[Scilab-users] repmat() slow compared to .* and .*. <= Re: More rapid calculation

[Samuel Gougeon]

Le 15/02/2018 à 11:45, Stéphane Mottelet a écrit :
> Le 15/02/2018 à 00:02, fujimoto2005 a écrit :
>> .../...
>
> Hello,
>
> In your code, most of the cpu time is spent between lines 40-54 
> (random generation of big matrices), then between lines 54-60, where 
> one of the bottlenecks is the use of repmat (and you use it twice) and 
> the "cumsum".In previous posts of Heinz Nabielek related to code 
> optimization, you may have noticed that using matrix multiplication by 
> a vector of ones gives the same result BUT uses the BLAS ! For 
> example, compare these timings, with size(timePoints_V)=[1 25000] and 
> sample=5000) :
>
> tic;repmat(timePoints_V,2*sample,1);disp(toc())
>
>    12.372273
>
> tic;ones(2*sample,1)*timePoints_V;disp(toc())
>
>    1.823105
>
> On my machine (MacPro, OSX, Scilab 6.0.0), this last piece of code 
> uses 100% cpu (four cores).

Thank you Stéphane for having pointed out the repmat() slowness.

Additional tests show that the Kronecker product is even slightly faster 
than .*

A new version of repmat() is proposed on review: 
https://codereview.scilab.org/19782
It is rewritten mainly using .*., which simplifies a lot the code.

This version is more than 7x faster than the current one, and uses both 
CPU of my PC.
It is roughly the ratio 12.37/1.82 ~ 6.8 that you give here-above.

Best regards
Samuel