LAM: Theorical question about parallel computing

Altough my question isnt directly about LAM/MPI it kind of
answers me a lot 
of things that I need to know if I need to use MPI or not.
Here it goes: Is 
it possible to make a parallel algorithm (using MPI) running
in a single 
machine/node (with many process running on it, i.e., mpirun
-np 10 main and 
only one hardware processor) to run faster than a serial one
on the same 
conditions? Thanks a lot for the atention. Any help would be
appreciated.

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Never say "never", but I would consider this
highly unlikely.


On Jul 11, 2007, at 10:57 PM, Pedro Petrovitch wrote:

> Altough my question isnt directly about LAM/MPI it kind
of answers  
> me a lot
> of things that I need to know if I need to use MPI or
not. Here it  
> goes: Is
> it possible to make a parallel algorithm (using MPI)
running in a  
> single
> machine/node (with many process running on it, i.e.,
mpirun -np 10  
> main and
> only one hardware processor) to run faster than a
serial one on the  
> same
> conditions? Thanks a lot for the atention. Any help
would be  
> appreciated.
>
>
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-- 
Jeff Squyres
Cisco Systems


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pedropetrovitchhotmail.com wrote:
> Altough my question isnt directly about LAM/MPI it kind
of answers me a lot 
> of things that I need to know if I need to use MPI or
not. Here it goes: Is 
> it possible to make a parallel algorithm (using MPI)
running in a single 
> machine/node (with many process running on it, i.e.,
mpirun -np 10 main and 
> only one hardware processor) to run faster than a
serial one on the same 
> conditions? Thanks a lot for the atention. Any help
would be appreciated.

Theoretically, possibly with some very strange application
designed so 
that a single process doesn't use all the resources, or
resources such 
as cache/memory can't be made available to a single process.

Practically, no, the single node MPI speedup of a normal
application is 
limited to the number of separate processors (e.g. cores) on
the node, 
and would normally be optimized by running one process per
core.
If there is an advantage for MPI over threading (e.g.
OpenMP) it is 
usually related to better cache and memory locality of the
MPI processes.
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On Jul 11, 2007, at 9:57 PM, Pedro Petrovitch wrote:

> Altough my question isnt directly about LAM/MPI it kind
of answers  
> me a lot
> of things that I need to know if I need to use MPI or
not. Here it  
> goes: Is
> it possible to make a parallel algorithm (using MPI)
running in a  
> single
> machine/node (with many process running on it, i.e.,
mpirun -np 10  
> main and
> only one hardware processor) to run faster than a
serial one on the  
> same
> conditions? Thanks a lot for the atention. Any help
would be  
> appreciated.

Not generally. If it did, chances are likely that you'd
actually just  
be abusing the scheduler into giving your user more CPU
cycles than  
you'd otherwise (i.e. with fewer processes) receive.

Normally though, even ignoring MPI-library overhead, you're
simply  
doing the same thing you'd do in serial except with the
addition of  
lots of unnecessary process swapping and extra memory
copying  
(between MPI processes) that you wouldn't need to do if
everything  
were in serial mode.

~Kyle

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