no parallel speedup with slepc krylovschur

[Jose E. Roman]

On 30/06/2009, Christian May wrote:

> Dear readers,
>
> I want to solve the following generalized eigenvalue problem in  
> parallel:
>
> http://www.phys.ethz.ch/~cmay/binaryoutput.tgz
> (gnu zipped tar of the binaryoutput you get using -eps_view_binary)
>
> I know the approximate value of the lowest eigenvalues I'm  
> interested in, so I'm using shift-and-invert with the following  
> options:
>
> -eps_type krylovschur -st_type sinvert -st_shift -0.228 -eps_ncv 32 - 
> eps_nev 16 -eps_tol 1e-10
>
>
> However, when I use multiple CPUs, I cannot see any speedup. I am  
> not sure whether this is a problem of the matrix or of my system.  
> Can anybody please have a look?
>
> Thanks
> Christian

With a shift-and-invert scheme, most of the computing time is spent in  
the solution of linear systems. In SLEPc 3.0.0 the default is to use  
KSP=redundant+PC=lu for the linear systems, so it is normal that you  
get little or no speedup with more than one processor. You should  
either use a truly parallel direct linear solver or an iterative  
linear solver. Please contact the SLEPc team directly if you need  
further assistance.

Best regards,
Jose