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Date: Tue, 25 Jan 2011 23:07:01 +0100
From: Georg Bauhaus <rm-host.bauhaus@maps.futureapps.de>
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Subject: Re: Loops and parallel execution
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Date: 2011-01-25T23:07:01+01:00
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On 1/25/11 10:32 PM, Dmitry A. Kazakov wrote:
> On Tue, 25 Jan 2011 18:36:29 +0100, Georg Bauhaus wrote:
>
>> If a compiler detects two independent paths inside a loop,
>
> I don't see any application for this. Can you remember the last time you
> wrote such loop? I cannot.

In fact, I have seen such a loop recently; it computes a Mandelbrot
set twice as fast.  (I am confident that the lessons learned in
finding this loop have found applications in other loops that
manipulate larger amounts of numeric data.)  The author has found a way
to split Ada's Complex type into its constituent parts (two FPT
objects) such that the program is a lot more efficient.  (One would wish
that types like Complex would be treated specially by the compiler.)


> The Occam's par-statement could be a better candidate, but I don't see how
> this could be useful under a modern general-purpose OS with their
> "vertical" parallelism, when each task is assigned to one core.  The thing
> you propose is "horizontal" parallelism, when a task/process would run on
> all cores simultaneously. Inmos' Occam ran under no true OS, and the
> processor architecture was well suited for such ad-hoc parallelism. Modern
> processors are very different from T805 and I doubt that they would allow
> an efficient implementation of this.


I have recently seen small boards carrying one processor each
that could be connected to one another on all sides, IIRC.
What matters thens is, I think, the efficiency of
(a) distribution of small computation, and
(b) the delivery of results at some nodes.
Is it therefore so unthinkable to have something like a transputer
these days?

BTW, FUD places the whole idea (from the early days, I guess)
subject to patent lawyerly action, nowadays, under names such
as map-reduce...