Date: Tue, 7 Jan 2025 11:23:47 -0600
From: Nathan Bossart <nathandbossart@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andres Freund <andres@anarazel.de>, pgsql-hackers@lists.postgresql.org,
	"Imseih (AWS), Sami" <simseih@amazon.com>,
	Justin Pryzby <pryzby@telsasoft.com>,
	"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: allow changing autovacuum_max_workers without restarting
Message-ID: <Z31jI10tLLd6thfu@nathan>
References: <Z3xFE7DQTSRJw2FM@nathan>
 <1346002.1736198977@sss.pgh.pa.us>
 <Z3xQIPX3aeri4DiT@nathan>
 <Z3xV_b3wp-XQR3Rm@nathan>
 <7B8590B5-3A23-4CF6-B21D-4AA8C0177C2C@anarazel.de>
 <1353350.1736202977@sss.pgh.pa.us>
 <Z3xlNmC3qtwf1lz1@nathan>
 <1359669.1736206603@sss.pgh.pa.us>
 <Z3yVFfKG_RAh8FgP@nathan>
 <1386858.1736220547@sss.pgh.pa.us>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="fBsFehpR9B6WlT8V"
Content-Disposition: inline
In-Reply-To: <1386858.1736220547@sss.pgh.pa.us>
Archived-At: <https://www.postgresql.org/message-id/Z31jI10tLLd6thfu%40nathan>
Precedence: bulk


--fBsFehpR9B6WlT8V
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline

On Mon, Jan 06, 2025 at 10:29:07PM -0500, Tom Lane wrote:
> +1 for simplicity ... but on reflection, what do you think about
> using max_connections / 6?  That would keep autovacuum_worker_slots
> at 100 / 6 = 16 for the vast majority of systems.  For the worst case
> *BSD machines, we'd select 25 / 6 = 4 which results in consuming one
> more semaphore than where we were yesterday.  I'm willing to accept
> that outcome though, since we still have 3 or so to spare.

WFM.  I'm kicking myself for not having thought of that...

> Other than the specific magic number, your patch LGTM.

Here's a new version of the patch with some small cosmetic changes
(including more commentary about the formula) and the constant changed to
6.  I'll go commit this shortly.

-- 
nathan

--fBsFehpR9B6WlT8V
Content-Type: text/plain; charset=us-ascii
Content-Disposition: attachment;
	filename="v3-0001-Lower-default-value-of-autovacuum_worker_slots-in.patch"

From d8b9fe6d3c166c14d9a8d17f43418be33c4e0784 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Tue, 7 Jan 2025 11:15:22 -0600
Subject: [PATCH v3 1/1] Lower default value of autovacuum_worker_slots in
 initdb as needed.

TODO

Reported-by: Tom Lane
Suggested-by: Andres Freund
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/1346002.1736198977%40sss.pgh.pa.us
---
 doc/src/sgml/config.sgml |  5 +++--
 src/bin/initdb/initdb.c  | 40 ++++++++++++++++++++++++++++++++++------
 2 files changed, 37 insertions(+), 8 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 740ff5d5044..8683f0bdf53 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -8639,8 +8639,9 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       <listitem>
        <para>
         Specifies the number of backend slots to reserve for autovacuum worker
-        processes.  The default is 16.  This parameter can only be set at server
-        start.
+        processes.  The default is typically 16 slots, but might be less if
+        your kernel settings will not support it (as determined during initdb).
+        This parameter can only be set at server start.
        </para>
        <para>
         When changing this value, consider also adjusting
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index 4e4b7ede190..f2b9d50e9b3 100644
--- a/src/bin/initdb/initdb.c
+++ b/src/bin/initdb/initdb.c
@@ -196,6 +196,7 @@ static char *pgdata_native;
 
 /* defaults */
 static int	n_connections = 10;
+static int	n_av_slots = 16;
 static int	n_buffers = 50;
 static const char *dynamic_shared_memory_type = NULL;
 static const char *default_timezone = NULL;
@@ -273,7 +274,8 @@ static void check_input(char *path);
 static void write_version_file(const char *extrapath);
 static void set_null_conf(void);
 static void test_config_settings(void);
-static bool test_specific_config_settings(int test_conns, int test_buffs);
+static bool test_specific_config_settings(int test_conns, int test_av_slots,
+										  int test_buffs);
 static void setup_config(void);
 static void bootstrap_template1(void);
 static void setup_auth(FILE *cmdfd);
@@ -1118,6 +1120,18 @@ test_config_settings(void)
 	 */
 #define MIN_BUFS_FOR_CONNS(nconns)	((nconns) * 10)
 
