[Lustre-discuss] Shared pointers, file locking and MPI-IO

Robert Latham robl at mcs.anl.gov
Wed Nov 12 09:46:33 PST 2008


On Thu, Oct 23, 2008 at 04:06:33PM +0200, Sébastien Buisson wrote:
> That being said, does anybody in the Lustre community know about current 
> alternatives to the MPI-IO shared pointers read/write routines, which do 
> not use the file locking mechanism but provide the same functionality?
> 
> By the way, do we have an idea of the Lustre version that will provide 
> the reliable version of the flock consistent mode?

Hi Sebastien

We implemented shared file pointers in ROMIO with MPI one-sided
operations a few years back.  It's possibly more work than you had in
mind, but you can get really good performance if you coordinate with
these memory regions instead of through the file system.

http://www.mcs.anl.gov/~robl/papers/latham_rmaops.pdf

That paper also discsusses an approach to the ordered-mode operations
that uses MPI_SCAN to share file offset information instead of locking
a file.

We could not make these part of our MPI-IO implementation because it
is common for an MPI implementation to not make "background progress"
in this passive-target RMA mode, and if a process went off and did
something computationally intensive for a very long time it could hang
up all the other processes.

But if you as the application know such an "out to lunch" situation
won't happen, then you'll have no problems with this approach.

Hope these help you out
==rob

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



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