[Pvfs2-developers] Re: noncontig-test

[Robert Latham]

On Wed, Aug 08, 2007 at 06:47:41PM -0500, Sam Lang wrote:
> Also, should we increase the limit of request segments allowed?  It  
> might be inefficient for a user to create an MPI indexed dataype with  
> that many elements, but there are users that will probably do it  
> anyway.  Alternatively, we could consider more efficiently encoding  
> each request segment in PVFS.

Bear in mind that Joe User is probably going to create these PVFS
types through ROMIO.  Since we take a list-io approach and turn all
types into offset-length pairs (the indexed type), what could be a
very compact MPI type might become a quite long PVFS type (for
example, some of the types created by the HDF5 test programs contain
512 offset-length pairs once they make their way down to
ADIOI_PVFS2_WriteStrided)

I'd like to see a few things:

- a way to ask PVFS how many segments romio can produce.  that's
  hard-coded to 64 now, but clearly should have been less in this case
  and more for TCP.

- a way to announce to PVFS "hey, i'm about to send you a giant
  request".  I'm 100% ok with a rendezvous approach if those
  additional messages mean a single call to PVFS_sys_io

- Real datatype I/O in ROMIO.  Avery's code has been languishing in a
  branch for way too long, pending creation of a thourough ROMIO test
  suite.  We have tested the PVFS_request_hindexed approach very well,
  but other PVFS request types have not been used nearly as often.  

==rob

-- 
Rob Latham
Mathematics and Computer Science Division    A215 0178 EA2D B059 8CDF
Argonne National Lab, IL USA                 B29D F333 664A 4280 315B