[FLASH-USERS] inerfacing PFFT and Multigrid

[Mateusz Ruszkowski]

   Hi all,

I am trying to better understand the performance of the PFFT+Multigrid 
(MG) hybrid solver. The F4.0 manual quantifies the gains in performance 
when PFFT+MG is used instead of MG, and discusses the comparison of cases 
such that, e.g., (1) lrefine_min = 4 and lrefine_max = 5, or (2) 
lrefine_min = 4 and lrefine_max = 6.

My question is: how does the performance gain depend on the volume filling 
of the highly refined region. For example, consider two hypothetical 
situations where lrefine_min = 4 and lrefine_max = 7:

(a) the volume occupied by lref=5,6,7 regions is << the domain size (e.g., 
a cosmological simulation with a number of small isolated and better 
refined halos).

(b) the volume occupied by lref=5,6,7 regions is ~ the domain size (e.g., 
some turbulence simulation where refinement is more uniform).

Question: is the speed improvement expected to be better in case (a) when 
we replace MG by PPFT+MG, or is it likely that the relative gain in speed 
will be better in case (b) when we switch from MG to PFFT+MG there.

  Thanks,
    Mateusz