Testing the performance of `matmul` under default compiler settings

[CRquantum]

That is because I used Intel MKL’s matmul which automatically uses multiple cores. So call cpu_time() will not show real wall time.

Does LAPACK/BLAS automatically use multi cores or threads?

I found one problem, it seems call cpu_time() does not measure the time correctly when multiple threads are involved. When using Intel OneAPI with MKL, I use John Burkardt’s wtime() instead, so I run the code below, program kxk integer, parameter :: wp=selected_real_kind(14), n=5000 real(wp) :: a(n,n), b(n,n), c(n,n) real(wp) :: cpu1, cpu0 call random_number( a ); a = a - 0.5_wp call random_number( b ); b = b - 0.5_wp c = 0.0_wp cpu0 = wtime() c = matmul( a, b ) …

About showing true wall time, you may check,

How to show the true wall time instead of cpu_time?

Deal all, A quick question, I want to see how long the subroutine XXX took, so should be t2-t1 below, call cpu_time(t1) call XXX() call cpu_time(t2) write(6,*) 'time = ', t2-t1 But, when I use intel fortran, with /qparallel flag, I found sometime the cpu_time no longer tell the true wall time. Eg, it actually took 1 second wall time, but with /qparallel, t2-t2 can show 10 seconds, obviously it show all the cpu_time involved. But I only want the true wall time So, is there a way to show the…