## Two stage algorithm is slower than one stage

Open discussion for MAGMA library (Matrix Algebra on GPU and Multicore Architectures)

### Two stage algorithm is slower than one stage

Hi!
I tried to compare computational time magma_zheevdx_2stage and magma_zheevdx to find both eigenvectors and eigenvalues and I expected to see that "two stage" algorithm is faster. But I found out that it is 30% slower. Is there any way to make magma_zheevdx_2stage work faster?
alexander_p1987

Posts: 5
Joined: Sun Jan 06, 2013 4:55 pm

### Re: Two stage algorithm is slower than one stage

If you are computing ALL the eigenvectors, then it is possible that the 2-stage algorithm is slower (though I'm a bit surprised that it is 30% slower -- depends on your hardware). If you need only a small portion, say 10%, of the eigenvectors, then the 2-stage approach should be faster. The 2-stage algorithm is much faster at computing the eigenvalues, but it adds an extra back-transform step in computing eigenvectors, which offsets the speed gains.
-mark
mgates3

Posts: 782
Joined: Fri Jan 06, 2012 2:13 pm

### Re: Two stage algorithm is slower than one stage

Thank you for this useful information :)
alexander_p1987

Posts: 5
Joined: Sun Jan 06, 2013 4:55 pm