read the same stuff and wondered the same
thing.  At that point in time, Inmos stated that there was a bit delay but that
transputers that supported overlapped acks (re T800s) would not experience any
significant delay.

     Horse hockey!  If you try a simple unidirectional throughput test between 
two T800 with a C004 and without a C004 (hardwired connections), you find the 
following difference in throughput rate (links at 20Mbps, using fairly large 
buffers >= 1Kbyte):

     without C004 : 1.75 Mbytes/sec
     with    C004 : 1.33 Mbytes/sec

Inmos does acknowledge this fact now, but back then it was a surprise to some
even at Inmos. Things only get worse if you go through 2 C004s.  I haven't 
verified it myself but Inmos claims it goes down to around 0.8 Mbytes/sec.

     Conclusion: if you have a really high-throughput problem that requires
the maximum amount of throughput, don't use the C004s.

P.S.  I hope things like this don't occur again with the H1 and C104.

Ken Koontz
The Johns Hopkins University
  Applied Physics Laboratory
Laurel, MD
email: koontz@capsrv.jhuapl.edu