Transpose in BLAS or do it myself first?

Question

Hi there.

I'm putting together some scientific code in Fortran 77, and I am having a debate on what would be faster.

Basically, I have an MxN matrix, let's call it A. M is larger than N. Later on in the code, I need to multiply transpose(A) by a bunch of vectors.

My question is, would it be faster to take A, transpose it on my own and store that, or when I call BLAS, just give it the transpose flag?

Thanks! -Patrick

Answer 1

my gut feeling tells me to use transpose flag. in that case you doing lots of dot products with stride of one.

In reality, it's very hard to tell without actually running codes. modern blas employs cache blocking techniques which make simple analysis difficult at best.