From ea59d2ebde818ddc2a9111858b3d956cbcc7bff2 Mon Sep 17 00:00:00 2001
From: Andrey Borodin <amborodin@acm.org>
Date: Mon, 15 Feb 2021 21:51:56 +0500
Subject: [PATCH v=18 1/3] Make all SLRU buffer sizes configurable.

Provide new GUCs to set the number of buffers, instead of using hard
coded defaults.

Remove the limits on xact_buffers and commit_ts_buffers.  The default
sizes for those caches are ~0.2% and ~0.1% of shared_buffers, as before,
but now there is no cap at 128 and 16 buffers respectively (unless
track_commit_timestamp is disabled, in the latter case, then we might as
well keep it tiny).  Sizes much larger than the old limits have been
shown to be useful on modern systems, and an earlier commit replaced a
linear search with a hash table to avoid problems with extreme cases.

Author: Andrey M. Borodin <x4mmm@yandex-team.ru>
Reviewed-by: Anastasia Lubennikova <a.lubennikova@postgrespro.ru>
Reviewed-by: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: Gilles Darold <gilles@darold.net>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/2BEC2B3F-9B61-4C1D-9FB5-5FAB0F05EF86%40yandex-team.ru
---
 doc/src/sgml/config.sgml                      | 135 ++++++++++++++++++
 src/backend/access/transam/clog.c             |  23 ++-
 src/backend/access/transam/commit_ts.c        |   5 +
 src/backend/access/transam/multixact.c        |   8 +-
 src/backend/access/transam/subtrans.c         |   5 +-
 src/backend/commands/async.c                  |   8 +-
 src/backend/storage/lmgr/predicate.c          |   4 +-
 src/backend/utils/init/globals.c              |   8 ++
 src/backend/utils/misc/guc.c                  |  99 +++++++++++++
 src/backend/utils/misc/postgresql.conf.sample |   9 ++
 src/include/access/clog.h                     |  10 ++
 src/include/access/commit_ts.h                |   1 -
 src/include/access/multixact.h                |   4 -
 src/include/access/slru.h                     |   5 +
 src/include/access/subtrans.h                 |   2 -
 src/include/commands/async.h                  |   5 -
 src/include/miscadmin.h                       |   7 +
 src/include/storage/predicate.h               |   4 -
 18 files changed, 299 insertions(+), 43 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index afbb6c35e30..57d9696abe8 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1952,6 +1952,141 @@ include_dir 'conf.d'
        </para>
       </listitem>
      </varlistentry>
+     
+    <varlistentry id="guc-multixact-offsets-buffers" xreflabel="multixact_offsets_buffers">
+      <term><varname>multixact_offsets_buffers</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>multixact_offsets_buffers</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the amount of shared memory to use to cache the contents
+        of <literal>pg_multixact/offsets</literal> (see
+        <xref linkend="pgdata-contents-table"/>).
+        If this value is specified without units, it is taken as blocks,
+        that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
+        The default value is <literal>8</literal>.
+        This parameter can only be set at server start.
+       </para>
+      </listitem>
+     </varlistentry>
+
+    <varlistentry id="guc-multixact-members-buffers" xreflabel="multixact_members_buffers">
+      <term><varname>multixact_members_buffers</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>multixact_members_buffers</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the amount of shared memory to use to cache the contents
+        of <literal>pg_multixact/members</literal> (see
+        <xref linkend="pgdata-contents-table"/>).
+        If this value is specified without units, it is taken as blocks,
+        that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
+        The default value is <literal>16</literal>.
+        This parameter can only be set at server start.
+       </para>
+      </listitem>
+     </varlistentry>
+     
+    <varlistentry id="guc-subtrans-buffers" xreflabel="subtrans_buffers">
+      <term><varname>subtrans_buffers</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>subtrans_buffers</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the amount of shared memory to use to cache the contents
+        of <literal>pg_subtrans</literal> (see
+        <xref linkend="pgdata-contents-table"/>).
+        If this value is specified without units, it is taken as blocks,
+        that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
+        The default value is <literal>8</literal>.
+        This parameter can only be set at server start.
+       </para>
+      </listitem>
+     </varlistentry>
+     
+    <varlistentry id="guc-notify-buffers" xreflabel="notify_buffers">
+      <term><varname>notify_buffers</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>notify_buffers</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the amount of shared memory to use to cache the contents
+        of <literal>pg_notify</literal> (see
+        <xref linkend="pgdata-contents-table"/>).
+        If this value is specified without units, it is taken as blocks,
+        that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
+        The default value is <literal>8</literal>.
+        This parameter can only be set at server start.
+       </para>
+      </listitem>
+     </varlistentry>
+     
+    <varlistentry id="guc-serial-buffers" xreflabel="serial_buffers">
+      <term><varname>serial_buffers</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>serial_buffers</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the amount of shared memory to use to cache the contents
+        of <literal>pg_serial</literal> (see
+        <xref linkend="pgdata-contents-table"/>).
+        If this value is specified without units, it is taken as blocks,
+        that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
+        The default value is <literal>16</literal>.
+        This parameter can only be set at server start.
+       </para>
+      </listitem>
+     </varlistentry>
+     
+    <varlistentry id="guc-xact-buffers" xreflabel="xact_buffers">
+      <term><varname>xact_buffers</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>xact_buffers</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the amount of shared memory to use to cache the contents
+        of <literal>pg_xact</literal> (see
+        <xref linkend="pgdata-contents-table"/>).
+        If this value is specified without units, it is taken as blocks,
+        that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
+        The default value is <literal>0</literal>, which requests
+        <varname>shared_buffers</varname> / 512, but not fewer than 4 blocks.
+        This parameter can only be set at server start.
+       </para>
+      </listitem>
+     </varlistentry>
+     
+    <varlistentry id="guc-commit-ts-buffers" xreflabel="commit_ts_buffers">
+      <term><varname>commit_ts_buffers</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>commit_ts_buffers</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the amount of memory to use to cache the cotents of
+        <literal>pg_commit_ts</literal> (see
+        <xref linkend="pgdata-contents-table"/>).
+        If this value is specified without units, it is taken as blocks,
+        that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
+        The default value is <literal>0</literal>, which requests
+        <varname>shared_buffers</varname> / 1024, but not fewer than 4 blocks.
+        This parameter can only be set at server start.
+       </para>
+      </listitem>
+     </varlistentry>
 
