[petsc-users] MatSetValues timing

[Jacek Miloszewski]

Dear PETSc users,

I use PETSc to assembly a square matrix (in the attached example it is n =
4356) which has around 12% of non-zero entries. I timed my code using
various number of process (data in table). Now I have 2 questions:

1. Why with doubling number of processes the speed-up is 4x? I would expect
2x at the most.

2. Is there a way to speed-up matrix construction in general? I attach
piece of my fortran code at the bottom. At compilation time I have the
following knowledge about the matrix: the total number of non-zero matrix
elements, all diagonal elements are non-zero.

Timing data:

number of proc time [s] speedup ratio
1 2044.941
2 504.692 4.051859352
4 149.678 3.371851575
8 64.102 2.334997348
16 17.296 3.706174838
32 4.43 3.904288939
64 1.096 4.041970803

Code:

call MatCreate(PETSC_COMM_WORLD, a, ierr)
call MatSetSizes(a, PETSC_DECIDE, PETSC_DECIDE, n, n, ierr)
call MatSetFromOptions(a, ierr)
call MatSetUp(a, ierr)

call MatGetOwnershipRange(a, Istart, Iend, ierr)

call system_clock(t1)
t_dim = h_dim*e_dim
do row = Istart, Iend - 1 ! row
   do col = 0, t_dim - 1 ! col
      call h_ij(row + 1, col + 1, n_h, n_e, b_h, b_e, h_dim, e_dim, e_sp,
v, basis, ht, hs, info_1)
      if (hs) then
         hh = ht
         call MatSetValues(a, 1, row, 1, col, hh, INSERT_VALUES, ierr)
      end if
   end do
end do
call system_clock(t2, ct)
if (rank == 0) then
   write(*, '(a, f0.3)') 'Matrix assembly time: ', real((t2 - t1),
r8)/real(ct, r8)
end if

-- 
Best Wishes
Jacek Miloszewski
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