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From: Thomas Munro <[email protected]>
To: Joel Jacobson <[email protected]>
Cc: pgsql-hackers <[email protected]>
Cc: Heikki Linnakangas <[email protected]>
Cc: Rishu Bagga <[email protected]>
Subject: Re: Optimize LISTEN/NOTIFY
Date: Wed, 23 Jul 2025 14:44:29 +1200
Message-ID: <CA+hUKGLrMGkWDB0cwTa0RqD+AF7O-Ywgck8aVYKwOQnZgYRRug@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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On Wed, Jul 23, 2025 at 1:39 PM Joel Jacobson <[email protected]> wrote:
> In their patch, in asyn.c's SignalBackends(), they do
> SendInterrupt(INTERRUPT_ASYNC_NOTIFY, procno) instead of
> SendProcSignal(pid, PROCSIG_NOTIFY_INTERRUPT, procnos[i]). They don't
> seem to check if the backend is already signalled or not, but maybe
> SendInterrupt() has signal coalescing built-in so it would be a noop
> with almost no cost?

Yeah:

+ old_pending = pg_atomic_fetch_or_u32(&proc->pendingInterrupts, interruptMask);
+
+ /*
+ * If the process is currently blocked waiting for an interrupt to arrive,
+ * and the interrupt wasn't already pending, wake it up.
+ */
+ if ((old_pending & (interruptMask | SLEEPING_ON_INTERRUPTS)) ==
SLEEPING_ON_INTERRUPTS)
+     WakeupOtherProc(proc);

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