$ time NUM_THREADS=2 ./grep-count-chunk "[aA].*[eE].*[iI].*[oO].*[uU]" /tmp/big.txt
parallel (2) Total Lines: 105000000, Matching Lines: 2195760

N-thds  2           5           10          20          30
real    0m8.017s    0m3.257s    0m1.537s    0m0.801s    0m0.618s
user    0m14.524s   0m14.997s   0m14.906s   0m15.397s   0m17.519s
sys     0m1.010s    0m1.019s    0m0.412s    0m0.515s    0m0.649s