Path: utzoo!attcan!uunet!crdgw1!sunroof!hammondr
From: hammondr@sunroof.crd.ge.com (richard a hammond)
Newsgroups: comp.arch
Subject: Re: DMA on RISC-based systems
Message-ID: <696@crdgw1.crd.ge.com>
Date: 8 Jun 89 20:31:06 GMT
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Reply-To: hammondr@sunroof.crd.ge.com (richard a hammond)
Organization: General Electric Corp. R&D, Schenectady, NY
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One might want to look at "An Analysis of TCP Processing Overhead"
by Clark, Jacobson, Romkey, and Salwen in the June 1989 issue
of IEEE Communications Magazine (Vol. 27, No. 6).  They point out
that the CPU has to do a checksum of the data during processing and
propose (as one alternative) removing the DMA controller from the
network controller and letting the CPU do the byte copy and move
together in a single loop.

Also, the disk throughput numbers of interest are not peak numbers
but avverage use over applications.  In previous jobs I've helped
collect the processing and disk I/O requirements of months of jobs on
Convex computers (i.e. big enough to do lots of number crunching
and I/O) and there were very few jobs which were both CPU intensive
and I/O intensive at the same time.

So - DMA on RISC based workstations might have a different cost
function than simply the cost of the CPU lost, one has to consider
whether the CPU cycles lost to doing the data movement would be
of use by another process.  I estimate that the answer is yes
only some fraction of the time, and so the numbers given are
upper bounds on the costs.

Note that I'm talking about workstations and not large mainframes
or number crunchers, where the load or applications may be different.

Definte numbers for applications which do both
CPU crunching and require some sort of I/O AT THE SAME TIME
would be interesting, since the Convex wasn't a personal workstation.

Rich Hammond