commit 67053e7204b5044a7147d878eec4c66217495298
Author: anastasia <a.lubennikova@postgrespro.ru>
Date:   Fri Aug 28 19:15:41 2020 +0300

    Make MultiXact local cache size configurable

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 7a7177c550..14eea77f2b 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1821,6 +1821,22 @@ include_dir 'conf.d'
       </listitem>
      </varlistentry>

+    <varlistentry id="guc-multixact-local-cache-entries" xreflabel="multixact_local-cache-entries">
+      <term><varname>multixact_local_cache_entries</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>multixact_local_cache_entries</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the number cached MultiXact by backend. Any SLRU lookup is preceeded
+        by cache lookup. Higher numbers of cache size help to deduplicate lock sets, while
+        lower values make cache lookup faster.
+        It defaults to 256.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-max-stack-depth" xreflabel="max_stack_depth">
       <term><varname>max_stack_depth</varname> (<type>integer</type>)
       <indexterm>
diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index b8bedca04a..99a2bc7c80 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -1589,7 +1589,7 @@ mXactCachePut(MultiXactId multi, int nmembers, MultiXactMember *members)
 	qsort(entry->members, nmembers, sizeof(MultiXactMember), mxactMemberComparator);

 	dlist_push_head(&MXactCache, &entry->node);
-	if (MXactCacheMembers++ >= MAX_CACHE_ENTRIES)
+	if (MXactCacheMembers++ >= multixact_local_cache_entries)
 	{
 		dlist_node *node;
 		mXactCacheEnt *entry;
diff --git a/src/backend/utils/init/globals.c b/src/backend/utils/init/globals.c
index 6ab8216839..9ca71933dc 100644
--- a/src/backend/utils/init/globals.c
+++ b/src/backend/utils/init/globals.c
@@ -149,3 +149,5 @@ int			VacuumCostBalance = 0;	/* working state for vacuum */
 bool		VacuumCostActive = false;

 double		vacuum_cleanup_index_scale_factor;
+
+int         multixact_local_cache_entries = 256;
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index de87ad6ef7..6f3027fd66 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -2257,6 +2257,16 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},

+	{
+		{"multixact_local_cache_entries", PGC_SUSET, RESOURCES_MEM,
+			gettext_noop("Sets the number of cached MultiXact by backend."),
+			NULL
+		},
+		&multixact_local_cache_entries,
+		256, 2, INT_MAX / 2,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
diff --git a/src/include/miscadmin.h b/src/include/miscadmin.h
index 72e3352398..01af61c963 100644
--- a/src/include/miscadmin.h
+++ b/src/include/miscadmin.h
@@ -162,6 +162,8 @@ extern PGDLLIMPORT int MaxConnections;
 extern PGDLLIMPORT int max_worker_processes;
 extern PGDLLIMPORT int max_parallel_workers;

+extern PGDLLIMPORT int multixact_local_cache_entries;
+
 extern PGDLLIMPORT int MyProcPid;
 extern PGDLLIMPORT pg_time_t MyStartTime;
 extern PGDLLIMPORT TimestampTz MyStartTimestamp;