Hi rob, I look at the src/mpi/romio/test/coll_perf.c test, and I experimented with the collective and non-collective versions, in single machine collective read is slower than non-collective read, ,my example is behind. #include <stdio.h> #include <stdlib.h> #include "mpi.h" #define FILE_LENGTH 100000000 #define UNSIGNEDCHAR_PER_BLK 10 int main( int argc, char **argv ) { int myid, numprocs; double startTime,readTime,newReadTime; unsigned char* buf; int bufLen; MPI_Datatype fileType; MPI_File fh; MPI_Status status; MPI_Init(&argc,&argv); MPI_Comm_rank(MPI_COMM_WORLD,&myid); MPI_Comm_size(MPI_COMM_WORLD,&numprocs); bufLen=FILE_LENGTH/numprocs; buf=(unsigned char*) malloc(bufLen); MPI_File_open(MPI_COMM_WORLD,"g://test.txt",MPI_MODE_RDWR,MPI_INFO_NULL,&fh); MPI_Type_vector(bufLen/UNSIGNEDCHAR_PER_BLK,UNSIGNEDCHAR_PER_BLK,numprocs*UNSIGNEDCHAR_PER_BLK,MPI_UNSIGNED_CHAR,&fileType); MPI_Type_commit(&fileType); MPI_File_set_view(fh, myid*UNSIGNEDCHAR_PER_BLK, MPI_UNSIGNED_CHAR, fileType, "native", MPI_INFO_NULL); startTime=MPI_Wtime(); //non-collective read //MPI_File_read(fh,buf,bufLen,MPI_UNSIGNED_CHAR,&status); //collective read MPI_File_read_all(fh,buf,bufLen,MPI_UNSIGNED_CHAR,&status); readTime=MPI_Wtime()-startTime; MPI_Allreduce(&readTime, &newReadTime, 1, MPI_DOUBLE, MPI_MAX,MPI_COMM_WORLD); if (myid == 0) { printf("%d processes read time = %f sec, read bandwidth = %f Mbytes/sec\n",numprocs, newReadTime, FILE_LENGTH/(1024*1024*newReadTime)); } free(buf); MPI_File_close(&fh); MPI_Finalize(); } the result is non-collective read 4 processes read time = 1.380783 sec, read bandwidth = 69.067633 Mbytes/sec collective read 4 processes read time = 2.256306 sec, read bandwidth = 42.267062 Mbytes/sec I am interested in collective read/write, mybe I should experiment collective I/O in cluster, thank you for your answer. Best Regard, Ice. At 2016-04-20 01:49:08, "Rob Latham" <[email protected]> wrote:

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).

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