From 7c803ff8e9e4025e690ebc8529792a79017efa78 Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Wed, 19 Mar 2025 10:15:20 -0400
Subject: [PATCH v2.13 16/28] Enable IO concurrency on all systems

Previously effective_io_concurrency and maintenance_io_concurrency could
not be set above 0 on machines without fadvise support. AIO enables
IO concurrency without such support.

Currently only subsystems using the read stream API will take advantage
of this. Other users of maintenance_io_concurrency (like recovery
prefetching) which leverage OS advice directly will not benefit from
this change. In those cases, maintenance_io_concurrency will have no
effect on I/O behavior.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/CAAKRu_atGgZePo=_g6T3cNtfMf0QxpvoUh5OUqa_cnPdhLd=gw@mail.gmail.com
---
 src/include/storage/bufmgr.h                  |  6 ----
 src/include/utils/guc_hooks.h                 |  4 ---
 src/backend/access/common/reloptions.c        |  8 -----
 src/backend/commands/variable.c               | 30 -------------------
 src/backend/utils/misc/guc_tables.c           |  4 +--
 src/backend/utils/misc/postgresql.conf.sample |  4 +--
 src/bin/initdb/initdb.c                       |  5 ----
 doc/src/sgml/config.sgml                      | 16 +++++-----
 doc/src/sgml/ref/alter_tablespace.sgml        |  2 +-
 doc/src/sgml/ref/create_tablespace.sgml       |  2 +-
 10 files changed, 14 insertions(+), 67 deletions(-)

diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 7abb3bdfd9d..784df8b00cb 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -156,14 +156,8 @@ extern PGDLLIMPORT int bgwriter_lru_maxpages;
 extern PGDLLIMPORT double bgwriter_lru_multiplier;
 extern PGDLLIMPORT bool track_io_timing;
 
-/* only applicable when prefetching is available */
-#ifdef USE_PREFETCH
 #define DEFAULT_EFFECTIVE_IO_CONCURRENCY 16
 #define DEFAULT_MAINTENANCE_IO_CONCURRENCY 16
-#else
-#define DEFAULT_EFFECTIVE_IO_CONCURRENCY 0
-#define DEFAULT_MAINTENANCE_IO_CONCURRENCY 0
-#endif
 extern PGDLLIMPORT int effective_io_concurrency;
 extern PGDLLIMPORT int maintenance_io_concurrency;
 
diff --git a/src/include/utils/guc_hooks.h b/src/include/utils/guc_hooks.h
index 0f1e74f96c9..799fa7ace68 100644
--- a/src/include/utils/guc_hooks.h
+++ b/src/include/utils/guc_hooks.h
@@ -61,8 +61,6 @@ extern bool check_default_text_search_config(char **newval, void **extra, GucSou
 extern void assign_default_text_search_config(const char *newval, void *extra);
 extern bool check_default_with_oids(bool *newval, void **extra,
 									GucSource source);
-extern bool check_effective_io_concurrency(int *newval, void **extra,
-										   GucSource source);
 extern bool check_huge_page_size(int *newval, void **extra, GucSource source);
 extern void assign_io_method(int newval, void *extra);
 extern bool check_io_max_concurrency(int *newval, void **extra, GucSource source);
@@ -83,8 +81,6 @@ extern bool check_log_stats(bool *newval, void **extra, GucSource source);
 extern bool check_log_timezone(char **newval, void **extra, GucSource source);
 extern void assign_log_timezone(const char *newval, void *extra);
 extern const char *show_log_timezone(void);
-extern bool check_maintenance_io_concurrency(int *newval, void **extra,
-											 GucSource source);
 extern void assign_maintenance_io_concurrency(int newval, void *extra);
 extern void assign_io_max_combine_limit(int newval, void *extra);
 extern void assign_io_combine_limit(int newval, void *extra);
diff --git a/src/backend/access/common/reloptions.c b/src/backend/access/common/reloptions.c
index 645b5c00467..46c1dce222d 100644
--- a/src/backend/access/common/reloptions.c
+++ b/src/backend/access/common/reloptions.c
@@ -361,11 +361,7 @@ static relopt_int intRelOpts[] =
 			RELOPT_KIND_TABLESPACE,
 			ShareUpdateExclusiveLock
 		},
-#ifdef USE_PREFETCH
 		-1, 0, MAX_IO_CONCURRENCY
-#else
-		0, 0, 0
-#endif
 	},
 	{
 		{
@@ -374,11 +370,7 @@ static relopt_int intRelOpts[] =
 			RELOPT_KIND_TABLESPACE,
 			ShareUpdateExclusiveLock
 		},
-#ifdef USE_PREFETCH
 		-1, 0, MAX_IO_CONCURRENCY
-#else
-		0, 0, 0
-#endif
 	},
 	{
 		{
diff --git a/src/backend/commands/variable.c b/src/backend/commands/variable.c
index f550a3c0c63..d3567aa7f27 100644
--- a/src/backend/commands/variable.c
+++ b/src/backend/commands/variable.c
@@ -1145,7 +1145,6 @@ check_cluster_name(char **newval, void **extra, GucSource source)
 void
 assign_maintenance_io_concurrency(int newval, void *extra)
 {
-#ifdef USE_PREFETCH
 	/*
 	 * Reconfigure recovery prefetching, because a setting it depends on
 	 * changed.
@@ -1153,7 +1152,6 @@ assign_maintenance_io_concurrency(int newval, void *extra)
 	maintenance_io_concurrency = newval;
 	if (AmStartupProcess())
 		XLogPrefetchReconfigure();
-#endif
 }
 
