[Yt-dev] parallelism when ngrids < nprocs

[John Wise]

Hi all,

I was calculating some quantities for a halo list with 32 processors, 
and it broke when it tried to calculate derived quantities for small 
halos with less than 32 grids within its sphere.  I fixed it for enzo 
datasets and posted the diff here.

http://paste.enzotools.org/show/1511/

First it broke in the preload function when the grid list is null, and 
it tried to get a field list from the first grid.  That was easily fixed 
by returning data = None.  Then I had to modify the mpi_catarray 
function to consider a null dataset (len(data) == 0).

I don't like how I modified the preload function because it only applies 
for the enzo frontend.  Is there another routine that can be modified 
that affects all data formats, so this fix can be generalized?

Thanks,
John