Date: Mon, 6 Jan 2025 17:20:22 -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: <Z3xlNmC3qtwf1lz1@nathan>
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On Mon, Jan 06, 2025 at 05:36:17PM -0500, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
>> How about trying the higher setting first in initdb? On any sane system
>> that won't cost anything because it'll succeed with the higher value.
> 
> That might be a good compromise.

+1, I like the idea.

> You'd have to think about how
> it should interact with initdb's probes for workable values of
> max_connections.  My first thought about that is to have initdb
> set autovacuum_worker_slots to max_connections / 8 or thereabouts
> as it works down the list of max_connections values to try.  Or
> you could do something more complicated, but I don't see a reason
> to make it too complex.

My first instinct was just to set it to the lowest default we'd consider
during the max_connections tests (which I'm assuming is 3 due to the
current default for autovacuum_max_workers).  That way, the max_connections
default won't change from version to version on affected systems, but you
might get some extra autovacuum slots.

-- 
nathan

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	filename="v1-0001-Lower-default-value-of-autovacuum_worker_slots-in.patch"

From e35b4c5425cbe1120d3c38619be4f1523ae1f2c9 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Mon, 6 Jan 2025 17:14:54 -0600
Subject: [PATCH v1 1/1] Lower default value of autovacuum_worker_slots in
 initdb as needed.

---
 doc/src/sgml/config.sgml |  5 +++--
 src/bin/initdb/initdb.c  | 41 +++++++++++++++++++++++++++++++++++-----
 2 files changed, 39 insertions(+), 7 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..1214f1b492a 100644
--- a/src/bin/initdb/initdb.c
+++ b/src/bin/initdb/initdb.c
@@ -197,6 +197,7 @@ static char *pgdata_native;
 /* defaults */
 static int	n_connections = 10;
 static int	n_buffers = 50;
+static int	n_av_slots = 16;
 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_buffs,
+										  int test_av_slots);
 static void setup_config(void);
 static void bootstrap_template1(void);
 static void setup_auth(FILE *cmdfd);
@@ -1118,6 +1120,13 @@ test_config_settings(void)
 	 */
 #define MIN_BUFS_FOR_CONNS(nconns)	((nconns) * 10)
 
+	/*
+	 * This macro defines the minimum default autovacuum_worker_slots we are
+	 * willing to consider.  It should be kept >= the default for
+	 * autovacuum_max_workers.
+	 */
+#define MIN_AV_WORKER_SLOTS	(3)
+
 	static const int trial_conns[] = {
 		100, 50, 40, 30, 25
 	};
@@ -1155,7 +1164,9 @@ test_config_settings(void)
 		test_conns = trial_conns[i];
 		test_buffs = MIN_BUFS_FOR_CONNS(test_conns);
 
-		if (test_specific_config_settings(test_conns, test_buffs))
+		if (test_specific_config_settings(test_conns,
+										  test_buffs,
+										  MIN_AV_WORKER_SLOTS))
 		{
 			ok_buffers = test_buffs;
 			break;
@@ -1180,7 +1191,9 @@ test_config_settings(void)
 			break;
 		}
 
-		if (test_specific_config_settings(n_connections, test_buffs))
+		if (test_specific_config_settings(n_connections,
+										  test_buffs,
+										  MIN_AV_WORKER_SLOTS))
 			break;
 	}
 	n_buffers = test_buffs;
@@ -1190,6 +1203,19 @@ test_config_settings(void)
 	else
 		printf("%dkB\n", n_buffers * (BLCKSZ / 1024));
 
+	printf(_("selecting default \"autovacuum_worker_slots\" ... "));
+	fflush(stdout);
+
+	for (; n_av_slots > MIN_AV_WORKER_SLOTS; n_av_slots--)
+	{
+		if (test_specific_config_settings(n_connections,
+										  n_buffers,
+										  n_av_slots))
+			break;
+	}
+
+	printf("%d\n", n_av_slots);
+
 	printf(_("selecting default time zone ... "));
 	fflush(stdout);
 	default_timezone = select_default_timezone(share_path);
@@ -1200,7 +1226,7 @@ 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_buffs, int test_av_slots)
 {
 	PQExpBufferData cmd;
 	_stringlist *gnames,
@@ -1214,9 +1240,10 @@ test_specific_config_settings(int test_conns, int test_buffs)
 					  "\"%s\" --check %s %s "
 					  "-c max_connections=%d "
 					  "-c shared_buffers=%d "
+					  "-c autovacuum_worker_slots=%d "
 					  "-c dynamic_shared_memory_type=%s",
 					  backend_exec, boot_options, extra_options,
-					  test_conns, test_buffs,
+					  test_conns, test_buffs, test_av_slots,
 					  dynamic_shared_memory_type);
 
 	/* Add any user-given setting overrides */
@@ -1289,6 +1316,10 @@ setup_config(void)
 	conflines = replace_guc_value(conflines, "shared_buffers",
 								  repltok, false);
 
+	snprintf(repltok, sizeof(repltok), "%d", n_av_slots);
+	conflines = replace_guc_value(conflines, "autovacuum_worker_slots",
+								  repltok, false);
+
 	conflines = replace_guc_value(conflines, "lc_messages",
 								  lc_messages, false);
 
-- 
2.39.5 (Apple Git-154)


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