Intel VML add is slow

Question

I wrote this small subroutine that compares simple vector mathematical functions, performed either with a loop:

f(i) = a(i) + b(i)

or direct:

f = a + b

or using Intel MKL VML:

vdAdd(n,a,b,f)

The timing results for n=50000000 are:

VML 0.9 sec direct 0.4 loop 0.4

And I dont understand, why VML takes twice as long as the other methods! (Loop is sometimes faster than direct)

Subroutine can be found under http://paste.ideaslabs.com/show/L6dVLdAOIf and called via

program test

  use vmltests
  implicit none

  call vmlTest()

end program

Answer 1

Your sample code have potential L2 cache issue, one can overcome it with blocking optimization. See Intel® Software Networks Forum answer for details: http://software.intel.com/en-us/forums/showthread.php?t=80041

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