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From: sharma@uicsg.UUCP
Newsgroups: net.arch
Subject: Cray-XMP v/s VP-200 - (nf)
Message-ID: <3200004@uicsg.UUCP>
Date: Fri, 11-May-84 16:12:00 EDT
Article-I.D.: uicsg.3200004
Posted: Fri May 11 16:12:00 1984
Date-Received: Fri, 18-May-84 00:30:15 EDT
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Nf-ID: #N:uicsg:3200004:000:1750
Nf-From: uicsg!sharma    May 11 15:12:00 1984

#N:uicsg:3200004:000:1750
uicsg!sharma    May 11 15:12:00 1984


It seems that the most important factor due to which the VP-200
outperforms the Cray-XMP is the lack of a good Vectorizing Compiler.
Hardware performance is also an issue, but this factor is expected 
to be a constant and has been observed to be between 1.0 and
about 1.5.

Preliminary results on an experiment which compared the two results
have been published in  :
	IEEE trans. on Computers, April '84 (pp374-375)


The present Cray-XMP Compiler has two problems, the first of 
which is of significance here.
	1. The Cray Vectorizing Compiler cannot automatically
	   handle loops with conditional statements in the body
	   of the loop. The VP-200 does a good job on such 
	   loops.
	2. The present Cray software cannot make use the two CPUs 
	   in the machine concurrently. Adequate software is expected
	   to be available soon.

There are two important hardware features that also contribute to
the edge that the Fujitsu machine has.
	1. The VP-200 has a Vector-register to Vector-pipe data
	   path width of 128 bits - which is twice that of the 
	   Cray-XMP. I wonder why ? - they couldn't have left
	   the data-path and functional-pipeline bandwidths 
	   on the Cray unmatched.
	2. The VP-200 has a main memory of 256 Mbytes. The Cray
	   has only 32Mbytes and uses a SSD ( Solid State Disk (or device))
	   as a staging memory.

The clock rates are not too different : 7.5nsec for VP-200 and 9.5nsec for
the Cray - a difference of 20%.

The point to be noted is that the Cray performance falls by a factor of
at least 2 when the loop has a conditional statement - only because the
Vectorizer fails. 


					- Madhumitra Sharma
					Computer Systems Group,
					Univ. of Illinois, Urbana.


UUCP address : ihnp4!uiucdcs!uicsg!sharma