[Chicago] MPI in Python?

Massimo Di Pierro mdipierro at cs.depaul.edu
Wed Feb 27 21:26:24 CET 2008

Sorry, sometimes I type fast and I do not understand myself.

MPI is a Message Passing Protocol (you send a string form one CPU and  
you receive the string on another CPU). depending on the MPI  
implementation (MPICH, LamMPI, MPI-Myranet, etc.) it may or many not  
take advantage of your architecture/network. If some CPUs share RAM  
memory MPI should take advantage of it but it doesn't.

1) case (1xCPU+RAM) x n -> You use MPI

2) case (m x CPU + RAM)  x n -> You still use MPI but lose efficiency  
if m>n

3) case (m x CPU + RAM) x 1 -> you use threads, there is no need for  
message passing

(n is the number of nodes, m is the number of CPUs per node)

In case 1) and 2) you can treat the n nodes as a shared memory  
machine by using a program called OpenMP. I do not suggest it.


On Feb 27, 2008, at 2:15 PM, skip at pobox.com wrote:

>     Massimo> Is the CPU don't share the same RAM threading is not  
> an option
>     Massimo> unless you use OpenMP, which emulates shared memory  
> but it is
>     Massimo> very slow compared with MPI. Nobody uses it anymore.
>     Skip> So MPI means "Message Passing Interface" where the  
> messages must
>     Skip> be passed via shared memory?  *sigh* If so, I'll look  
> elsewhere
>     Skip> for solutions.
>     Mike> MPI means "Message Passing Interface" where you shouldn't  
> have to
>     Mike> worry about the protocol used.  MPI will transparently  
> use shared
>     Mike> memory, pthreads, some other ipc mechanism, or sockets  
> based on
>     Mike> the configuration when you built the MPI library and/or  
> what is
>     Mike> the best supported mechanism on your operating system.
> So I obviously misunderstood what Massimo wrote.  What did he mean?  I
> interpreted his statement as meaning you can't use MPI without a  
> shared
> memory architecture unless you are using OpenMP which nobody uses  
> anymore.
> Skip
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