Description

markshannon

opened on Nov 13, 2025

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The backoff counter, _Py_BackoffCounter, supports exponential backoff and a value up to 4095.
Backoff is exponential in powers 2.
There are two issues with this:

• It is a bit slow to backoff: specialization and JIT optimizer failures are rarely transient, so we are wasting effort with backoffs like 2, 4, 8,...
• It has a relatively low limit of 4095 forcing re-specialization/trace recording every 4095 iteration at most.

The fix is simple, increase the power from 2 to 4. This means we only need 3 bits for the backoff and we get a higher maximum of 8191.
We can also use a lookup table, to ensure that the number of iteration until the next specialization/jit is a prime number

Backoff	Value	Iterations (value + 1)
0	1	2
1	6	7
6	8190	8191
7	UNREACHABLE_BACKOFF

Linked PRs

• gh-141498: Change backoff counter to use powers of 4 instead of powers of 2 #141589
• gh-141498: Change backoff counter to use prime numbers instead of powers of 2 #141591

Reacted by Mikhail Efimov

Reacted by stonebig and Sacul

Activity

1. markshannon

   added

   performancePerformance or resource usagePerformance or resource usage

   topic-JIT

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   3.15bugs and security fixesbugs and security fixes

   on Nov 13, 2025
2. markshannon

   self-assigned this

   on Nov 13, 2025
3. efimov-mikhail commented on Nov 15, 2025

   

   efimov-mikhail

   on Nov 15, 2025

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   I'd like to work on this, if you're not against it, @markshannon.
4. efimov-mikhail commented on Nov 15, 2025

   

   efimov-mikhail

   on Nov 15, 2025

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   I've created a PR with minimal fix: 12 + 4 bits -> 13 + 3 bits, value = 2**backoff - 1 -> value = 2**(2*backoff+1) - 1.
   I can add using prime numbers to the same PR, if we want to.

   CC @Fidget-Spinner
5. markshannon commented on Nov 19, 2025

   

   markshannon

   on Nov 19, 2025

   MemberAuthor

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   I'd like to work on this, if you're not against it, @markshannon.

   I did some work on it, but I'm happy for you to do it. It looks like you already are 🙂

   Reacted by Mikhail Efimov and Savannah Ostrowski
6. markshannon

   added a commit that references this issue on Nov 21, 2025

   gh-141498: Change backoff counter to use prime numbers instead of pow…

   a3b78a3
7. markshannon

   closed this as completedon Nov 21, 2025
8. StanFromIreland

   added a commit that references this issue on Dec 6, 2025

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

   0fcedd2
9. ashm-dev

   added a commit that references this issue on Dec 8, 2025

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

   6387a5e

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