You cannot change the number of threads during run time using environment variables. However, you can call OpenMP API functions from your program to change the number of threads during run time. The following sample code shows how to change the number of threads during run time using the omp_set_num_threads() routine. See also Techniques to Set the Number of Threads.
The following example shows both C and Fortran code examples. To run this example in the C language, use the omp.h header file from the Intel(R) compiler package. If you do not have the Intel compiler but wish to explore the functionality in the example, use Fortran API for omp_set_num_threads() rather than the C version. For example, omp_set_num_threads_( &i_one );
// ******* C language ******* #include "omp.h" #include "mkl.h" #include <stdio.h> #define SIZE 1000 void main(int args, char *argv[]){ double *a, *b, *c; a = new double [SIZE*SIZE]; b = new double [SIZE*SIZE]; c = new double [SIZE*SIZE]; double alpha=1, beta=1; int m=SIZE, n=SIZE, k=SIZE, lda=SIZE, ldb=SIZE, ldc=SIZE, i=0, j=0; char transa='n', transb='n'; for( i=0; i<SIZE; i++){ for( j=0; j<SIZE; j++){ a[i*SIZE+j]= (double)(i+j); b[i*SIZE+j]= (double)(i*j); c[i*SIZE+j]= (double)0; } } cblas_dgemm(CblasRowMajor, CblasNoTrans, CblasNoTrans, m, n, k, alpha, a, lda, b, ldb, beta, c, ldc); printf("row\ta\tc\n"); for ( i=0;i<10;i++){ printf("%d:\t%f\t%f\n", i, a[i*SIZE], c[i*SIZE]); } omp_set_num_threads(1); for( i=0; i<SIZE; i++){ for( j=0; j<SIZE; j++){ a[i*SIZE+j]= (double)(i+j); b[i*SIZE+j]= (double)(i*j); c[i*SIZE+j]= (double)0; } } cblas_dgemm(CblasRowMajor, CblasNoTrans, CblasNoTrans, m, n, k, alpha, a, lda, b, ldb, beta, c, ldc); printf("row\ta\tc\n"); for ( i=0;i<10;i++){ printf("%d:\t%f\t%f\n", i, a[i*SIZE], c[i*SIZE]); } omp_set_num_threads(2); for( i=0; i<SIZE; i++){ for( j=0; j<SIZE; j++){ a[i*SIZE+j]= (double)(i+j); b[i*SIZE+j]= (double)(i*j); c[i*SIZE+j]= (double)0; } } cblas_dgemm(CblasRowMajor, CblasNoTrans, CblasNoTrans, m, n, k, alpha, a, lda, b, ldb, beta, c, ldc); printf("row\ta\tc\n"); for ( i=0;i<10;i++){ printf("%d:\t%f\t%f\n", i, a[i*SIZE], c[i*SIZE]); } delete [] a; delete [] b; delete [] c; }
// ******* Fortran language ******* PROGRAM DGEMM_DIFF_THREADS INTEGER N, I, J PARAMETER (N=1000) REAL*8 A(N,N),B(N,N),C(N,N) REAL*8 ALPHA, BETA INTEGER*8 MKL_MALLOC integer ALLOC_SIZE integer NTHRS ALLOC_SIZE = 8*N*N A_PTR = MKL_MALLOC(ALLOC_SIZE,128) B_PTR = MKL_MALLOC(ALLOC_SIZE,128) C_PTR = MKL_MALLOC(ALLOC_SIZE,128) ALPHA = 1.1 BETA = -1.2 DO I=1,N DO J=1,N A(I,J) = I+J B(I,J) = I*j C(I,J) = 0.0 END DO END DO CALL DGEMM('N','N',N,N,N,ALPHA,A,N,B,N,BETA,C,N) print *,'Row A C' DO i=1,10 write(*,'(I4,F20.8,F20.8)') I, A(1,I),C(1,I) END DO CALL OMP_SET_NUM_THREADS(1); DO I=1,N DO J=1,N A(I,J) = I+J B(I,J) = I*j C(I,J) = 0.0 END DO END DO CALL DGEMM('N','N',N,N,N,ALPHA,A,N,B,N,BETA,C,N) print *,'Row A C' DO i=1,10 write(*,'(I4,F20.8,F20.8)') I, A(1,I),C(1,I) END DO CALL OMP_SET_NUM_THREADS(2); DO I=1,N DO J=1,N A(I,J) = I+J B(I,J) = I*j C(I,J) = 0.0 END DO END DO CALL DGEMM('N','N',N,N,N,ALPHA,A,N,B,N,BETA,C,N) print *,'Row A C' DO i=1,10 write(*,'(I4,F20.8,F20.8)') I, A(1,I),C(1,I) END DO STOP END
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