[mpich-discuss] collective read slower than non-collective read

[Rob Latham]

On 04/19/2016 07:45 AM, 冰 wrote:
> Hi,
> I run programs in one computer, In my tests here, the MPI_File_read_all
> have been slower than MPI_File_read, could you send me a simple routine
> that collective read is faster than non-collective read?

Need more information.

The collective calls provide two important optimizations:  reduce the 
number of file system clients and increase the typical request size.

On a single processor, though, it's possible the collective overhead 
outweighs the benefits.

How much slower are you seeing?  how many MPI processes?  what kind of 
file view and memory type?

You might want to look at the src/mpi/romio/test/coll_perf.c test and 
experiment with the collective and non-collective versions (you'll have 
to modify the test or pass in hints).

==rob
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