      <varlistentry id="guc-max-stack-depth" xreflabel="max_stack_depth">
       <term><varname>max_stack_depth</varname> (<type>integer</type>)
diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c
index 3ea16a270a8..ca28ada75fa 100644
--- a/src/backend/access/transam/clog.c
+++ b/src/backend/access/transam/clog.c
@@ -58,8 +58,8 @@
 
 /* We need two bits per xact, so four xacts fit in a byte */
 #define CLOG_BITS_PER_XACT	2
-#define CLOG_XACTS_PER_BYTE 4
-#define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)
+StaticAssertDecl((CLOG_BITS_PER_XACT * CLOG_XACTS_PER_BYTE) == BITS_PER_BYTE,
+				 "CLOG_BITS_PER_XACT and CLOG_XACTS_PER_BYTE are inconsistent");
 #define CLOG_XACT_BITMASK	((1 << CLOG_BITS_PER_XACT) - 1)
 
 #define TransactionIdToPage(xid)	((xid) / (TransactionId) CLOG_XACTS_PER_PAGE)
@@ -664,23 +664,16 @@ TransactionIdGetStatus(TransactionId xid, XLogRecPtr *lsn)
 /*
  * Number of shared CLOG buffers.
  *
- * On larger multi-processor systems, it is possible to have many CLOG page
- * requests in flight at one time which could lead to disk access for CLOG
- * page if the required page is not found in memory.  Testing revealed that we
- * can get the best performance by having 128 CLOG buffers, more than that it
- * doesn't improve performance.
- *
- * Unconditionally keeping the number of CLOG buffers to 128 did not seem like
- * a good idea, because it would increase the minimum amount of shared memory
- * required to start, which could be a problem for people running very small
- * configurations.  The following formula seems to represent a reasonable
- * compromise: people with very low values for shared_buffers will get fewer
- * CLOG buffers as well, and everyone else will get 128.
+ * By default, we'll use 2MB of for every 1GB of shared buffers, up to the
+ * theoretical maximum useful value, but always at least 4 buffers.
  */
 Size
 CLOGShmemBuffers(void)
 {
-	return Min(128, Max(4, NBuffers / 512));
+	/* Use configured value if provided. */
+	if (xact_buffers > 0)
+		return Max(4, xact_buffers);
+	return Min(CLOG_MAX_ALLOWED_BUFFERS, Max(4, NBuffers / 512));
 }
 
