Why is printing to a terminal slow?

General Usage Performance
22

A different use of printing to stdout/stderr is to pipe output through less (or similar pager) to be able to inspect the output interactively without having to do a full (long) run. E.g. grep for certain strings in the output to check a condition, then letting the program continue until the next matching line, etc. I find this very useful during development.

In this case you will also hit upon the slower output of Julia compared to Python. A simple (rough, due to the user keyboard interaction) test of printing one integer per line for 1 - 10,000,000 and piping through less then using > and q to jump to the end of the output to let the script finish:

paulm@cmstorm 09:38:/data/examples/julia$ cat print_integers.py 
for i in range(1, 10000001):
    print(i)

paulm@cmstorm 09:38:/data/examples/julia$ cat print_integers.jl 
for i in 1 : 10000000
    println(i)
end

paulm@cmstorm 09:38:/data/examples/julia$ time python print_integers.py | less
<press ">" followed by "q" and wait for script to finish>

real	0m3.474s
user	0m3.723s
sys	0m0.080s

paulm@cmstorm 09:38:/data/examples/julia$ time julia -O3 print_integers.jl | less
<press ">" followed by "q" and wait for script to finish>

real	0m53.254s
user	0m39.575s
sys	0m47.758s

This is actually quite an eye-opener, to see the insane number of syscalls Julia does compared to Python:

paulm@cmstorm 09:43:/data/examples/julia$ strace -c -o out.python python print_integers.py | less
<press ">" followed by "q" and wait for script to finish>

paulm@cmstorm 09:43:/data/examples/julia$ strace -c -o out.julia julia -O3 print_integers.jl | less
<press ">" followed by "q" and wait for script to finish>

paulm@cmstorm 09:54:/data/examples/julia$ grep write out.python 
 18.68    0.001243           0      9626           write
paulm@cmstorm 09:54:/data/examples/julia$ grep wait out.python 
paulm@cmstorm 09:54:/data/examples/julia$ grep write out.julia 
 41.49   49.752976           2  20000003           write
  0.00    0.000010           0        13           pwrite64
paulm@cmstorm 09:55:/data/examples/julia$ grep wait out.julia 
 58.49   70.127743           1  40000001           epoll_pwait
2 Likes