$ g++ -o llil4vec-omp -std=c++20 -fopenmp -Wall -O3 llil4vec.cpp -I./fast_io/include

$ time OMP_NUM_THREADS=30 ./llil4vec-omp big?.txt >out.txt

llil4vec (fixed string length=6) start
use OpenMP
get properties      time : 0.937948 secs
sort properties     time : 0.494592 secs
emplace set sort    time : 1.02668 secs
write stdout        time : 0.50569 secs
total               time : 2.96497 secs

real  0m3.271s   <-- total wall time, performs similarly
user  0m16.823s  <-- nothing unusual, fans seem quiet
sys   0m4.188s