[FLASH-USERS] Improving the load balancing in FLASH

[Tomasz Plewa]

Early versions of Paramesh provided support for individual level time 
stepping. Putting that functionality back into the code is not an 
impossible task, but quite tedious and boring. It would also require 
essentially doubling the code memory, in the simplest approach.

Taking into account the number of time steps required to evolve a given 
level into consideration when load balancing is less important then the 
information about refinement level. This is because the number of parent 
blocks is always smaller than the number of offspring blocks. But it 
could be included at no cost. At some point data locality would 
interfere and become a serious problem. This is a common problem for all 
Berger-Colella AMR implementations. Paramesh is a very restricted subset 
of that approach and those simplifications make it efficient for most 
well-balanced physics applications.

Level-based load balancing would be helpful in general. One example is a 
multigrid solver. That has been bypassed algorithmically in Enzo, if I 
am not mistaken. I was told Chombo provides level-based load balancing.

Tomek
--
Ross Parkin wrote:
> Hello,
>
> I've been thinking about the load balancing method used in FLASH. The  
> timestep used by the hydro units is the same on all refinement levels  
> and so blocks are essentially distributed by paramesh so that all  
> processors have the roughly the smae number of blocks. Has anyone  
> tried modifying FLASH so that each refinement level has its own  
> timestep? To do this properly you would need to modify how PARAMESH  
> distributes blocks amongst processors, taking account of the relative  
> work done by each block on each refinement level, i.e. a block on a  
> finer refinement level may need to do twice as many hydro loops before  
> its simulation time equals that of a coarser block.
>
> Any ideas guys?
>
> Cheers,
>
> Ross Parkin
>   
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