[petsc-users] MatMatMul inefficient

[Guido Margherita]

Hi all, 

I realised that performing a matrix-matrix multiplication using the function MatMatMult it is not at all computationally efficient with respect to performing N times a matrix-vector multiplication with MatMul, being N the number of columns of the second matrix in the product. 
When I multiply I matrix A  46816 x 46816 to a matrix Q  46816 x 6, the MatMatMul function is indeed 6 times more expensive than 6 times a call to MatMul, when performed sequentially (0.04056  s vs 0.0062 s ). When the same code is run in parallel the gap grows even more, being10 times more expensive.
  Is there an explanation for it? 


t1 = MPI_Wtime()
call MatMatMult(A,Q,MAT_INITIAL_MATRIX, PETSC_DEFAULT_REAL, AQ, ierr )
t2 = MPI_Wtime() 
t_MatMatMul = t2-t1

t_MatMul=0.0
do j = 0, m-1
	call MatGetColumnVector(Q, q_vec, j,ierr)

	t1 = MPI_Wtime()
	call MatMult(A, q_vec, aq_vec, ierr) 
	t2 = MPI_Wtime()

	t_MatMul = t_MatMul + t2-t1
end do

Thank you, 
Margherita Guido