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GeezerVison is showing github.com — gh-141498: Change backoff counter to use prime numbers instead of powers of 2 by efimov-mikhail · Pull Request #141591 · python/cpython

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efimov-mikhail commented Nov 15, 2025 •

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Issue: Change backoff counter to have larger range and backoff more quickly. #141498

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efimov-mikhail added 4 commits

November 15, 2025 13:34


          Some refactor

86de63a


          Changes in backoff bit pattern

4eb3231


          Add NEWS


          NEWS fix

96de415

bedevere-app Bot added the awaiting review label

Nov 15, 2025

bedevere-app Bot mentioned this pull request

Nov 15, 2025

Change backoff counter to have larger range and backoff more quickly. #141498

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efimov-mikhail requested review from brandtbucher and markshannon

November 15, 2025 11:14

efimov-mikhail marked this pull request as draft

November 15, 2025 11:30

bedevere-app Bot removed the awaiting review label

Nov 15, 2025


          Change ITEMS count in test

ce3e14f

efimov-mikhail commented

Nov 15, 2025

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Lib/test/test_opcache.py Resolved

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efimov-mikhail marked this pull request as ready for review

November 15, 2025 12:45

bedevere-app Bot added the awaiting review label

Nov 15, 2025

markshannon reviewed

Nov 19, 2025

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Looks good, but I think it is worth using a table.
Using a table allows us to tweak the values to get better performance profile in future, and is easier to understand.

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Include/internal/pycore_backoff.h Outdated

		int backoff = counter.value_and_backoff & BACKOFF_MASK;
		if (backoff < MAX_BACKOFF) {
		return make_backoff_counter((1 << (backoff + 1)) - 1, backoff + 1);
		return make_backoff_counter((1 << (2 * backoff + 3)) - 1, backoff + 1);

markshannon Nov 19, 2025

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Since we are changing this, we might as well use a table. It is only 8 entries of 2 bytes each.

See #141498 (comment)
We want 0 to map to 1, and MAX_BACKOFF to map to near 8k.

I think you can make this branchless as well. Something like:

assert(backoff <= MAX_BACKOFF);
value = LOOKUP_TABLE[backoff]
backoff = backoff == MAX_BACKOFF ? backoff : backoff + 1;
return make_backoff_counter(value, backoff);

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markshannon Nov 19, 2025

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In fact, if we store the counter, both value and backoff, in the table. It can be even simpler:

assert(backoff <= MAX_BACKOFF);
return LOOKUP_TABLE[backoff];

And have LOOKUP_TABLE[UNREACHABLE_BACKOFF] == LOOKUP_TABLE[MAX_BACKOFF] so non-debug builds do something sensible.

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efimov-mikhail Nov 19, 2025

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Of course, I'll add using LOOKUP_TABLE in this PR.

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efimov-mikhail changed the title ~~gh-141498: Change backoff counter to use powers of 4 instead of powers of 2~~ gh-141498: Change backoff counter to use prime numbers instead of powers of 2

Nov 19, 2025

efimov-mikhail added 14 commits

November 19, 2025 16:28


          Refactor restart_backoff_counter

e17b195


          Remove unneed assertion and function

44b4eb8


          Use lookup_table

6d44333


          Use MAKE_BACKOFF_COUNTER macro

0e05712


          Comment change

f3c9287


          Fix Windows build: remove const keyword

e16b368


          Simplify obtaining next backoff

6b1fe66


          Remove forge_backoff_counter


          Another try to fix Windows build

ec27072


          Revert make_backoff_counter

401b6da


          Revert changes to pycore_code.h

3418be1


          Comment fix

ceddb6a


          Temporarily disable assert

a89d6de


          Revert forge_backoff_counter

c7873c2


          Fix CI, NEWS updated

5b7bfd9

efimov-mikhail requested a review from markshannon

November 19, 2025 17:54

sergey-miryanov reviewed

Nov 19, 2025

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Include/internal/pycore_backoff.h Resolved

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sergey-miryanov approved these changes

Nov 20, 2025

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LGTM

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Nov 20, 2025

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Nov 20, 2025

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The code looks good, but the values in the table need to be moved one position:

Instead of 6, 30, ... , 8190, 8190, 8190 it should be 1, 6, 30, ... , 8190, 8190

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Include/internal/pycore_backoff.h Outdated

		// We only use values x and backoffs b such that
		// x + 1 is near to 2*(2b+1) and x + 1 is prime.
		static const uint16_t value_and_backoff_next[] = {
		MAKE_VALUE_AND_BACKOFF(6, 1),

markshannon Nov 20, 2025

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This needs to be MAKE_VALUE_AND_BACKOFF(1, 1) so that code is re-specialized quickly when the cooldown counter expires.
See https://github.com/python/cpython/blob/main/Include/internal/pycore_code.h#L445

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bedevere-app Bot added awaiting changes and removed awaiting core review labels

Nov 20, 2025

bedevere-app Bot commented Nov 20, 2025

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A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

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          Review addressed: change values and comment

dda4cdb

efimov-mikhail commented Nov 20, 2025

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Oops, sorry for this mistake.
I've commited fix to values and small change for comments.

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Nov 20, 2025

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Looks good now.
Thanks

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bedevere-app Bot added awaiting merge and removed awaiting changes labels

Nov 20, 2025

efimov-mikhail commented Nov 20, 2025

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CC @Fidget-Spinner

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markshannon merged commit a3b78a3 into python:main

Nov 21, 2025

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bedevere-app Bot removed the awaiting merge label

Nov 21, 2025

StanFromIreland pushed a commit to StanFromIreland/cpython that referenced this pull request

Dec 6, 2025


          pythongh-141498: Change backoff counter to use prime numbers instead …

0fcedd2

…of powers of 2 (pythonGH-141591)

ashm-dev pushed a commit to ashm-dev/cpython that referenced this pull request

Dec 8, 2025


          pythongh-141498: Change backoff counter to use prime numbers instead …

6387a5e

…of powers of 2 (pythonGH-141591)

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efimov-mikhail commented Nov 15, 2025 • edited by bedevere-app Bot Loading Uh oh! There was an error while loading. Please reload this page.

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efimov-mikhail commented Nov 15, 2025 •

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