Intel VML add is slow - fortran

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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