[prev in list] [next in list] [prev in thread] [next in thread] 

List:       python-capi-sig
Subject:    [capi-sig] Re: How to access the various levels of runtime state (e.g. PyInterpreterState, _PyRuntim
From:       Victor Stinner <vstinner () redhat ! com>
Date:       2019-05-24 21:26:04
Message-ID: CA+3bQGH-dyMWe9RxNFw+YqkPi3Oq8Pyx9wcvKigGAsLO_=TKwg () mail ! gmail ! com
[Download RAW message or body]

Le ven. 24 mai 2019 à 19:29, Steve Dower <steve.dower@python.org> a écrit :
> > What do you plan to put in such PyContext?
>
> We hadn't figured that out yet, but given that Yury found we needed
> contextvars because thread locals weren't sufficient after many years, I
> don't want to assume that the thread state is sufficient.
>
> If we always pass the context struct by pointer, we can even expand its
> contents without breaking existing code. It just seems like a good
> engineering design to allow for future growth here.

If we pass a structure by copy, the structure should be copied to
every call. It might have a negative impact on performances,
especially if the structure contains multiple fields.

If we pass the structure by reference (pointer), it would add yet
another indirection to every memory access to the context. It can also
have an impact on performance. By the way, my first implementation of
_PyBytesWriter was to put everything in a single structure, like OOP.
But it was way more inefficient. I had hard time to understand why
simply moving pointers into registers made the code way more
inefficient. It's something about aliasing... I took some notes there:

https://vstinner.github.io/pybyteswriter.html
https://bugs.python.org/issue17742
https://gcc.gnu.org/ml/gcc-help/2013-04/msg00192.html

Since this bad experience, I'm less excited by structures for hot code path :-(

--

In Python 3.7, PyThreadState_GET() is implemented as:
_Py_atomic_load_relaxed(&_PyThreadState_Current).

In Python 3.8, it's implemented as
_Py_atomic_load_relaxed(&_PyRuntime.gilstate.tstate_current).

_Py_atomic_load_relaxed() should be efficient, but it's called very
frequently and it requires memory fences. I expect that not reading an
atomic variable by passing directly 'tstate' is more efficient.

Victor
-- 
Night gathers, and now my watch begins. It shall not end until my death.
_______________________________________________
capi-sig mailing list -- capi-sig@python.org
To unsubscribe send an email to capi-sig-leave@python.org

[prev in list] [next in list] [prev in thread] [next in thread]