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?
MPI_File_read/write does not return an error (but MPI_Get_count states
that only 2147479552 bytes are considered). Find attached a C++ code
example which tries to write and read 2^31-1 bytes.
I am using Open MPI 1.4.2 compiled with the Intel compiler.
Best regards,
German
#include <fstream>
#include <iostream>
#include <iomanip>
#include <math.h>
#ifndef NO_MPI
#include <mpi.h>
#endif
#include <limits>
using namespace std;
int main( int nargs, char *args[] )
{
int mpi_rank, mpi_size;
#ifndef NO_MPI
MPI_Init( &nargs, &args );
MPI_Comm_rank( MPI_COMM_WORLD, &mpi_rank );
MPI_Comm_size( MPI_COMM_WORLD, &mpi_size );
#else
mpi_rank = 0;
mpi_size = 1;
#endif
int nbuf = std::numeric_limits<int>::max();
if ( nargs < 2 ){
if ( mpi_rank == 0 ){
cerr<<"usage:\n"
<<"------\n"
<<args[0]<<" filename"<<endl
<<"description:\n"
<<"------------\n"
<<"write 2 x "<<nbuf<<" bytes to file\n"
<<"read 1 x "<<nbuf<<" bytes from file\n";
}
#ifndef NO_MPI
MPI_Finalize();
#endif
return 0;
}
// filename ---------
char *sfile = args[1];
#ifndef NO_MPI
int error, count;
MPI_File fp;
MPI_Status stat;
MPI_Offset fpos;
if ( mpi_rank == 0 ){
// allocate buffer
char *buf = new char[nbuf];
// open for write
error = MPI_File_open( MPI_COMM_SELF, sfile, MPI_MODE_WRONLY | MPI_MODE_CREATE,
MPI_INFO_NULL, &fp );
if( error != MPI_SUCCESS ) {
cerr<<"error: MPI_File_open for file "<<sfile
<<" in mode write|create returns code "<<error<<endl;
MPI_Abort( MPI_COMM_WORLD, 1 );
}
MPI_File_set_size( fp, 0 );
// write (initial)
cout<<"write "<<nbuf<<" bytes"<<endl;
error = MPI_File_write( fp, buf, nbuf, MPI_BYTE, &stat );
MPI_Get_count( &stat, MPI_BYTE, &count );
cout<<"MPI_File_write returns code: "<<error<<" count: "<<count<<endl;
// write (add)
fpos = (MPI_Offset) count;
cout<<"write: add "<<nbuf<<" bytes at file position "<<count<<endl;
error = MPI_File_write_at( fp, fpos, buf, nbuf, MPI_BYTE, &stat );
MPI_Get_count( &stat, MPI_BYTE, &count );
cout<<"MPI_File_write returns code: "<<error<<" count: "<<count<<endl;
// close
MPI_File_close( &fp );
// open for read
error = MPI_File_open( MPI_COMM_SELF, sfile, MPI_MODE_RDONLY,
MPI_INFO_NULL, &fp );
if( error != MPI_SUCCESS ) {
cerr<<"error: MPI_File_open for file "<<sfile
<<" in mode read returns code "<<error<<endl;
MPI_Abort( MPI_COMM_WORLD, 1 );
}
// read
cout<<"read "<<nbuf<<" bytes"<<endl;
error = MPI_File_read( fp, buf, nbuf, MPI_BYTE, &stat );
MPI_Get_count( &stat, MPI_BYTE, &count );
cout<<"MPI_File_read returns code: "<<error<<" count: "<<count<<endl;
// close
MPI_File_close( &fp );
delete[] buf;
}
MPI_Barrier( MPI_COMM_WORLD );
MPI_Finalize();
#else
size_t count;
FILE *fp = fopen( sfile, "w" );
if ( !fp ){
cerr<<"error: could not open "<<sfile<<" in write mode\n";
return 1;
}
// allocate buffer
char *buf = new char[nbuf];
cout<<"write "<<nbuf<<" bytes"<<endl;
count = fwrite( buf, 1, nbuf, fp );
cout<<"fwrite returns count: "<<count<<endl;
cout<<"write: add "<<nbuf<<" bytes"<<endl;
count = fwrite( buf, 1, nbuf, fp );
cout<<"fwrite returns "<<count<<endl;
fclose( fp );
fp = fopen( sfile, "r" );
if ( !fp ){
cerr<<"error: could not open "<<sfile<<" in read mode\n";
return 1;
}
cout<<"read "<<nbuf<<" bytes"<<endl;
count = fread( buf, 1, nbuf, fp );
cout<<"read returns "<<count<<endl;
fclose( fp );
delete[] buf;
#endif
return 0;
}
