[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[h-vetinari]

Change by h-vetinari :


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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[Dong-hee Na]

Change by Dong-hee Na :


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pull_requests: +23375
pull_request: https://github.com/python/cpython/pull/24597

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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[Dong-hee Na]

Change by Dong-hee Na :


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pull_requests: +23374
pull_request: https://github.com/python/cpython/pull/24596

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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

STINNER Victor  added the comment:


New changeset 62078101ea1be5d2fc472a3f0d9d135e0bd5cd38 by Victor Stinner in 
branch 'master':
bpo-40522: Replace PyThreadState_GET() with PyThreadState_Get() (GH-24575)
https://github.com/python/cpython/commit/62078101ea1be5d2fc472a3f0d9d135e0bd5cd38


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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

Change by STINNER Victor :


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pull_requests: +23354
pull_request: https://github.com/python/cpython/pull/24575

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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[hai shi]

Change by hai shi :


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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[Dong-hee Na]

Change by Dong-hee Na :


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nosy: +corona10

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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

STINNER Victor  added the comment:

One GIL per interpreter requires to store the tstate per thread. I don't see 
any other option. We need to replace the global _PyRuntime atomic variable with 
a TLS variable. I'm trying to reduce the overhead, but it's heard to beat the 
performance of an atomic variable.

That's also we I modified many functions to pass explicitly tstate to 
subfunctions in internal C functions, to avoid any possible overhead of getting 
tstate.

https://vstinner.github.io/cpython-pass-tstate.html


Pablo:
> In MacOS is quite challenging to activate LTO, so normally optimized builds 
> are only done with PGO.

Oh right, I forgot macOS. I should check how TLS is compiled on macOS. IMO wwo 
MOV instead of MOV is not a major performance bottleneck.

The best would be to be able to avoid pthread_getspecific() function which is 
less efficient than a TLS variable. The glibc implementation uses an array for 
a few variables (first 32 variables?) and then a slower hash table.


Pablo:
> Also in Windows I am not sure is possible to use LTO. Same for many other 
> platforms.

I will check how it's implemented on Windows.

We cannot use TLS on all platforms, since it requires C11 features which are 
not available on all platforms. Also, the implementation depends on the 
architecture.

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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

> An optimized Python should always be built with LTO.

In MacOS is quite challenging to activate LTO, so normally optimized builds are 
only done with PGO. Also in Windows I am not sure is possible to use LTO. Same 
for many other platforms

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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

STINNER Victor  added the comment:

Mark Shannon experiment using __thread:

* 
https://mail.python.org/archives/list/python-...@python.org/thread/RPSTDB6AEMIACJFZKCKIRFTVLAJQLAS2/

* 
https://github.com/python/cpython/compare/master...markshannon:threadstate_in_tls

He added " extern __thread struct _ts *_Py_tls_tstate;" to 
Include/cpython/pystate.h.

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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

STINNER Victor  added the comment:

PR 23976 should make _PyInterpreterState_GET() more efficient, and it prepares 
the code base for one GIL per interpreter (which is purpose of this issue ;-)).

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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

STINNER Victor  added the comment:

PR 23976 stores the currrent interpreter and the current Python thread state 
into a Thread Local Storage (TLS) using GCC/clang __thread keyword.

On x86-64 using LTO and GCC -O3, _PyThreadState_GET() and 
_PyInterpreterState_GET() become a single MOV.

Assembly code using LTO and gcc -O3.

_PyThreadState_GET() in _PySys_GetObjectId():

   0x004adabe <+14>: mov rbx,QWORD PTR fs:0xfff8

_PyThreadState_GET() in PyThreadState_Get():

   0x0046b660 <+0>: mov  rax,QWORD PTR fs:0xfff8

_PyInterpreterState_GET() in PyTuple_New():

   0x0048dfcc <+12>: mov  rax,QWORD PTR fs:0xfff0

_PyInterpreterState_GET() in PyState_FindModule():

   0x0044bf20 <+16>: mov  rax,QWORD PTR fs:0xfff0

---

Note: Without LTO, sometimes there is an indirection:

_PyThreadState_GET() in _PySys_GetObjectId(), 2 MOV (PIC indirection):

   movrax,QWORD PTR [rip+0x1eb270]# 0x713fe0
   # rax = 0xfff0 (-16)
   movr13,QWORD PTR fs:[rax]

_PyInterpreterState_GET() in PyTuple_New(), 2 MOV (PIC indirection):

  movrax,QWORD PTR [rip+0x294d95]# 0x713ff8
  movrax,QWORD PTR fs:[rax]

An optimized Python should always be built with LTO.

