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Re: Octave 3.6.0 on Windows XP plot fails.

From: Martin Helm
Subject: Re: Octave 3.6.0 on Windows XP plot fails.
Date: Wed, 29 Feb 2012 16:09:53 +0100
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Am Mittwoch, 29. Februar 2012, 16:01:57 schrieb Michael Goffioul:
> On Wed, Feb 29, 2012 at 2:28 PM, Martin Helm <address@hidden> wrote:
> > Am 29.02.2012 14:45, schrieb Xianyi Zhang:
> >> Hi,
> >> 
> >> The matrix multiplication cannot obtain the performance from
> >> hyperthreading.
> >> 
> >> Thanks
> >> 
> >> Xianyi
> > 
> > Why not? Is this a limitation of the mingw compiler, the windows
> > environment or the BLAS library in question?
> No, I think it's because of the principle of hyperthreading. HT does
> not mean you magically have 4 independent cores out of 2. You still
> have only 2 physical cores, but some parts of each core are duplicated
> such that they can appear as 4 instead of 2 at the OS level. However,
> the processing unit is not duplicated: so within a single physical
> core, each logical CPU will have to wait its turn on the processing
> unit.
> So roughly, if you have 4 operations that you can perform in parallel,
> only 2 will be done really simultaneously.
> HT can increase performances when you have 2 threads, one of which is
> stuck for some reason (waiting for data for instance). Then the other
> thread can use the processing unit while the first one is waiting. But
> if the 2 threads need the processing unit at the same time, you won't
> gain anything.
> So I think that what  Xianyi Zhang means is that matrix multiplication
> is this kind of operations that cannot really benefit from HT. For the
> record, I've tried a 2-threaded ATLAS on a P4-HT (1 physical CPU, 2
> logical CPUs) and the performances were lower than a single-threaded
> one.
> Michael.

Performing now my previous trivial example on my atom netbook (dual core atom 
with hyperthreading, htop shows 4 threads are used) ATLAS gives me a speedup 
by a factor 2.4 (so more than double!) when using multi vs single threaded 

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