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Subject: Re: New crap statement ?

Author: Bruce Moreland

Date: 15:36:57 09/25/01

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On September 25, 2001 at 10:29:56, Robert Hyatt wrote:

>I don't see this as a problem.  The issue still remains, "can you do something
>parallel that executes more than 2x faster than what you do serially?"  If the
>answer is "yes" then there is something basically wrong with your serial
>algorithm...

If I make a minor change to my program, the same search might take half as long,
or twice as long, or whatever.

If I run a given search serially, with a particular version, it takes X time.

If I run the same search with a two-processor parallel version, it might take
1/2X time, or 1/4X time, or X time, or even >X time.  I believe this happens for
the same reason that when I make a minor change to my serial version, I get
different search times.

If I could consistently get less than 1/2X time with two processors, that'd be
something to be happy about, and it would also raise some serious questions,
obviously.

But I don't think it is necessarily true that this would violate the laws of
physics.

If the two halves of the problem were absolutely independent, then of course,
you'd have a 2X speedup at best.

Since there is interaction between the two threads, it is not necessarily true
that you can have at most a 2X speedup.  It is possible to construct a
hypothetical case where one thread helps the other to over-achieve, and perhaps
there are problem domains where these hypothetical cases would be the rule,
rather than the exception.

Chess programs are deeply flawed from the start, so I'd believe that anything
might be *seen*.  Whether it's seen for a reason, or due to random chance, I
don't know.

But I don't think that people who argue for super-scalar speedups are
necessarily arguing in favor of perpetual motion machines.

bruce



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