[Numpy-discussion] Numpy and OpenMP

[Gnata Xavier]

Robert Kern wrote:
> On Sat, Mar 15, 2008 at 2:48 PM, Gnata Xavier <xavier.gnata@gmail.com> wrote:
>   
>> Hi,
>>
>>  Numpy is great : I can see several IDL/matlab projects switching to numpy :)
>>  However, it would be soooo nice to be able to put some OpenMP into the
>>  numpy code.
>>
>>  It would be nice to be able to be able to use several CPU using the
>>  numpy syntax ie A=sqrt(B).
>>
>>  Ok, we can use some inline C/C++ code but it is not so easy.
>>  Ok, we can split the data over several python executables (one per CPU)
>>  but A=sqrt(B) is so simple...
>>
>>  numpy + recent gcc with OpenMP  --> :) ?
>>  Any comments ?
>>     
>
> Eric Jones tried to use multithreading to split the computation of
> ufuncs across CPUs. Ultimately, the overhead of locking and unlocking
> made it prohibitive for medium-sized arrays and only somewhat
> disappointing improvements in performance for quite large arrays. I'm
> not familiar enough with OpenMP to determine if this result would be
> applicable to it. If you would like to try, we can certainly give you
> pointers as to where to start.
>
>   
Well of course if the arrays are too small it will be slower but it *is* 
much faster on large arrays.
In many cases, there is no need lock/unlock : Look at  A=sqrt(A) : it is 
obvious to speed-up such a compuation in pure C using OpenMP. From a 
simple minded point of view, I would say that somewhere in numpy, there 
should be such a C loop.

Why do we really (IMHO) need that ? Because "all" the machines (even 
laptops) are now multicore/cpu.

Using IDL, it is possible to develop a quite large image processing 
project working on large images on a 8core machine without *any* 
konwledge of semaphore/lock.
All this piece of software uses the simple syntaxe of IDL (ugly ones 
compare to numpy :))
I used to be very sceptical about the performances but I had a look and 
*it just works well*. It scales just nicely up to 6 cores.

Small arrays computation shall *not* be threaded, large ones should be 
if we look at the multicores trend.

Any comments ?

Xavier
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