# strange timing result of LinearAlgebraBLAS in mathematica? [closed]

BLAS is short for "Basic Linear Algebra Subprograms". It is a famous collection of routines for doing linear algebra. I just know from Oleksandr R. that mma can directly call BLAS under the context "LinearAlgebraBLAS". I don't know why mma makes it undocumented.

Oleksandr R. provide an example of using GER

$$\mathrm{GER}: \alpha, \vec{x}, \vec{y}, \mathbf{A} : \mathbf{A} \leftarrow \alpha \vec{x} {\vec{y}}^\mathrm{T} + \mathbf{A}$$

like this

A = RandomReal[1., {40000000, 2}];
alpha = 1.;
x = ConstantArray[1., Length[A]];
y = {1., 2.};
LinearAlgebraBLASGER[alpha, x, y, A]; // AbsoluteTiming


to achieve the same result of

Transpose[{1., 2.} + Transpose@A]; // AbsoluteTiming


Oleksandr R.'s timing shows that BLAS approach is faster. While strangely, on my computer, I tried many times BLAS is much slower than double transpose with mma 10.3, windows system. I also tried it on an HPC with linux version mma 10.3 installed At this moment, it seems that maybe it is the problem of windows version 10.3. But after I tried on my friend's computer, I know it is not. Here is timing on his computer, also windows system So what is wrong, how to explain this? My cpu is Intel Core i3-4500U, and 64 bit win8 system. My memory is 8GB, and there is 1.2GB for ramdisk

• Try adding parentheses, e.g. (LinearAlgebraBLASGER[alpha, x, y, A]; // AbsoluteTiming) to be sure you are not timing something else. Also keep in mind, as @OleksandrR mentioned, that it is not a fair comparison if you are overwriting A. Nov 21 '15 at 17:00
• Does your computer have an AMD processor? How about the HPC cluster? It seems unlikely, but perhaps this could explain it. Nov 21 '15 at 21:10
• @OleksandrR. Yeah, unlikely, both are intel Nov 22 '15 at 0:50
• @ilian Actually, I didn't see any significant different whether to add overwriting A or not. So I just omitted. And after all, this is apparently not the point. Since on my friend's computer, the code is exactly the same Nov 22 '15 at 0:53
• @matheorem In your Linux screenshot, the timing measured for In definitely includes generating A which is not correct. As for the Windows difference, I haven't been able to reproduce it. Could you perhaps include the exact CPU models and the amount of RAM for both machines? I'd also suggest setting \$HistoryLength=0` first. Nov 22 '15 at 1:12