problems writing vars with pnetcdf

[Katie Antypas]

Yes, the work around with the offset set to 0 when there are zero 
particles on a proc successfully ran.

Thanks very much for the help and suggestions.

Katie


On Sat, 4 Dec 2004, Rob Ross wrote:

> Hi Katie,
> 
> It sounds like you and Jianwei have come up with a couple of different 
> solutions to this problem.
> 
> It is best if you can maintain use of the collective mode, because as 
> Jianwei mentioned, there are optimizations that can be applied in that 
> case (in the libraries, transparent to the application) that cannot be 
> applied in the independent mode, because the libraries know less about 
> what you are doing as a whole in the independent mode.
> 
> We will fix our code so that your zero-particle writes work as desired.  
> In the mean time, Jianwei came up with a little hack that will get around 
> the problem for the moment.
> 
> Will that work for you?
> 
> Thanks!
> 
> Rob
> 
> On Fri, 3 Dec 2004, Katie Antypas wrote:
> 
> > Thanks for the email.  I'll try to make that fix.  
> > 
> > We had one other idea for a fix that currently doesn't work, but let me
> > run it by you.  Pnetcdf allows you to work in collective and independent
> > data modes.  Right now we are doing everything in collective mode (ie
> > all put calls end in _all).  We thought that possibly we could get
> > around this bug by writing the particles out in the independent mode.  
> > That way a processor with zero particles wouldn't make the put_vars call
> > at all and then the syncronization wouldn't get messed up.(?)
> > 
> > This seems to be more of the way that hdf5 works for us.  We don't write a 
> > zero length array, instead the processor with zero particles doesn't make 
> > the h5_write call.
> > 
> > I've been reading the bit of documenation on this which talks very 
> > briefly about setting MPI_File_set_view as a file handle for collective 
> > operations and MPI_COMM_SELF as the handler for independent mode.
> > 
> > There is this mysterious line in documentation though, 'It is difficult
> > if not impossible in the general case to ensure consistency of access
> > when a collection of processes are using multiple MPI_File handles to
> > access the same file with mixed independent and collective
> > operations....'
> > 
> > which sounds like this might be a more complicated fix.
> > 
> > any thoughts? do you think using independent mode could fix this?
> 

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