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GH-130328: pasting in new REPL is slow on Windows#132884

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ambv merged 7 commits into
python:mainfrom
chris-eibl:fix_wait
Apr 29, 2025
Merged

GH-130328: pasting in new REPL is slow on Windows#132884
ambv merged 7 commits into
python:mainfrom
chris-eibl:fix_wait

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@chris-eibl

@chris-eibl chris-eibl commented Apr 24, 2025 •
edited by bedevere-app Bot
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The reason why pasting is so slow on Windows (especially in the legacy console case where virtual terminal mode - and thus bracketed paste - is disabled):

cpython/Lib/_pyrepl/reader.py

Lines 702 to 706 in 9f5994b

while True:
# We use the same timeout as in readline.c: 100ms
self.run_hooks()
self.console.wait(100)
event = self.console.get_event(block=False)

and

cpython/Lib/_pyrepl/windows_console.py

Lines 523 to 532 in 9f5994b

def wait(self, timeout: float | None) -> bool:
"""Wait for an event."""
# Poor man's Windows select loop
start_time = time.time()
while True:
if msvcrt.kbhit(): # type: ignore[attr-defined]
return True
if timeout and time.time() - start_time > timeout / 1000:
return False
time.sleep(0.01)

It is interesting, that msvcrt.kbhit() returns True in case of pasting, but if that weren't the case, we'd hit the 100ms timeout and would be even way slower. However, msvcrt.kbhit() is very slow in case of the legacy console, and it is called at least twice as often, because we get a key up and a key down event - but only a key down event in the virtual terminal case. If the pasted input contains upper case letters, we additionally get a "shift key pressed" event - so in the worst case, msvcrt.kbhit() gets called 3 times more often (and each call is slower).

Output of python.bat -m cProfile -m _pyrepl when pasting the "test value" given in the OP for a legacy console:

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
     50/1    0.010    0.000   17.202   17.202 {built-in method builtins.exec}
        1    0.000    0.000   17.202   17.202 <frozen runpy>:201(run_module)
        1    0.000    0.000   17.199   17.199 <frozen runpy>:65(_run_code)
        1    0.000    0.000   17.199   17.199 __main__.py:1(<module>)
        1    0.000    0.000   17.112   17.112 main.py:24(interactive_console)
        1    0.000    0.000   17.112   17.112 simple_interact.py:99(run_multiline_interactive_console)
        5    0.000    0.000   17.111    3.422 readline.py:375(multiline_input)
        5    0.004    0.001   17.111    3.422 reader.py:741(readline)
     3090    0.036    0.000   17.103    0.006 reader.py:694(handle1)
     6210    0.013    0.000   13.714    0.002 windows_console.py:523(wait)
     6236   13.433    0.002   13.433    0.002 {built-in method msvcrt.kbhit}

Here the relevant line in case of virtual terminal:

  3270    0.554    0.000    0.554    0.000 {built-in method msvcrt.kbhit}

The fix is to use WaitForSingleObject

    def wait(self, timeout: float | None) -> bool:
        """Wait for an event."""
        ret = WaitForSingleObject(InHandle, int(timeout))
        if ret == WAIT_FAILED:
            raise WinError(ctypes.get_last_error())

which speeds up especially the legacy console (times in seconds):

legacy terminal virtual terminal
before 15.6 0.89
after 1.8 0.26

Note, see also #132440 (comment):

  • legacy terminal: manually start cmd.exe. Timings gotten by pasting
import time
t1 = time.time()
<"test value" given in the OP>
print(time.time() - t1)
  • virtual terminal: power shell via Windows terminal. Timings gotton by temporarily patching commands.py:
import time
class enable_bracketed_paste(Command):
    def do(self) -> None:
        self.reader.bp_begin = time.perf_counter()
        self.reader.paste_mode = True
        self.reader.in_bracketed_paste = True

class disable_bracketed_paste(Command):
    def do(self) -> None:
        print("bracketed_paste took %5.2f s" % (time.perf_counter() - self.reader.bp_begin, ))
        self.reader.paste_mode = False
        self.reader.in_bracketed_paste = False
        self.reader.dirty = True

@bedevere-app bedevere-app Bot mentioned this pull request Apr 24, 2025
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@chris-eibl chris-eibl added the needs backport to 3.13 bugs and security fixes label Apr 24, 2025
@sergey-miryanov

sergey-miryanov commented Apr 24, 2025

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Good analysis and speedup! 👍

@chris-eibl chris-eibl mentioned this pull request Apr 24, 2025
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@chris-eibl chris-eibl added OS-windows topic-repl Related to the interactive shell labels Apr 24, 2025
@Fidget-Spinner

Fidget-Spinner commented Apr 24, 2025

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We generally don't backport perf fixes unless they lead to a denial-of-service problem (so usually like an exponential-time algorithm), or result in a serious bug. So I'm removing the backport label.

@Fidget-Spinner Fidget-Spinner removed the needs backport to 3.13 bugs and security fixes label Apr 24, 2025
@Fidget-Spinner

Fidget-Spinner commented Apr 24, 2025

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On second thought, "slow" is perhaps an understatement. So once this is merged, let's ping Thomas to ask for his permission to backport to 3.13

@chris-eibl

chris-eibl commented Apr 24, 2025 •
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Yeah, in case of the legacy console pasting of the 30 + 3 lines given by the OP takes 15 seconds, and this brings it down to 1.8. Most people will hopefully use a virtual terminal, where the situation is much less severe. After all, the new REPL can be disabled using https://docs.python.org/3/using/cmdline.html#envvar-PYTHON_BASIC_REPL.

3 more lines added by me for easier benchmarking #130328 (comment)

import time
t1 = time.time()
"test value" given in the OP

"""1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890"""

print(time.time() - t1)

@pablogsal

pablogsal commented Apr 25, 2025

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If @ambv has not checked this this in a couple of days ping me and I will do a pass and land it. Thanks a lot for looking into this @chris-eibl ❤️

@chris-eibl chris-eibl mentioned this pull request Apr 25, 2025
Merged
@ambv ambv merged commit acb222c into python:main Apr 29, 2025
@ambv

ambv commented Apr 29, 2025

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Thanks for the fix, Chris!

@Fidget-Spinner

Fidget-Spinner commented Apr 29, 2025

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@Yhg1s seeking permission to backport this to 3.13.

According to Chris, it takes 15 seconds to paste 58 lines in the new repl in 3.13. Do you think that's slow enough to be worth backporting?

@chris-eibl chris-eibl deleted the fix_wait branch May 2, 2025 14:01
@chris-eibl

chris-eibl commented May 20, 2025

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@Yhg1s friendly ping and also asking @ambv whether this is worth backporting to 3.13?

@KristofferC KristofferC mentioned this pull request Oct 9, 2025
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@chris-eibl @sergey-miryanov @Fidget-Spinner @pablogsal @ambv @tomasr8