+	/*
+	 * This macro defines the default value of autovacuum_worker_slots we want
+	 * for a given max_connections value.  Note that it has been carefully
+	 * crafted to provide specific values for the associated values in
+	 * trial_conns.  We want it to return autovacuum_worker_slot's initial
+	 * default value (16) for the maximum value in trial_conns (100), and we
+	 * want it to return close to the minimum value we'd consider (3, which is
+	 * the default of autovacuum_max_workers) for the minimum value in
+	 * trial_conns (25).
+	 */
+#define AV_SLOTS_FOR_CONNS(nconns)	((nconns) / 6)
+
 	static const int trial_conns[] = {
 		100, 50, 40, 30, 25
 	};
@@ -1145,7 +1159,8 @@ test_config_settings(void)
 
 	/*
 	 * Probe for max_connections before shared_buffers, since it is subject to
-	 * more constraints than shared_buffers.
+	 * more constraints than shared_buffers.  We also choose the default
+	 * autovacuum_worker_slots here.
 	 */
 	printf(_("selecting default \"max_connections\" ... "));
 	fflush(stdout);
@@ -1153,9 +1168,10 @@ test_config_settings(void)
 	for (i = 0; i < connslen; i++)
 	{
 		test_conns = trial_conns[i];
+		n_av_slots = AV_SLOTS_FOR_CONNS(test_conns);
 		test_buffs = MIN_BUFS_FOR_CONNS(test_conns);
 
-		if (test_specific_config_settings(test_conns, test_buffs))
+		if (test_specific_config_settings(test_conns, n_av_slots, test_buffs))
 		{
 			ok_buffers = test_buffs;
 			break;
@@ -1167,6 +1183,13 @@ test_config_settings(void)
 
 	printf("%d\n", n_connections);
 
+	/*
+	 * We chose the default for autovacuum_worker_slots during the
+	 * max_connections tests above, but we print a progress message anyway.
+	 */
+	printf(_("selecting default \"autovacuum_worker_slots\" ... %d\n"),
+		   n_av_slots);
+
 	printf(_("selecting default \"shared_buffers\" ... "));
 	fflush(stdout);
 
@@ -1180,7 +1203,7 @@ test_config_settings(void)
 			break;
 		}
 
-		if (test_specific_config_settings(n_connections, test_buffs))
+		if (test_specific_config_settings(n_connections, n_av_slots, test_buffs))
 			break;
 	}
 	n_buffers = test_buffs;
@@ -1200,7 +1223,7 @@ test_config_settings(void)
  * Test a specific combination of configuration settings.
  */
 static bool
-test_specific_config_settings(int test_conns, int test_buffs)
+test_specific_config_settings(int test_conns, int test_av_slots, int test_buffs)
 {
 	PQExpBufferData cmd;
 	_stringlist *gnames,
@@ -1213,10 +1236,11 @@ test_specific_config_settings(int test_conns, int test_buffs)
 	printfPQExpBuffer(&cmd,
 					  "\"%s\" --check %s %s "
 					  "-c max_connections=%d "
+					  "-c autovacuum_worker_slots=%d "
 					  "-c shared_buffers=%d "
 					  "-c dynamic_shared_memory_type=%s",
 					  backend_exec, boot_options, extra_options,
-					  test_conns, test_buffs,
+					  test_conns, test_av_slots, test_buffs,
 					  dynamic_shared_memory_type);
 
 	/* Add any user-given setting overrides */
@@ -1280,6 +1304,10 @@ setup_config(void)
 	conflines = replace_guc_value(conflines, "max_connections",
 								  repltok, false);
 
+	snprintf(repltok, sizeof(repltok), "%d", n_av_slots);
+	conflines = replace_guc_value(conflines, "autovacuum_worker_slots",
+								  repltok, false);
+
 	if ((n_buffers * (BLCKSZ / 1024)) % 1024 == 0)
 		snprintf(repltok, sizeof(repltok), "%dMB",
 				 (n_buffers * (BLCKSZ / 1024)) / 1024);
-- 
2.39.5 (Apple Git-154)


--fBsFehpR9B6WlT8V--