 /*
@@ -1249,34 +1247,6 @@ check_default_with_oids(bool *newval, void **extra, GucSource source)
 	return true;
 }
 
-bool
-check_effective_io_concurrency(int *newval, void **extra, GucSource source)
-{
-#ifndef USE_PREFETCH
-	if (*newval != 0)
-	{
-		GUC_check_errdetail("\"%s\" must be set to 0 on platforms that lack support for issuing read-ahead advice.",
-							"effective_io_concurrency");
-		return false;
-	}
-#endif							/* USE_PREFETCH */
-	return true;
-}
-
-bool
-check_maintenance_io_concurrency(int *newval, void **extra, GucSource source)
-{
-#ifndef USE_PREFETCH
-	if (*newval != 0)
-	{
-		GUC_check_errdetail("\"%s\" must be set to 0 on platforms that lack support for issuing read-ahead advice.",
-							"maintenance_io_concurrency");
-		return false;
-	}
-#endif							/* USE_PREFETCH */
-	return true;
-}
-
 bool
 check_ssl(bool *newval, void **extra, GucSource source)
 {
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 989825d3a9c..5d729102f46 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -3245,7 +3245,7 @@ struct config_int ConfigureNamesInt[] =
 		&effective_io_concurrency,
 		DEFAULT_EFFECTIVE_IO_CONCURRENCY,
 		0, MAX_IO_CONCURRENCY,
-		check_effective_io_concurrency, NULL, NULL
+		NULL, NULL, NULL
 	},
 
 	{
@@ -3259,7 +3259,7 @@ struct config_int ConfigureNamesInt[] =
 		&maintenance_io_concurrency,
 		DEFAULT_MAINTENANCE_IO_CONCURRENCY,
 		0, MAX_IO_CONCURRENCY,
-		check_maintenance_io_concurrency, assign_maintenance_io_concurrency,
+		NULL, assign_maintenance_io_concurrency,
 		NULL
 	},
 
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 2246ccb85a7..ac6ae0a9489 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -198,8 +198,8 @@
 # - I/O -
 
 #backend_flush_after = 0		# measured in pages, 0 disables
-#effective_io_concurrency = 16		# 1-1000; 0 disables prefetching
-#maintenance_io_concurrency = 16	# 1-1000; 0 disables prefetching
+#effective_io_concurrency = 16		# 1-1000; 0 disables issuing multiple simultaneous IO requests
+#maintenance_io_concurrency = 16	# 1-1000; same as effective_io_concurrency
 #io_max_combine_limit = 128kB		# usually 1-128 blocks (depends on OS)
 					# (change requires restart)
 #io_combine_limit = 128kB		# usually 1-128 blocks (depends on OS)
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index 22b7d31b165..c17fda2bc81 100644
--- a/src/bin/initdb/initdb.c
+++ b/src/bin/initdb/initdb.c
@@ -1402,11 +1402,6 @@ setup_config(void)
 								  repltok, true);
 #endif
 
