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Re: rogue comment: IEEE 754R and Reproducibility
"David James" <dvj@xxxxxxxxxxxx> wrote:
You are therefore demanding a global, absolute, precise clock model
- and nobody in the parallel field believes that such a thing is
even theoretically possible,
The possibility of synchronizing the clocks to within less than
one instruction (on a many-core chip) or one read/write transaction
(on a many-chip configuration) is certainly possible and practical.
Firstly, while what you say is just about possible (at CONSIDERABLE
cost) today, nobody in the area is convinced that will be true even
tomorrow, let alone the day after. The difference between 4 cores
and 64 cores is more than you realise.
Secondly, and more importantly, that is not enough and does not meet
the requirement I described. If you have N cores and a basic time
of T, you need clocks precise to T/N to ensure consistent ordering
of the CPUs. That is a gross simplification, of course.
Now consider 64 cores, each running at 4 GHz - just HOW are you
going to keep those clocks consistent?
Its probably not the right solution, though.
Well, THERE you are right.
in combination with even tolerable efficiency OR scalability.
Its not clear to me that one must serialize all operations.
I'm not an expert, but it would seem to be sufficient
to ensure that the dependency graphs are the same,
and these do allow for parallelism.
If you can invent an algorithm that is guaranteed to preserve the
dependency graph of an arbitrary serial program when allowing it to
be parallelised to even good efficiency, I will bow down and worship
you as at least a minor deity. For heaven's sake - does nobody EVER
learn anything from history?
Well, as we all know, the classic answer is "no". People like me
don't count. You are making EXACTLY the same mistake that Intel and
HP did with the Itanic in the mid-1990s - I told them that they
couldn't deliver the compiler technology, and I was right. I am not
a second Alan Turing, but merely someone who had observed 25 years
of dismal failure by the best computer scientists in the world!
Admittedly not as fast as possible, if associativity and commutativity
were to be assumed, but not as slow as serialized operations.
Yeah. Typically you can get something like O(log(N)) speedup by using
N cores. Big deal.
Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email: nmm1@xxxxxxxxx
Tel.: +44 1223 334761 Fax: +44 1223 334679