 /*
diff --git a/src/backend/access/transam/commit_ts.c b/src/backend/access/transam/commit_ts.c
index cbbe19fea83..cb2e0ceb1c3 100644
--- a/src/backend/access/transam/commit_ts.c
+++ b/src/backend/access/transam/commit_ts.c
@@ -511,10 +511,15 @@ pg_xact_commit_timestamp_origin(PG_FUNCTION_ARGS)
  * We use a very similar logic as for the number of CLOG buffers (except we
  * scale up twice as fast with shared buffers, and the maximum is twice as
  * high); see comments in CLOGShmemBuffers.
+ * By default, we'll use 1MB of for every 1GB of shared buffers, up to the
+ * maximum value that slru.c will allow, but always at least 4 buffers.
  */
 Size
 CommitTsShmemBuffers(void)
 {
+	/* Use configured value if provided. */
+	if (commit_ts_buffers > 0)
+		return Max(4, commit_ts_buffers);
 	return Min(256, Max(4, NBuffers / 256));
 }
 
diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index e6c70ed0bc2..a29ab4769dc 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -1834,8 +1834,8 @@ MultiXactShmemSize(void)
 			 mul_size(sizeof(MultiXactId) * 2, MaxOldestSlot))
 
 	size = SHARED_MULTIXACT_STATE_SIZE;
-	size = add_size(size, SimpleLruShmemSize(NUM_MULTIXACTOFFSET_BUFFERS, 0));
-	size = add_size(size, SimpleLruShmemSize(NUM_MULTIXACTMEMBER_BUFFERS, 0));
+	size = add_size(size, SimpleLruShmemSize(multixact_offsets_buffers, 0));
+	size = add_size(size, SimpleLruShmemSize(multixact_members_buffers, 0));
 
 	return size;
 }
@@ -1851,13 +1851,13 @@ MultiXactShmemInit(void)
 	MultiXactMemberCtl->PagePrecedes = MultiXactMemberPagePrecedes;
 
 	SimpleLruInit(MultiXactOffsetCtl,
-				  "MultiXactOffset", NUM_MULTIXACTOFFSET_BUFFERS, 0,
+				  "MultiXactOffset", multixact_offsets_buffers, 0,
 				  MultiXactOffsetSLRULock, "pg_multixact/offsets",
 				  LWTRANCHE_MULTIXACTOFFSET_BUFFER,
 				  SYNC_HANDLER_MULTIXACT_OFFSET);
 	SlruPagePrecedesUnitTests(MultiXactOffsetCtl, MULTIXACT_OFFSETS_PER_PAGE);
 	SimpleLruInit(MultiXactMemberCtl,
-				  "MultiXactMember", NUM_MULTIXACTMEMBER_BUFFERS, 0,
+				  "MultiXactMember", multixact_offsets_buffers, 0,
 				  MultiXactMemberSLRULock, "pg_multixact/members",
 				  LWTRANCHE_MULTIXACTMEMBER_BUFFER,
 				  SYNC_HANDLER_MULTIXACT_MEMBER);
diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/subtrans.c
index 6a8e521f894..785f2520fde 100644
--- a/src/backend/access/transam/subtrans.c
+++ b/src/backend/access/transam/subtrans.c
@@ -31,6 +31,7 @@
 #include "access/slru.h"
 #include "access/subtrans.h"
 #include "access/transam.h"
+#include "miscadmin.h"
 #include "pg_trace.h"
 #include "utils/snapmgr.h"
 
