--------
one thread: harm sum: N=5E+07
harm: summing from 1 to 5E+07 ...
harm: summing from 1 to 5E+07 gives 18.3047492382933
one thread: harm sum from 1 to 5E+07 = 18.3047492382933
real 4.21
user 4.13
sys 0.04
---------
two threads: harm sum: N=5E+07
harm: summing from 2.5E+07 to 5E+07 ...
harm: summing from 1 to 2.5E+07 ...
harm: summing from 2.5E+07 to 5E+07 gives 0.69314721056011
harm: summing from 1 to 2.5E+07 gives 17.6116020277299
two threads: harm sum from 1 to 5E+07 = 18.30474923829
real 2.69
user 5.02
sys 0.02
----------
parallel for: harm sum: N=5E+07
parallel for: harm sum from 1 to 50000000 = 14.5290954602666
real 6.85
user 25.43
sys 0.16