On Thu, May 29, 2008 at 04:48:49PM -0300, Davi Vercillo C. Garcia wrote: > > Oh, I see you want to use ordered i/o in your application. PVFS > > doesn't support that mode. However, since you know how much data each > > process wants to write, a combination of MPI_Scan (to compute each > > processes offset) and MPI_File_write_at_all (to carry out the > > collective i/o) will give you the same result with likely better > > performance (and has the nice side effect of working with pvfs). > > I don't understand very well this... what do I need to change in my code ?
MPI_File_write_ordered has an interesting property (which you probably know since you use it, but i'll spell it out anyway): writes end up in the file in rank-order, but are not necessarily carried out in rank-order. Once each process knows the offsets and lengths of the writes the other process will do, that process can writes its data. Observe that rank 0 can write immediately. Rank 1 only needs to know how much data rank 0 will write. and so on. Rank N can compute its offset by knowing how much data the proceeding N-1 processes want to write. The most efficent way to collect this is to use MPI_Scan and collect a sum of data: http://www.mpi-forum.org/docs/mpi-11-html/node84.html#Node84 Once you've computed these offsets, MPI_File_write_at_all has enough information to cary out a collective write of the data. ==rob -- Rob Latham Mathematics and Computer Science Division A215 0178 EA2D B059 8CDF Argonne National Lab, IL USA B29D F333 664A 4280 315B