@@ -184,14 +185,14 @@ SubTransGetTopmostTransaction(TransactionId xid)
 Size
 SUBTRANSShmemSize(void)
 {
-	return SimpleLruShmemSize(NUM_SUBTRANS_BUFFERS, 0);
+	return SimpleLruShmemSize(subtrans_buffers, 0);
 }
 
 void
 SUBTRANSShmemInit(void)
 {
 	SubTransCtl->PagePrecedes = SubTransPagePrecedes;
-	SimpleLruInit(SubTransCtl, "Subtrans", NUM_SUBTRANS_BUFFERS, 0,
+	SimpleLruInit(SubTransCtl, "Subtrans", subtrans_buffers, 0,
 				  SubtransSLRULock, "pg_subtrans",
 				  LWTRANCHE_SUBTRANS_BUFFER, SYNC_HANDLER_NONE);
 	SlruPagePrecedesUnitTests(SubTransCtl, SUBTRANS_XACTS_PER_PAGE);
diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index 85570085450..7f2b7598449 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -117,7 +117,7 @@
  * frontend during startup.)  The above design guarantees that notifies from
  * other backends will never be missed by ignoring self-notifies.
  *
- * The amount of shared memory used for notify management (NUM_NOTIFY_BUFFERS)
+ * The amount of shared memory used for notify management (notify_buffers)
  * can be varied without affecting anything but performance.  The maximum
  * amount of notification data that can be queued at one time is determined
  * by slru.c's wraparound limit; see QUEUE_MAX_PAGE below.
@@ -235,7 +235,7 @@ typedef struct QueuePosition
  *
  * Resist the temptation to make this really large.  While that would save
  * work in some places, it would add cost in others.  In particular, this
- * should likely be less than NUM_NOTIFY_BUFFERS, to ensure that backends
+ * should likely be less than notify_buffers, to ensure that backends
  * catch up before the pages they'll need to read fall out of SLRU cache.
  */
 #define QUEUE_CLEANUP_DELAY 4
@@ -521,7 +521,7 @@ AsyncShmemSize(void)
 	size = mul_size(MaxBackends + 1, sizeof(QueueBackendStatus));
 	size = add_size(size, offsetof(AsyncQueueControl, backend));
 
-	size = add_size(size, SimpleLruShmemSize(NUM_NOTIFY_BUFFERS, 0));
+	size = add_size(size, SimpleLruShmemSize(notify_buffers, 0));
 
 	return size;
 }
@@ -569,7 +569,7 @@ AsyncShmemInit(void)
 	 * Set up SLRU management of the pg_notify data.
 	 */
 	NotifyCtl->PagePrecedes = asyncQueuePagePrecedes;
-	SimpleLruInit(NotifyCtl, "Notify", NUM_NOTIFY_BUFFERS, 0,
+	SimpleLruInit(NotifyCtl, "Notify", notify_buffers, 0,
 				  NotifySLRULock, "pg_notify", LWTRANCHE_NOTIFY_BUFFER,
 				  SYNC_HANDLER_NONE);
 
diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index 4f4d5b0d20f..c5e66757643 100644
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -872,7 +872,7 @@ SerialInit(void)
 	 */
 	SerialSlruCtl->PagePrecedes = SerialPagePrecedesLogically;
 	SimpleLruInit(SerialSlruCtl, "Serial",
-				  NUM_SERIAL_BUFFERS, 0, SerialSLRULock, "pg_serial",
+				  serial_buffers, 0, SerialSLRULock, "pg_serial",
 				  LWTRANCHE_SERIAL_BUFFER, SYNC_HANDLER_NONE);
 #ifdef USE_ASSERT_CHECKING
 	SerialPagePrecedesLogicallyUnitTests();
@@ -1396,7 +1396,7 @@ PredicateLockShmemSize(void)
 