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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

Change by STINNER Victor :


--
pull_requests: +22821
pull_request: https://github.com/python/cpython/pull/23976

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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

STINNER Victor  added the comment:

>_PyInterpreterState_GET():
>   movrax,QWORD PTR [rip+0x22a7dd]# 0x743118 <_PyRuntime+568>
>   movrax,QWORD PTR [rax+0x10]

While working on bpo-39465, I wrote PR 20767 to optimize 
_PyInterpreterState_GET(): single instruction instead of two:

* Add _PyRuntimeState.interp_current member: atomic variable
* _PyThreadState_Swap() sets _PyRuntimeState.interp_current

But I failed to measure any performance difference.

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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

STINNER Victor  added the comment:

There are many ways to get the current interpreter (interp) and the current 
Python thread state (tstate).

Public C API, opaque function call:

* PyInterpreterState_Get() (new in Python 3.9)
* PyThreadState_Get()

Internal C API, static inline functions:

* _PyInterpreterState_GET()
* _PyThreadState_GET()

There are so many variants that I wrote notes for myself:
https://pythondev.readthedocs.io/pystate.html

This issue is about optimizing _PyInterpreterState_GET() and 
_PyThreadState_GET() which are supposed to be the most efficient 
implementations.

Currently, _PyInterpreterState_GET() is implemented as 
_PyThreadState_GET()->interp, and _PyThreadState_GET() is implemented as:

_Py_atomic_load_relaxed(_PyRuntime.gilstate.tstate_current)

--

To find the _PyInterpreterState_GET() machine code, I read the 
PyInterpreterState_Get() assembly code (not optimized, it adds tstate==NULL 
test) and PyTuple_New() assembly code, since PyTuple_New() now needs to get the 
current interpreter:

static struct _Py_tuple_state *
get_tuple_state(void)
{
PyInterpreterState *interp = _PyInterpreterState_GET();
return >tuple;
}

To find the _PyThreadState_GET() machine code, I read the PyThreadState_Get() 
assembly code.

I looked at the x86-64 machine code generated by GCC -O3 (no LTO, no PGO, it 
should not be relevant here), using GCC 10.2.1 on Fedora 33.

_PyThreadState_GET():

   movrax,QWORD PTR [rip+0x2292b1]   # 0x743118 <_PyRuntime+568>

_PyInterpreterState_GET():

   movrax,QWORD PTR [rip+0x22a7dd]# 0x743118 <_PyRuntime+568>
   movrax,QWORD PTR [rax+0x10]

By default, Python is built with -fPIC: _PyRuntime variable does not have a 
fixed address.

$ objdump -t ./python|grep '\<_PyRuntime\>'
00742ee0 g O .bss   02a0  _PyRuntime

The "[rip+0x2292b1] # 0x743118 <_PyRuntime+568>" indirection is needed by PIC.

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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[Sebastian Berg]

Change by Sebastian Berg :


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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

STINNER Victor  added the comment:

See also bpo-15751: "Make the PyGILState API compatible with subinterpreters".

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[issue40522] [subinterpreters] Get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

Change by STINNER Victor :


--
components: +Subinterpreters -Interpreter Core
title: Subinterpreters: get the current Python interpreter state from Thread 
Local Storage (autoTSSkey) -> [subinterpreters] Get the current Python 
interpreter state from Thread Local Storage (autoTSSkey)

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[issue40522] Subinterpreters: get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

STINNER Victor  added the comment:


New changeset e838a9324c1719bb917ca81ede8d766b5cb551f4 by Victor Stinner in 
branch 'master':
bpo-40522: _PyThreadState_Swap() sets autoTSSkey (GH-19939)
https://github.com/python/cpython/commit/e838a9324c1719bb917ca81ede8d766b5cb551f4


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[issue40522] Subinterpreters: get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

Change by STINNER Victor :


--
keywords: +patch
pull_requests: +19254
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/19939

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[issue40522] Subinterpreters: get the current Python interpreter state from Thread Local Storage (autoTSSkey)

[STINNER Victor]

New submission from STINNER Victor :

_PyThreadState_GET() gets the current Python thread state from 
_PyRuntime.gilstate.tstate_current atomic variable.

When I experimented per-interpreter GIL (bpo-40512), I got issues with 
_PyThreadState_GET() which didn't return the expected Python thread state.

I propose to modify _PyThreadState_GET() in the exprimental isolated 
subinterpreters mode to get and set the current Python thread state using a 
Thread Local Storage: autoTSSkey.

--
components: Interpreter Core
messages: 368182
nosy: vstinner
priority: normal
severity: normal
status: open
title: Subinterpreters: get the current Python interpreter state from Thread 
Local Storage (autoTSSkey)
versions: Python 3.9

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