Scaling of hydro codes

[Craig Tierney]

On Thu, Apr 10, 2003 at 10:47:04AM +0200, Wolfgang Dobler wrote:
> We have a 3-d finite-difference hydro code and find that the time per time
> step and grid point scales almost linearly,
>   t_step ~ Ncpu^(-1) ,
> on an Origin3000 from 1 up to 64 CPUs.
> 
> On our Linux cluster (Gbit ethernet, 8x2 CPUs) however, we get a scaling
> that is well represented by
>   t_step ~ Ncpu^(-0.75) .
> More or less the same scaling is obtained on another machine (100Mbit, 128
> nodes), and also for another hydro code (parallelized using Cactus).
> Note that the number of grid points was adapted for these timings, so that
> the problem size per CPU is roughly constant.

Did determine this number scaling from 1 to 16 cpus, or from 2 to 16 cpus?
You aren't going to get good scaling from 1 to 2 because lack of memory bandwidth
(this is usually the case).  Scale from 1 to 8 nodes (2 to 16 processors) to
see how the code scales due to the interconnect.

Craig


> 
> My question is: do others find the same type of scaling for hydro codes?
> If so, how can this be understood?
> 
> I don't expect latency to play a role for these timings, as we are only
> communicating a reasonably low number of large arrays in every time step;
> I suppose, Cactus does the same.
>   And if saturation of the switch played a role, I would expect a
> well-defined drop at some critical value of Ncpu, not a power law.
> 
> 
> W o l f g a n g
> 
> -- 
> 
>  -------------------------------------------------------------------------
> |  Wolfgang Dobler                           Phone: ++49/(0)761/3198-224  |
> |  Kiepenheuer Institute for Solar Physics   Fax:   ++49/(0)761/3198-111  |
> |  Sch?neckstra?e 6                                                       |
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-- 
Craig Tierney (ctierney at hpti.com)
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