 	/* Shared memory structures for SLRU tracking of old committed xids. */
 	size = add_size(size, sizeof(SerialControlData));
-	size = add_size(size, SimpleLruShmemSize(NUM_SERIAL_BUFFERS, 0));
+	size = add_size(size, SimpleLruShmemSize(serial_buffers, 0));
 
 	return size;
 }
diff --git a/src/backend/utils/init/globals.c b/src/backend/utils/init/globals.c
index 381d9e548d1..c83151d5ab5 100644
--- a/src/backend/utils/init/globals.c
+++ b/src/backend/utils/init/globals.c
@@ -150,3 +150,11 @@ int64		VacuumPageDirty = 0;
 
 int			VacuumCostBalance = 0;	/* working state for vacuum */
 bool		VacuumCostActive = false;
+
+int			multixact_offsets_buffers = 8;
+int			multixact_members_buffers = 16;
+int			subtrans_buffers = 32;
+int			notify_buffers = 8;
+int			serial_buffers = 16;
+int			xact_buffers = 0;
+int			commit_ts_buffers = 0;
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index bff949a40bc..eb6bf4c0a04 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -32,9 +32,11 @@
 #endif
 #include <unistd.h>
 
+#include "access/clog.h"
 #include "access/commit_ts.h"
 #include "access/gin.h"
 #include "access/rmgr.h"
+#include "access/slru.h"
 #include "access/tableam.h"
 #include "access/toast_compression.h"
 #include "access/transam.h"
@@ -202,6 +204,8 @@ static const char *show_tcp_keepalives_idle(void);
 static const char *show_tcp_keepalives_interval(void);
 static const char *show_tcp_keepalives_count(void);
 static const char *show_tcp_user_timeout(void);
+static const char *show_xact_buffers(void);
+static const char *show_commit_ts_buffers(void);
 static bool check_maxconnections(int *newval, void **extra, GucSource source);
 static bool check_max_worker_processes(int *newval, void **extra, GucSource source);
 static bool check_autovacuum_max_workers(int *newval, void **extra, GucSource source);
@@ -2366,6 +2370,83 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"multixact_offsets_buffers", PGC_POSTMASTER, RESOURCES_MEM,
+			gettext_noop("Sets the number of shared memory buffers used for the MultiXact offset SLRU cache."),
+			NULL,
+			GUC_UNIT_BLOCKS
+		},
+		&multixact_offsets_buffers,
+		8, 2, SLRU_MAX_ALLOWED_BUFFERS,
+		NULL, NULL, NULL
+	},
+
+	{
+		{"multixact_members_buffers", PGC_POSTMASTER, RESOURCES_MEM,
+			gettext_noop("Sets the number of shared memory buffers used for the MultiXact member SLRU cache."),
+			NULL,
+			GUC_UNIT_BLOCKS
+		},
+		&multixact_members_buffers,
+		16, 2, SLRU_MAX_ALLOWED_BUFFERS,
+		NULL, NULL, NULL
+	},
+
+	{
+		{"subtrans_buffers", PGC_POSTMASTER, RESOURCES_MEM,
+			gettext_noop("Sets the number of shared memory buffers used for the sub-transaction SLRU cache."),
+			NULL,
+			GUC_UNIT_BLOCKS
+		},
+		&subtrans_buffers,
+		32, 2, SLRU_MAX_ALLOWED_BUFFERS,
+		NULL, NULL, NULL
+	},
+
+	{
+		{"notify_buffers", PGC_POSTMASTER, RESOURCES_MEM,
+			gettext_noop("Sets the number of shared memory buffers used for the NOTIFY message SLRU cache."),
+			NULL,
+			GUC_UNIT_BLOCKS
+		},
+		&notify_buffers,
+		8, 2, SLRU_MAX_ALLOWED_BUFFERS,
+		NULL, NULL, NULL
+	},
+
+	{
+		{"serial_buffers", PGC_POSTMASTER, RESOURCES_MEM,
+			gettext_noop("Sets the number of shared memory buffers used for the serializable transaction SLRU cache."),
+			NULL,
+			GUC_UNIT_BLOCKS
+		},
+		&serial_buffers,
+		16, 2, SLRU_MAX_ALLOWED_BUFFERS,
+		NULL, NULL, NULL
+	},
+
+	{
+		{"xact_buffers", PGC_POSTMASTER, RESOURCES_MEM,
+			gettext_noop("Sets the number of shared memory buffers used for the transaction status SLRU cache."),
+			NULL,
+			GUC_UNIT_BLOCKS
+		},
+		&xact_buffers,
+		0, 0, CLOG_MAX_ALLOWED_BUFFERS,
+		NULL, NULL, show_xact_buffers
+	},
+
+	{
+		{"commit_ts_buffers", PGC_POSTMASTER, RESOURCES_MEM,
+			gettext_noop("Sets the size of the dedicated buffer pool used for the commit timestamp SLRU cache."),
+			NULL,
+			GUC_UNIT_BLOCKS
+		},
+		&commit_ts_buffers,
+		0, 0, SLRU_MAX_ALLOWED_BUFFERS,
+		NULL, NULL, show_commit_ts_buffers
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
@@ -12074,6 +12155,24 @@ show_tcp_user_timeout(void)
 	return nbuf;
 }
 
