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SMP multithreading
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Date: 2010-11-17 (11:12)
From: Goswin von Brederlow <goswin-v-b@w...>
Subject: Re: [Caml-list] Re: SMP multithreading
Sylvain Le Gall <> writes:

> Hi,
> On 15-11-2010, Wolfgang Draxinger <> wrote:
>> Hi,
>> I've just read
>> in particular this paragraph:
>>| What about hyperthreading?  Well, I believe it's the last convulsive
>>| movement of SMP's corpse :-)  We'll see how it goes market-wise.  At
>>| any rate, the speedups announced for hyperthreading in the Pentium 4
>>| are below a factor of 1.5; probably not enough to offset the overhead
>>| of making the OCaml runtime system thread-safe.
>> This reads just like the "640k ought be enough for everyone". Multicore
>> systems are the standard today. Even the cheapest consumer machines
>> come with at least two cores. Once can easily get 6 core machines today.
>> Still thinking SMP was a niche and was dying?
> Hyperthreading was never remarkable about performance or whatever and is
> probably not pure SMP (emulated SMP maybe?).

Hyperthreading is a hack to better utilize idle cpu sub units. The CPU
has multiple complete sets of registers, one per hyper thread. Execution
of the threads is interleaved. Now when one thread is doing some
floating point operation the cpu switches over to another thread and
lets it do some integer aritmetic. But that assumes the threads are
using different sub units. If they are using the same unit then they
just block each other and no speedup occurs.

The speedup of hyperthreading is purely from avoiding dead cycles when
one thread waits for something. On te other hand the cache is shared
between threads so per thread it is smaller and more easily
trashed. Hyperthreading can be much slower too.