[netcdf-hdf] collective I/O ?

[MuQun Yang]

Hi Rob,

Perhaps you can share your testing program,machine and compiler 
information with both Ed and me. I may steal some time to reproduce here.

Thanks,

Kent
At 04:32 PM 9/18/2007, Ed Hartnett wrote:
>MuQun Yang <ymuqun at hdfgroup.org> writes:
>
> > Hi Rob,
> >
> > I've done some parallel NetCDF4 performance tests with ROMS model
> > almost a year ago with NetCDF4 alpha release. At that time, I am
> > pretty sure NetCDF-4 can successfully go  to collective IO calls
> > (MPI_File_write_all with set_file_view) in HDF5 layer. But I
> > remembered there are some parameters being set wrong inside NetCDF-4
> > that I have to change so that collective IO calls can be passed to
> > HDF5. I think Ed may have already fixed that but I may be wrong.
>
>The issue that Kent found was related to the size of unlimited
>dimensions. I was storing this information in a special
>attribute. But, as Kent pointed out, that meant that every time the
>data were appended along that dimension, the attribute had to change,
>and that was not good.
>
>That has been fixed (for many moons) so that the attribute is no
>longer involved when a record is written to the file.
>
> > Another possibility is that HDF5 "magically" figure out your case is
> > not good or not possible for collective IO and it will change the
> > route to do an independent IO call instead. To verify that, we have
> > to get your program, the platform, mpi-io compiler information.
>
> >>
> >>I can verify in Jumpshot that all (in this case) 4 processors are
> >>calling independent I/O routines (MPI_File_write_at), even though I
> >>was expecting to see collective I/O (MPI_File_write_all or
> >>MPI_File_write_at_all).
>
>It would be best if we can reproduce this, but at the moment I don't
>quite know how to tell if independent or collective I/O is actually
>being used. What is Jumpshot - your debugger?
>
>Thanks!
>
>Ed
>
>--
>Ed Hartnett  -- ed at unidata.ucar.edu