+static const char *
+show_xact_buffers(void)
+{
+	static char nbuf[16];
+
+	snprintf(nbuf, sizeof(nbuf), "%zu", CLOGShmemBuffers());
+	return nbuf;
+}
+
+static const char *
+show_commit_ts_buffers(void)
+{
+	static char nbuf[16];
+
+	snprintf(nbuf, sizeof(nbuf), "%zu", CommitTsShmemBuffers());
+	return nbuf;
+}
+
 static bool
 check_maxconnections(int *newval, void **extra, GucSource source)
 {
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index a1acd46b611..22bda4383ce 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -195,6 +195,15 @@
 #old_snapshot_threshold = -1		# 1min-60d; -1 disables; 0 is immediate
 					# (change requires restart)
 
+# - SLRU Buffers (change requires restart) -
+
+#xact_buffers = 0			# memory for pg_xact (0 = auto)
+#subtrans_buffers = 32			# memory for pg_subtrans
+#multixact_offsets_buffers = 8		# memory for pg_multixact/offsets
+#multixact_members_buffers = 16		# memory for pg_multixact/members
+#notify_buffers = 8			# memory for pg_notify
+#serial_buffers = 16			# memory for pg_serial
+#commit_ts_buffers = 0			# memory for pg_commit_ts (0 = auto)
 
 #------------------------------------------------------------------------------
 # WRITE-AHEAD LOG
diff --git a/src/include/access/clog.h b/src/include/access/clog.h
index 39b8e4afa8a..739a292f7f3 100644
--- a/src/include/access/clog.h
+++ b/src/include/access/clog.h
@@ -15,6 +15,16 @@
 #include "storage/sync.h"
 #include "lib/stringinfo.h"
 
+/*
+ * Don't allow xact_buffers to be set higher than could possibly be useful or
+ * SLRU would allow.
+ */
+#define CLOG_XACTS_PER_BYTE 4
+#define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)
+#define CLOG_MAX_ALLOWED_BUFFERS \
+	Min(SLRU_MAX_ALLOWED_BUFFERS, \
+		(((MaxTransactionId / 2) + (CLOG_XACTS_PER_PAGE - 1)) / CLOG_XACTS_PER_PAGE))
+
 /*
  * Possible transaction statuses --- note that all-zeroes is the initial
  * state.
diff --git a/src/include/access/commit_ts.h b/src/include/access/commit_ts.h
index a1538978c62..f86760f7240 100644
--- a/src/include/access/commit_ts.h
+++ b/src/include/access/commit_ts.h
@@ -16,7 +16,6 @@
 #include "replication/origin.h"
 #include "storage/sync.h"
 
-
 extern PGDLLIMPORT bool track_commit_timestamp;
 
 extern void TransactionTreeSetCommitTsData(TransactionId xid, int nsubxids,
diff --git a/src/include/access/multixact.h b/src/include/access/multixact.h
index 4bbb035eaea..97c0a463768 100644
--- a/src/include/access/multixact.h
+++ b/src/include/access/multixact.h
@@ -29,10 +29,6 @@
 
