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From: cik@l.cc.purdue.edu (Herman Rubin)
Newsgroups: comp.arch
Subject: Re: Cray & Amdahl Virtual memory debate
Summary: An example
Message-ID: <851@l.cc.purdue.edu>
Date: 26 Jul 88 12:15:54 GMT
References: <4232@cbmvax.UUCP> <76700035@p.cs.uiuc.edu> <34254@yale-celray.yale.UUCP>
Organization: Purdue University Statistics Department
Lines: 20


One of the examples given at the short course at Purdue on the CYBER 205
was the problem of multiplying two 1024x1024 matrices.  Because of the 
fact that there is a paged virtual memory, it was necessary to use a very
peculiar way of arranging the matrix elements to avoid thrashing.  On a
machine without virtual memory, assuming IO and computing can go on simul-
taneously, any competent programmer could arrange things so that, after
the initial loading of memory, very little time would be taken up by the
IO, and there would be no thrashing, and without using any non-obvious 
arrangement.

VM is a convenience for the programmer, but a problem for the machine if
much memory is involved, especially if paging is a problem.  It is also
a convenience if "standard" subroutines are to be kept in memory when
possible, but are removed if a space problem occurs.  A smart loader can
take care of this problem without VM, but do such exist?
-- 
Herman Rubin, Dept. of Statistics, Purdue Univ., West Lafayette IN47907
Phone: (317)494-6054
hrubin@l.cc.purdue.edu (Internet, bitnet, UUCP)