How to store A to get fastest performance of AT*x using cblas_dgemv?

Hello,

I am using cblas_dgemv to obtain A^T*x.  The size of the matrix A is about 10000 Rows x 20000 columns.  I am storing A in row major format.  A_i,j+1 is stored next to A_ij

My questions are as follows (in order to get fastest execution time):

1. What is better way to store A -- row major format or column major format? does it matter?
2. Is it better to store A and set TransA=CblasTrans or store A^T directly and use it with TransA=CblasNoTrans.
3. If answer to #2 is to use A^T directly, is it better to store A^T in rowmajor format or column major format?

Another related question I have has to do with byte alignment.  Let us say we are storing in A in row major format.  A has m rows and n columns.  I have read that, when doing multithreading using openmp, to avoid false sharing it is better if each row of A starts at a byte aligned boundary.  A common way of doing that is by padding the number of columns such that it is divisible by 8 (64 bytes for 8 doubles).  So LDA = n + (8 - n%8).  Does doing this help dgemv run faster?

Finally, For my calculation I need alpha=1 and beta=0.  Does cblas_dgemv optimize for this trivial case or does it do the extra and in this unneccessary calculations?

Thanks in advance for any help.