[Numpy-discussion] Unnecessarily bad performance of elementwise operators with Fortran-arrays

Travis E. Oliphant oliphant@enthought....
Thu Nov 8 22:44:11 CST 2007


David Cournapeau wrote:
> Travis E. Oliphant wrote:
>   
>> Christopher Barker wrote:
>>     
>>> This discussion makes me wonder if the basic element-wise operations 
>>> could (should?) be special cased for contiguous arrays, reducing them to 
>>>   simple pointer incrementing from the start to the finish of the data 
>>> block. The same code would work for C and Fortran order arrays, and be 
>>> pretty simple.
>>>
>>> This would address Hans' issue, no?
>>>
>>> It's a special case but a common one.
>>>
>>>   
>>>       
>> There is a special case for this already.  It's just that the specific 
>> operations he is addressing requires creation of output arrays that by 
>> default are in C-order.    This would need to change in order to take 
>> advantage of the special case.
>>     
> For copy and array creation, I understand this, but for element-wise 
> operations (mean, min, and max), this is not enough to explain the 
> difference, no ? For example, I can understand a 50 % or 100 % time 
> increase for simple operations (by simple, I mean one element operation 
> taking only a few CPU cycles), because of copies, but a 5 fold time 
> increase seems too big, no (mayb a cache problem, though) ? Also, the 
> fact that mean is slower than min/max for both cases (F vs C) seems a 
> bit counterintuitive (maybe cache effects are involved somehow ?).
>
> Again, I see huge differences between my Xeon PIV @ 3.2 Ghz and my 
> pentium M @ 1.2 Ghz for those operations: pentium M gives more 
> "intuitive results (and is almost as fast, and sometimes even faster 
> than my Xeon for arrays which can stay in cache).
>
>   

I wasn't talking about the min, mean, and max methods specifically.  
These are all implemented with the reduce method of a ufunc. 

But, there are special cases for the reduce method as well and so 
relatively smooth pathways for optimization. 

-Travis O.




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