Re: Octave 3.6.0 on Windows XP plot fails.

[José Luis García Pallero]

2012/2/29 Michael Goffioul <address@hidden>:
> 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.

Yes, HT should be use with care. In some cases HT can cause a penalty.
We need lots of tests with our programs to ensure that HT really
speeds the computations

>
> Michael.
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José Luis García Pallero
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