-#ifndef USE_PREFETCH
-	conflines = replace_guc_value(conflines, "effective_io_concurrency",
-								  "0", true);
-#endif
-
 #ifdef WIN32
 	conflines = replace_guc_value(conflines, "update_process_title",
 								  "off", true);
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 13773c05ff4..370cda13553 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -2585,8 +2585,7 @@ include_dir 'conf.d'
          session attempts to initiate in parallel.  The allowed range is
          <literal>1</literal> to <literal>1000</literal>, or
          <literal>0</literal> to disable issuance of asynchronous I/O requests.
-         The default is <literal>16</literal> on supported systems, otherwise
-         <literal>0</literal>.
+         The default is <literal>16</literal>.
         </para>
 
         <para>
@@ -2597,8 +2596,9 @@ include_dir 'conf.d'
         </para>
 
         <para>
-         On systems without prefetch advice support, attempting to configure
-         any value other than <literal>0</literal> will error out.
+         On systems with prefetch advice support,
+         <varname>effective_io_concurrency</varname> also controls the
+         prefetch distance.
         </para>
 
         <para>
@@ -2621,10 +2621,10 @@ include_dir 'conf.d'
          for maintenance work that is done on behalf of many client sessions.
         </para>
         <para>
-         The default is <literal>16</literal> on supported systems, otherwise
-         <literal>0</literal>.  This value can be overridden for tables in a
-         particular tablespace by setting the tablespace parameter of the same
-         name (see <xref linkend="sql-altertablespace"/>).
+         The default is <literal>16</literal>.  This value can be overridden
+         for tables in a particular tablespace by setting the tablespace
+         parameter of the same name (see <xref
+         linkend="sql-altertablespace"/>).
         </para>
        </listitem>
       </varlistentry>
diff --git a/doc/src/sgml/ref/alter_tablespace.sgml b/doc/src/sgml/ref/alter_tablespace.sgml
index 6ec863400d1..d0e08089ddb 100644
--- a/doc/src/sgml/ref/alter_tablespace.sgml
+++ b/doc/src/sgml/ref/alter_tablespace.sgml
@@ -88,7 +88,7 @@ ALTER TABLESPACE <replaceable>name</replaceable> RESET ( <replaceable class="par
       and <varname>maintenance_io_concurrency</varname>.
       Setting these values for a particular tablespace will override the
       planner's usual estimate of the cost of reading pages from tables in
-      that tablespace, and the executor's prefetching behavior, as established
+      that tablespace, and how many concurrent I/Os are issued, as established
       by the configuration parameters of the
       same name (see <xref linkend="guc-seq-page-cost"/>,
       <xref linkend="guc-random-page-cost"/>,
diff --git a/doc/src/sgml/ref/create_tablespace.sgml b/doc/src/sgml/ref/create_tablespace.sgml
index 9d5ab025261..b77e774c53f 100644
--- a/doc/src/sgml/ref/create_tablespace.sgml
+++ b/doc/src/sgml/ref/create_tablespace.sgml
@@ -110,7 +110,7 @@ CREATE TABLESPACE <replaceable class="parameter">tablespace_name</replaceable>
         and <varname>maintenance_io_concurrency</varname>.
         Setting these values for a particular tablespace will override the
         planner's usual estimate of the cost of reading pages from tables in
-        that tablespace, and the executor's prefetching behavior, as established
+        that tablespace, and how many concurrent I/Os are issued, as established
         by the configuration parameters of the
         same name (see <xref linkend="guc-seq-page-cost"/>,
         <xref linkend="guc-random-page-cost"/>,
-- 
2.48.1.76.g4e746b1a31.dirty

