Hi Izaak,
On Aug 12, 2011, at 11:32 AM, Izaak Beekman wrote:
> One more question about hdf5 parallel data set creation: here two
> alternatives are presenting for writing data from a subset of MPI ranks. I
> will be writting to a lustre filesystem, and and was wondering about the
> performance differences between the two techniques. I have ordered data in a
> 3D array, and the domain decomposition is done over the 2nd and 3rd indices
> (Fortran code). So each MPI rank has a 3d array. The domain is decomposed,
> and the processors layed out, as a 2D grid. I want to write 2D planar slices
> of data out with high frequency. If the slice is taken as j=const or k=const
> (where j and k are the 2nd and 3rd indices respectively, the indeces which
> the separate subdomains) then only a relatively small (compared to the total
> #) but still O(10-100) MPI ranks need to write data. I would say in general
> less than 50% but more than 10% of the MPI ranks will be involved in this
> write. Should I do it as a collective write where I guess there is some
> over-head as the MPI ranks are cycled through in a predetermined order, or
> should I hammer the lustre fs with uncoordinated individual writes,
> concurrently? Any insight here is appreciated.
I'm guessing that collective I/O will be a win still, but you may have
to experiment a little...
Quincey
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