On Thu, Sep 22, 2011 at 11:37:10PM +0200, German Hoecht wrote: > Hello, > > MPI_File_read/write functions uses an integer to specify the size of > the buffer, for instance: > int MPI_File_read(MPI_File fh, void *buf, int count, MPI_Datatype > datatype, MPI_Status *status) > with: > count Number of elements in buffer (integer). > datatype Data type of each buffer element (handle). > > However, using the maximum value of 32 bytes integers: > count = 2^31-1 = 2147483647 (and datatype = MPI_BYTE) > MPI_file_read only reads 2^31-2^12 = 2147479552 bytes. > This means that 4095 bytes are ignored. > > I am not aware of this specific limit for integers in (Open) MPI > function calls. Is this supposed to be correct?
Hi. I'm the ROMIO maintainer. OpenMPI more or less rolls up ROMIO into OpenMPI, so any problems with the MPI_File_* routines is in my lap, not OpenMPI. I'll be honest with you: i've not given any thought to just how big the biggest request could be. The independent routines, especially with a simple type like MPI_BYTE, are going to almost immediately call the underlying posix read() or write() call. I can confirm the behavior you observe with your test program. Thanks much for providing one. I'll dig around but I cannot think of something in ROMIO that would ignore these 4095 bytes. I do think it's legal by the letter of the standard to read or write less than requested. "Upon completion, the amount of data accessed by the calling process is returned in a status." Bravo to you for actually checking return values and the status. I don't think many non-library codes do that :> I should at least be able to explain the behavior, so I'll dig a bit. in general, if you plot "i/o performance vs blocksize", every file system tops out around several tens of megabytes. So, we have given the advice to just split up this nearly 2 gb request into several 1 gb requests. ==rob -- Rob Latham Mathematics and Computer Science Division Argonne National Lab, IL USA
