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Multithreaded scaling regression in 3.13.1t #131988

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colesbury opened this issue Apr 1, 2025 · 1 comment
Open

Multithreaded scaling regression in 3.13.1t #131988

colesbury opened this issue Apr 1, 2025 · 1 comment
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3.13 bugs and security fixes interpreter-core (Objects, Python, Grammar, and Parser dirs) performance Performance or resource usage release-blocker topic-free-threading type-bug An unexpected behavior, bug, or error

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@colesbury
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colesbury commented Apr 1, 2025 •
edited by bedevere-app bot
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Bug report

Bug description:

#127114 changed the logic to immortalize certain objects when the first thread is created instead of the first non-main thread.

3.13.0:

sgross@devfair0453:/raid/sgross/clean-cpython/Tools/ftscalingbench$ uv run -p 3.13.0t ftscalingbench.py
Running benchmarks with 20 threads
object_cfunction          14.3x faster
cmodule_function          14.3x faster
object_lookup_special     14.3x faster
mult_constant             13.8x faster
generator                 13.8x faster
pymethod                  14.3x faster
pyfunction                14.0x faster
module_function           14.2x faster
load_string_const          2.5x slower
load_tuple_const          13.5x faster
create_pyobject           14.0x faster
create_closure            14.0x faster
create_dict               13.0x faster
thread_local_read          3.0x slower

3.13.1:

sgross@devfair0453:/raid/sgross/clean-cpython/Tools/ftscalingbench$ uv run -p 3.13.1t ftscalingbench.py
Running benchmarks with 20 threads
object_cfunction           1.6x slower
cmodule_function           1.9x slower
object_lookup_special      1.7x slower
mult_constant             14.3x faster
generator                  2.7x faster
pymethod                   2.3x slower
pyfunction                 3.1x slower
module_function            2.1x slower
load_string_const          2.1x slower
load_tuple_const          13.1x faster
create_pyobject            3.6x slower
create_closure             4.3x slower
create_dict               15.0x faster
thread_local_read          3.0x slower

CPython versions tested on:

3.13

Operating systems tested on:

No response

Linked PRs
@colesbury colesbury added performance Performance or resource usage type-bug An unexpected behavior, bug, or error topic-free-threading 3.13 bugs and security fixes labels Apr 1, 2025
colesbury added a commit to colesbury/cpython that referenced this issue Apr 1, 2025
@colesbury
pythongh-131988: Fix a multithreaded scaling regression
22349fd 
The 3.13 free threaded build immortalizes certain objects to avoid
reference count contention. In pythongh-127114 the condition was
unintentionally changed to happen when the first thread was created
instead of the first non-main thread. The `interp->gc.immortalize` field
is then cleared again during `_PyGC_Init()`.

Change the condition so that we check if we should immortalize objects
using deferred reference counting whenever a non-main thread is created.
@bedevere-app bedevere-app bot mentioned this issue Apr 1, 2025
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@vstinner vstinner changed the title Mulitthreaded scaling regression in 3.13.1t Multithreaded scaling regression in 3.13.1t Apr 2, 2025
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vstinner commented Apr 2, 2025

I added "release-blocker" since it would be nice to get this important performance fix in the next 3.13 release.

@picnixz picnixz added the interpreter-core (Objects, Python, Grammar, and Parser dirs) label Apr 4, 2025
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3.13 bugs and security fixes interpreter-core (Objects, Python, Grammar, and Parser dirs) performance Performance or resource usage release-blocker topic-free-threading type-bug An unexpected behavior, bug, or error
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