 #define MaxMultiXactOffset	((MultiXactOffset) 0xFFFFFFFF)
 
-/* Number of SLRU buffers to use for multixact */
-#define NUM_MULTIXACTOFFSET_BUFFERS		8
-#define NUM_MULTIXACTMEMBER_BUFFERS		16
-
 /*
  * Possible multixact lock modes ("status").  The first four modes are for
  * tuple locks (FOR KEY SHARE, FOR SHARE, FOR NO KEY UPDATE, FOR UPDATE); the
diff --git a/src/include/access/slru.h b/src/include/access/slru.h
index dd52e8cec7e..793c045f160 100644
--- a/src/include/access/slru.h
+++ b/src/include/access/slru.h
@@ -17,6 +17,11 @@
 #include "storage/lwlock.h"
 #include "storage/sync.h"
 
+/*
+ * To avoid overflowing internal arithmetic and the size_t data type, the
+ * number of buffers should not exceed this number.
+ */
+#define SLRU_MAX_ALLOWED_BUFFERS ((1024 * 1024 * 1024) / BLCKSZ)
 
 /*
  * Define SLRU segment size.  A page is the same BLCKSZ as is used everywhere
diff --git a/src/include/access/subtrans.h b/src/include/access/subtrans.h
index d0ab44ae828..64fa86938e2 100644
--- a/src/include/access/subtrans.h
+++ b/src/include/access/subtrans.h
@@ -11,8 +11,6 @@
 #ifndef SUBTRANS_H
 #define SUBTRANS_H
 
-/* Number of SLRU buffers to use for subtrans */
-#define NUM_SUBTRANS_BUFFERS	32
 
 extern void SubTransSetParent(TransactionId xid, TransactionId parent);
 extern TransactionId SubTransGetParent(TransactionId xid);
diff --git a/src/include/commands/async.h b/src/include/commands/async.h
index f371ac896b9..99575974982 100644
--- a/src/include/commands/async.h
+++ b/src/include/commands/async.h
@@ -15,11 +15,6 @@
 
 #include <signal.h>
 
-/*
- * The number of SLRU page buffers we use for the notification queue.
- */
-#define NUM_NOTIFY_BUFFERS	8
-
 extern bool Trace_notify;
 extern volatile sig_atomic_t notifyInterruptPending;
 
diff --git a/src/include/miscadmin.h b/src/include/miscadmin.h
index 90a30160657..22d31546ad5 100644
--- a/src/include/miscadmin.h
+++ b/src/include/miscadmin.h
@@ -176,6 +176,13 @@ extern PGDLLIMPORT int MaxBackends;
 extern PGDLLIMPORT int MaxConnections;
 extern PGDLLIMPORT int max_worker_processes;
 extern PGDLLIMPORT int max_parallel_workers;
+extern PGDLLIMPORT int multixact_offsets_buffers;
+extern PGDLLIMPORT int multixact_members_buffers;
+extern PGDLLIMPORT int subtrans_buffers;
+extern PGDLLIMPORT int notify_buffers;
+extern PGDLLIMPORT int serial_buffers;
+extern PGDLLIMPORT int xact_buffers;
+extern PGDLLIMPORT int commit_ts_buffers;
 
 extern PGDLLIMPORT int MyProcPid;
 extern PGDLLIMPORT pg_time_t MyStartTime;
diff --git a/src/include/storage/predicate.h b/src/include/storage/predicate.h
index 152b6986114..c72779bd88d 100644
--- a/src/include/storage/predicate.h
+++ b/src/include/storage/predicate.h
@@ -26,10 +26,6 @@ extern int	max_predicate_locks_per_xact;
 extern int	max_predicate_locks_per_relation;
 extern int	max_predicate_locks_per_page;
 
-
-/* Number of SLRU buffers to use for Serial SLRU */
-#define NUM_SERIAL_BUFFERS		16
-
 /*
  * A handle used for sharing SERIALIZABLEXACT objects between the participants
  * in a parallel query.
-- 
2.33.1

