Thread

Re: [PATCH] Avoid pallocs in async.c's SignalBackends critical section

Joel Jacobson <joel@compiler.org> — 2025-11-25T10:15:58Z

On Mon, Nov 24, 2025, at 22:53, Joel Jacobson wrote:
> On Mon, Nov 24, 2025, at 17:06, Tom Lane wrote:
>> Unfortunately, releasing locks, sending notifies, etc is not all
>> that "noncritical" if you want the DB to keep functioning well.
>> But there's a good deal of code in there and making it all obey
>> the critical-section rules looks painful.
>
> I see why a critical-section is probably too painful. But since the
> direction in [1] is to avoid adding new possibly risky operations to
> AtCommit_Notify, I don't think it's completely unreasonable to consider
> moving some existing ones into PreCommit_Notify, when feasible.
>
> If it's preferable, I'm fine dropping this standalone patch and folding
> any such adjustments into v29 in [1], or I can just leave the existing
> code untouched?

With the following three changes, I think the only remaining
potentially-risky code in AtCommit_Notify, is the acquire/release of
locks.

* 0001-async-avoid-pallocs-in-critical-section-v2.patch:
Preallocate signal arrays to avoid pallocs AtCommit.

* 0002-async-avoid-pallocs-in-critical-section-v2.patch:
Move asyncQueueAdvanceTail from AtCommit to PreCommit.

* 0003-async-avoid-pallocs-in-critical-section-v2.patch
Convert listenChannels to hash table.
This is based on Heikki's suggestion

    "We really should turn that into a hash table."

from the bug fix thread [2] combined with Tom's idea of a boolean

    "is it REALLY listening?"

field [1].

Together, these patches allows us to gets rid of the following comments:

0001:
-	 * XXX in principle these pallocs could fail, which would be bad. Maybe
-	 * preallocate the arrays?  They're not that large, though.

0002:
- * This is (usually) called during CommitTransaction(), so it's important for
- * it to have very low probability of failure.

0003:
-	 * XXX It is theoretically possible to get an out-of-memory failure here,
-	 * which would be bad because we already committed.  For the moment it
-	 * doesn't seem worth trying to guard against that, but maybe improve this
-	 * later.

Please advise if we want these changes, and if so, if they should be
folded into [1] i.e. closing this thread, or if we want to keep this thread.

/Joel

[1] https://www.postgresql.org/message-id/flat/6899c044-4a82-49be-8117-e6f669765f7e@app.fastmail.com

[2] https://www.postgresql.org/message-id/flat/CAK98qZ3wZLE-RZJN_Y%2BTFjiTRPPFPBwNBpBi5K5CU8hUHkzDpw%40mail.gmail.com