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From: Chao Li <[email protected]>
To: Joel Jacobson <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: Optimize LISTEN/NOTIFY
Date: Tue, 18 Nov 2025 16:15:23 +0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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Hi Joel,


> On Nov 16, 2025, at 05:53, Joel Jacobson <[email protected]> wrote:
> 
> The attached v28 is the same as v27, except some comments have been
> fixed to accurately reflect the code.
> 
> /Joel<0001-optimize_listen_notify-v28.patch><0002-optimize_listen_notify-v28.patch>

Thanks for the continuous effort on this patch. Finally, I got some time, after revisiting v28 throughoutly, I think it’s much better now. Just got 2 more comments:

1
```
+	if (asyncQueueControl->channelHashDSH == DSHASH_HANDLE_INVALID)
+	{
+		/* Initialize dynamic shared hash table for channel hash */
+		channelDSA = dsa_create(LWTRANCHE_NOTIFY_CHANNEL_HASH);
+		dsa_pin(channelDSA);
+		dsa_pin_mapping(channelDSA);
+		channelHash = dshash_create(channelDSA, &channelDSHParams, NULL);
+
+		/* Store handles in shared memory for other backends to use */
+		asyncQueueControl->channelHashDSA = dsa_get_handle(channelDSA);
+		asyncQueueControl->channelHashDSH =
+			dshash_get_hash_table_handle(channelHash);
+	}
+	else if (!channelHash)
+	{
+		/* Attach to existing dynamic shared hash table */
+		channelDSA = dsa_attach(asyncQueueControl->channelHashDSA);
+		dsa_pin_mapping(channelDSA);
+		channelHash = dshash_attach(channelDSA, &channelDSHParams,
+									asyncQueueControl->channelHashDSH,
+									NULL);
+	}
```

DSA is created and pinned by the first backend and every backend isa_in_mapping, but I don’t see any unpin, is it a problem? If unpin is not needed, why are they provided?

2
```
+			entry = dshash_find(channelHash, &key, false);
+		}
+
+		if (entry == NULL)
+			continue;			/* No listeners registered for this channel */
+
+		listeners = (ProcNumber *) dsa_get_address(channelDSA,
+												   entry->listenersArray);
+
+		for (int j = 0; j < entry->numListeners; j++)
+		{
+			ProcNumber	i = listeners[j];
+			int32		pid;
+			QueuePosition pos;
+
+			if (QUEUE_BACKEND_WAKEUP_PENDING(i))
+				continue;
+
+			pos = QUEUE_BACKEND_POS(i);
+			pid = QUEUE_BACKEND_PID(i);
+
+			/* Skip if caught up */
 			if (QUEUE_POS_EQUAL(pos, QUEUE_HEAD))
 				continue;
+
+			Assert(pid != InvalidPid);
+
+			QUEUE_BACKEND_WAKEUP_PENDING(i) = true;
+			pids[count] = pid;
+			procnos[count] = i;
+			count++;
 		}
-		else
+
+		dshash_release_lock(channelHash, entry);
```

SignalBackends() now holds the dshash entry lock for long time, while other backend’s LISTEN/UNLISTEN all needs to acquire the lock. So, my suggestion is to copy the listeners array to local then quickly release the lock.


Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/

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