Parallel IO using MPI IO-Message Passing Interface Library-Lecture Slides, Slides of Microprocessor and Interfacing

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Dr. Hanif Durad

Lecture Outline

IntroMPI.ppt

Common Ways of Doing I/O inParallel Programs



Sequential I/O:



All processes send data to rank 0, and 0 writes it tothe file

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mpi-io_2.ppt

Example using C (2/2)

if (myrank != 0)

MPI_Send(buf, BUFSIZE, MPI_INT, 0, 99, MPI_COMM_WORLD);

else {

myfile = fopen("testfile1", "w");fwrite(buf, sizeof(int), BUFSIZE, myfile);for (i=1; i<numprocs; i++) {

MPI_Recv(buf, BUFSIZE, MPI_INT, i, 99, MPI_COMM_WORLD,

&status);

fwrite(buf, sizeof(int), BUFSIZE, myfile);

} fclose(myfile);

} MPI_Finalize();return 0;

}

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io0.c

Pros and Cons of Sequential I/O



Pros:



parallel machine may support I/O from only one process (e.g.,no common file system)



Some I/O libraries (e.g. HDF-4, NetCDF) not parallel



resulting single file is handy for

ftp

mv



big blocks improve performance



short distance from original, serial code



Cons:



lack of parallelism limits scalability, performance (singlenode bottleneck)

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What is Parallel I/O?



Multiple processes of a parallel programaccessing data (reading or writing) from a common

file

FILE

P

P

P

P(n-1)

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Why Parallel I/O?



Non-parallel I/O is simple but



Poor performance (single process writes to one file)or



Awkward and not interoperable with other tools (eachprocess writes a separate file)



Parallel I/O



Provides high performance



Can provide a single file that can be used with othertools (such as visualization programs)

Dr. Hanif Durad

MPI-IO Background



Marc Snir et al (IBM Watson) paper exploring MPI ascontext for parallel I/O (1994)



MPI-IO email discussion group led by J.-P. Prost (IBM)and Bill Nitzberg (NASA), 1994



MPI-IO group joins MPI Forum in June 1996



MPI-2 standard released in July 1997



MPI-IO is Chapter 9 of MPI-

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Using MPI for Simple I/O

FILE

P

P

P

P(n-1)

Each process needs to read a chunk of data from a common file

Arguments

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comm_old

existing communicator

ndims

number of dimensions

periods

logical array indicating whether a dimension is cyclic (If TRUE, cyclic boundary conditions)

reorder

logical (If FALSE, rank preserved)

(If TRUE, possible rank reordering)

comm_cart

new cartesian communicator