Update: a later version with improved get_properties():
$ NUM_THREADS=6 ./llil4vec2 big?.txt big?.txt big?.txt big?.txt big?.txt big?.txt >f4vec.tmp
llil4vec2 (fixed string length=6) start
use OpenMP
use boost sort
get properties      time :    4.392 secs
sort properties     time :    4.706 secs
vector reduce       time :    0.225 secs
vector stable sort  time :    0.491 secs
write stdout        time :    0.766 secs
total               time :   10.593 secs
$ NUM_THREADS=6 ./llil4map big?.txt big?.txt big?.txt big?.txt big?.txt big?.txt >f4map.tmp
llil4map start
use phmap::parallel_flat_hash_map
use OpenMP
use boost sort
get properties      time : 10.6884 secs
finish merging      time : 2.11571 secs
vector stable sort  time : 1.83454 secs
write stdout        time : 2.55906 secs
total               time : 17.198 secs
$ diff f4map.tmp f4vec.tmp