Why is sum(f,1) slower than sum(g,2) for g=f'?
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So I'm trying to optimize my code and I came across something that doesn't immediately make sense to me but I think is INCREDIBLY important for me to understand.
In the sample code below I found that summing the transpose of my matrix along the second dimmension is 3 times faster than summing the original matrix. Why is this?
f = rand(3,10000);
g = f';
tic
for k = 1:1000000
sum(f,1);
end
toc
%Elapsed time is 28.528823 seconds.
tic
for k = 1:1000000
sum(g,2);
end
toc
%Elapsed time is 9.325141 seconds.
2 Comments
Stephen23
on 15 Feb 2023
"Why is sum(f,1) slower than sum(f',2)?"
But that is not what you tested: your code excludes the transpose operation. For a fairer comparison include the transpose:
n = 1e5;
f = rand(3,10000);
tic
for k = 1:n
A = sum(f,1);
end
toc
%Elapsed time is 28.528823 seconds.
tic
for k = 1:n
B = sum(f.',2);
end
toc
Accepted Answer
Rik
on 15 Feb 2023
I expect this has to do with how a matrix is stored in memory. If I recall correctly, this is column major, so summing along row requires jumping around in the list, while summing along the columns is just following the raw data.
If you had put the conjugate in the test, I expect the result to flip.
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