10000 gh-119786: Add InternalDocs/qsbr.md. by nascheme · Pull Request #135411 · python/cpython · GitHub
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Merged
merged 10 commits into from
Jun 23, 2025
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gh-119786: Add InternalDocs/qsbr.md. #135411

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ccea600
Add InternalDocs/qsbr.md.
nascheme Jun 11, 2025
def9494
Update InternalDocs/qsbr.md
nascheme Jun 13, 2025
b192609
Update InternalDocs/qsbr.md
nascheme Jun 13, 2025
19c0efa
Update InternalDocs/qsbr.md
nascheme Jun 20, 2025
c80ac37
Update InternalDocs/qsbr.md
nascheme Jun 20, 2025
25b7140
Fix goal compare condition, format tweaks.
nascheme Jun 20, 2025
b313c1e
Merge 'origin/main' into qsbr-internal-doc
nascheme Jun 20, 2025
f23c1e3
Formatting adjustment based on review.
nascheme Jun 23, 2025
7b08578
More format changes.
nascheme Jun 23, 2025
00439e9
Merge branch 'main' into qsbr-internal-doc
nascheme Jun 23, 2025
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Fix goal compare condition, format tweaks.
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@nascheme
nascheme committed Jun 20, 2025
commit 25b714093e489e31de1db2980cf8ed0a3a140834
12 changes: 6 additions & 6 deletions InternalDocs/qsbr.md
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Expand Up @@ -39,15 +39,15 @@ mechanism to determine when it is safe to free the list's old backing array.

Specific use cases for QSBR include:

* Dictionary keys (PyDictKeysObject) and list arrays (_PyListArray): When a
* Dictionary keys (PyDictKeysObject) and list arrays (`_PyListArray`): When a
dictionary or list that may be shared between threads is resized, we use QSBR
to delay freeing the old keys or array until it's safe. For dicts and lists
that are not shared, their storage can be freed immediately upon resize.

* Mimalloc mi_page_t: Non-locking dictionary and list accesses require
* Mimalloc `mi_page_t`: Non-locking dictionary and list accesses require
cooperation from the memory allocator. If an object is freed and its memory is
reused, we must ensure the new object's reference count field is at the same
memory location. In practice, this means when a mimalloc page (mi_page_t)
memory location. In practice, this means when a mimalloc page (`mi_page_t`)
becomes empty, we don't immediately allow it to be reused for allocations of a
different size class. QSBR is used to determine when it's safe to repurpose the
page or return its memory to the OS.
Expand Down Expand Up @@ -91,9 +91,9 @@ Periodically, a polling mechanism processes this deferred-free list:
1. The minimum read sequence value across all active threads is calculated and
stored as the global `rd_seq`.

2. For each item on the deferred-free list, if its qsbr_goal is less than the
new `rd_seq`, its memory is freed, and it is removed from the list. Otherwise,
it remains on the list for a future attempt.
2. For each item on the deferred-free list, if its qsbr_goal is less than or
equal to the new `rd_seq`, its memory is freed, and it is removed from the
list. Otherwise, it remains on the list for a future attempt.


### Deferred Advance Optimization
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