[issue42197] Disable automatic update of frame locals during tracing

[Fabio Zadrozny]

New submission from Fabio Zadrozny <fabioz at users.sourceforge.net>:

Right now, when a debugger is active, the number of local variables can affect the tracing speed quite a lot.

For instance, having tracing setup in a program such as the one below takes 4.64 seconds to run, yet, changing all the variables to have the same name -- i.e.: change all assignments to `a = 1` (such that there's only a single variable in the namespace), it takes 1.47 seconds (in my machine)... the higher the number of variables, the slower the tracing becomes.

```
import time
t = time.time()

def call():
    a = 1
    b = 1
    c = 1
    d = 1
    e = 1
    f = 1

def noop(frame, event, arg):
    return noop

import sys
sys.settrace(noop)

for i in range(1_000_000):
    call()

print('%.2fs' % (time.time() - t,))
```

This happens because `PyFrame_FastToLocalsWithError` and `PyFrame_LocalsToFast` are called inside the `call_trampoline` (https://github.com/python/cpython/blob/master/Python/sysmodule.c#L946).

So, I'd like to simply remove those calls.

Debuggers can call  `PyFrame_LocalsToFast` when needed -- otherwise mutating non-current frames doesn't work anyways. As a note, pydevd already has such a call: https://github.com/fabioz/PyDev.Debugger/blob/0d4d210f01a1c0a8647178b2e665b53ab113509d/_pydevd_bundle/pydevd_save_locals.py#L57 and PyPy also has a counterpart.

As for `PyFrame_FastToLocalsWithError`, I don't really see any reason to call it at all.

i.e.: something as the code below prints the `a` variable from the `main()` frame regardless of that and I checked all pydevd tests and nothing seems to be affected (it seems that accessing f_locals already does this: https://github.com/python/cpython/blob/cb9879b948a19c9434316f8ab6aba9c4601a8173/Objects/frameobject.c#L35, so, I don't see much reason to call it at all).

```
def call():
    import sys
    frame = sys._getframe()
    print(frame.f_back.f_locals)
   
def main():
    a = 1
    call()
   
if __name__ == '__main__':
    main()
```

So, the idea is removing those lines (I expect that I'll be able to provide a pull request for this).

----------
components: Interpreter Core
messages: 379874
nosy: fabioz
priority: normal
severity: normal
status: open
title: Disable automatic update of frame locals during tracing
type: performance
versions: Python 3.10

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42197>
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