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From: carroll@ssc-vax (Jeff Carroll)
Newsgroups: comp.arch
Subject: Re: Anything wrong with the i860
Message-ID: <3986@ssc-bee.ssc-vax.UUCP>
Date: 15 May 91 17:12:56 GMT
References: <1991May7.145407.18417@midway.uchicago.edu> <3486@charon.cwi.nl>
Sender: news@ssc-vax.UUCP
Reply-To: carroll@ssc-vax.UUCP (Jeff Carroll)
Organization: Boeing Aerospace & Electronics
Lines: 77

In article <3486@charon.cwi.nl> dik@cwi.nl (Dik T. Winter) writes:
>In article <1991May7.145407.18417@midway.uchicago.edu> rtp1@quads.uchicago.edu (raymond thomas pierrehumbert) writes:
> > I have seen relatively little about the i860 chip on this newsgroup.
> > Also, compared to MIPS, it doesn't seem to be very popular as the
> > base processor for computers;  Alliant uses it in their shared-memory
> > machines (800 & 2800), Intel has the Touchstone experimental mpp machine,
> > and the i860 seems popular as a graphics coprocessor (e.g. in the NeXt),
> > but generally, I see surprisingly little interest in the chip.
>FPS and Stardent (will) use it as computational coprocessor (I think).

	FPS pitched this box to me more than a year ago, and if it's not
out yet it's probably never going to be. It's essentially the old
Celerity box with SPARCs where the Celerity chips used to be, and two
or three optional add-in boards, one of which is supposed to be up to
84 i860s sharing a single bank of memory. They claim that bandwidth to
memory is sufficient; I don't remember the actual numbers and will thus
leave this to the judgment of the reader, but I thought that claim a 
little hard to believe at the time...

> > 
> > Is there something wrong with the architecture?  As a platform,
> > what are its advantages and disadvantages over the competition?
> > I am particularly interested in this, as I am thinking of buying
> > an Alliant F/800; I did a lot of benchmarks, and the performance
> > seems extremely good compared to other RISC architectures (even
> > on a per-processor basis running on throughput rather than
> > parallelization), so I'm wondering if there isn't some "catch"
> > I haven't encountered yet.

	The i860, to the best of my knowledge, has the best floating-point
performance of any microprocessor in the world today (possible bugs
notwithstanding; I haven't seen 'em, but I don't build i860 systems.
I just use 'em.) The integer performance is closer to other recent RISC chips.

>The problem I see with the i860 is that it is very good with
>especially tuned code, but that it is extremely bothersome for
>compilers to get that performance.  (This holds for f-p only, if you
>are thinking non-f-p it can compete with the others.) 

But if you are thinking non-floating-point there might well be strong
reasons for going with another chip. As long as you don't have to write
the compilers, or live with them while the vendor is coming up the 
learning curve, you don't care how "bothersome" it is. In fact there
are some i860 compilers that are better than others, and I've been told
by people I trust that the Portland Group compilers are quite good
(maybe some day soon I'll be able to verify this myself).

> My opinion is
>that you would want the i860 only if your f-p work-load consist mainly
>of the use of standard libraries.  Do not expect such a good
>performance if you code everything yourself.  So for chemical/physical
>research it is quite good (if you use the standard libraries),

I use a canned library on our i860 boards, but I do it mostly to save time
in coding, not because it is especially fast (which it isn't). I'll
withhold the name in order to protect the guilty, and also the innocent
(namely myself).

> but
>when you are doing research in numerical mathematics it is less well
>suited.  On the other hand Alliants model for parallellism is very
>good to do basic research in parallel algorithms (in that case
>performance is not the main problem; we are still on an FX/4; talking
>about performance :-)).

Actually the i860 made possible some work on parallel numerical algorithms
that would have been orders of magnitude more expensive (in both time and
dollars) without it. In particular an Intel/Boeing team found that it
was possible to achieve very nearly peak performance from the i860 using
very little assembly code, on certain problems.


-- 
Jeff Carroll
carroll@ssc-vax.boeing.com

"Do you think I care? ... I have an infinite amount of money."	-Bill Gates