Re: mixed linear/logical indexing question

[Olaf Till]

On Wed, Dec 19, 2018 at 11:09:51AM -0800, Rik wrote:
> Recently, sliding windows of statistics functions (movXXX) were added to
> Octave.  All of these functions depend on a base function movfun.  As part
> of implementing movfun I used the profiler to examine the performance and I
> found a hot spot responsible for 70% of the run time.  Unfortunately, I
> couldn't see any way to improve things without jumping out of the Octave
> language in to C++.  I'm hoping someone else might look at this and have an
> idea.
> 
> The code is below
> 
> --- Code Start ---
> ## Apply "shrink" boundary conditions
> ## Function is not applied to any window elements outside the original data.
> function y = shrink_bc (fcn, x, idxp, win, wlen, odim)
>   N   = length (x);
>   idx = idxp + win;
>   tf  = (idx > 0) & (idx <= N);  # idx inside boundaries
> 
>   n   = length (idxp);
>   y   = zeros (n, odim);
>   ## FIXME: This nested for loop accounts for 70% of running time.
>   ##        Given that "shrink" is the default Endpoint value this
>   ##        code needs to be reworked.
>   for i = 1:n
>     k      = idx(tf(:,i),i);
>     y(i,:) = fcn (x(k));
>   endfor
> endfunction
> --- Code End ---
> 
> In my case, the following conditions are a suitable example
> 
> +verbatim+
> fcn = @mean;
> x = randi (1e4, [1000, 1]);
> idxp = 1:25;
> win = (-25:25).';
> wlen = [51, 51];
> odim = 1;
> -verbatim-
> 
> Indexing in Octave is insanely fast, but this is a weird case because it is
> not simply linear indexing [x(5)], nor is it logical indexing [x(x < 1)]. 
> It is a hybrid where the indices are linear (as would be returned from
> find()) but only some of them are valid so they need to be masked with a
> logical value.
> 
> When I run this code repeatedly in a benchmark it requires ~3.0
> milliseconds, but in the overall code it is called 2000 times so the
> function is taking ~6 seconds which feels long.
> 
> In this example, the value of k for i = 1 is 1:26, for i = 2, 1:27, etc., etc.
> 
> I tried recoding using cellfun
> 
> idx = cellfun (@(n) (1:n), num2cell ((wlen(1) - idxp(end)):(wlen(end)-1)),
> 'uniformoutput', false);
> y = cellfun (@(i) fcn (x(i)), idx);
> 
> but this takes ~4.5 milliseconds to run.

At my system:

--- Code Start ---
k = 1:30;
x_k = x(k).';
inp = num2cell (1:25); # 1 : length (idxp)
tic;
cellfun (@ (id) mean (x_k), inp, "UniformOutput", false);
toc
--- Code End ---

takes roughly the same time as calling the original 'shrink_bc'
function with:

--- Code Start ---
tic;
shrink_bc (fcn, x, idxp, win, wlen, odim);
toc
--- Code End ---

(both times 32--33 ms, 5 times slower than at your system).

So neither indexing nor for-loops seem to be the cause, just the
repeated calling of the function 'mean'...

Olaf

-- 
public key id EAFE0591, e.g. on x-hkp://pool.sks-keyservers.net

signature.asc
Description: PGP signature