What is the relation between BLAS, LAPACK and ATLAS
I don't understand how BLAS, LAPACK and ATLAS are related and how I should use them together! I have been looking through all of their manuals and I have a general idea of BLAS and LAPACK and how to use them with the very few examples I find, but I can't find any actual examples using ATLAS to see how it is related with these two.
I am trying to do some low level work on matrixes and my primary language is C. First I wanted to use GSL, but it says that if you want the best performance you should use BLAS and ATLAS. Is there any good webpage giving some nice examples of how to use these (in C) all together? In other words I am looking for a tutorial on using these three (or any subset of them!). In short I am confused!
Answer
BLAS is a collection of low-level matrix and vector arithmetic operations (“multiply a vector by a scalar”, “multiply two matrices and add to a third matrix”, etc ...).
LAPACK is a collection of higher-level linear algebra operations. Things like matrix factorizations (LU, LLt, QR, SVD, Schur, etc) that are used to do things like “find the eigenvalues of a matrix”, or “find the singular values of a matrix”, or “solve a linear system”. LAPACK is built on top of the BLAS; many users of LAPACK only use the LAPACK interfaces and never need to be aware of the BLAS at all. LAPACK is generally compiled separately from the BLAS, and can use whatever highly-optimized BLAS implementation you have available.
ATLAS is a portable reasonably good implementation of the BLAS interfaces, that also implements a few of the most commonly used LAPACK operations.
What “you should use” depends somewhat on details of what you’re trying to do and what platform you’re using. You won't go too far wrong with “use ATLAS + LAPACK”, however.