> As for why the naive rust version is slower, it's because without adding a Buf...

glandium · on Oct 12, 2017

Your bytes version outputs lines of, literally, `b'y'`.

The characters versions is still a clear win for python2 on my machine (8.9MiB/s vs. 5.6MiB/s)

It's also worth noting that the buffering behavior of python is only happening because the output is a pipe to pv. If it were the terminal, it would be line buffered, like the naive rust version.

thechewanater · on Oct 12, 2017

python3 seems to do much better than either of those for me when using an unbuffered write(1, ...) syscall (plus it prints the correct thing)

    $ cat yes3.py
    stdout = open(1, 'wb')
    while True:
        stdout.write(b'y\n')
    $ python3 yes3.py | pv -r > /dev/null
    [13.7MiB/s]

    $ cat yes2.py
    import os
    stdout = os.fdopen(1, 'wb')
    while True:
        stdout.write('y\n')
    $ python2 yes2.py | pv -r > /dev/null
    [7.77MiB/s]

glandium · on Oct 12, 2017

For better comparison with my numbers, I ran your scripts on my machine:

  $ python3 yes3.py | pv -r > /dev/null
  [18.4MiB/s]
  $ python2 yes2.py | pv -r > /dev/null
  [10.2MiB/s]

In both cases, a 4KiB buffer is used by python. That's still way slower than the equivalent rust code with a 4KiB buffer (use BufWriter::with_capacity(4096, stdout.lock()) instead of BufWriter::new(stdout.lock())).

Twirrim · on Oct 12, 2017

Out of curiosity:

yes2.py:

  import os
  stdout = os.fdopen(1, 'wb')
  while True:
      stdout.write('y\n')


  $ python yes2.py | pv -r > /dev/null
  [9.12MiB/s]

  $ pypy yes2.py | pv -r > /dev/null
  [45.5MiB/s]

So pypy does a good job of speeding it up.

Off a quick 9 second run, python2 with profiling:

     ncalls  tottime  percall  cumtime  percall filename:lineno(function)
          1    4.272    4.272    9.301    9.301 yes2.py:1(<module>)
   30856080    5.029    0.000    5.029    0.000 {method 'write' of 'file' objects}
          1    0.000    0.000    0.000    0.000 {posix.fdopen}
          1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}