From bd1788051b02cfddcd9ef0e2fd094972f372b8fd Mon Sep 17 00:00:00 2001
From: Michael Paquier
Date: Wed, 8 Sep 2021 12:02:30 +0900
Subject: Introduce GUC shared_memory_size

This runtime-computed GUC shows the size of the server's main shared
memory area, taking into account the amount of shared memory allocated
by extensions as this is calculated after processing
shared_preload_libraries.

Author: Nathan Bossart
Discussion: https://postgr.es/m/F2772387-CE0F-46BF-B5F1-CC55516EB885@amazon.com
---
 src/backend/postmaster/postmaster.c |  7 +++++++
 src/backend/storage/ipc/ipci.c      | 22 ++++++++++++++++++++++
 src/backend/utils/misc/guc.c        | 12 ++++++++++++
 3 files changed, 41 insertions(+)

(limited to 'src/backend')

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 63043ed8d1f..b2fe420c3cf 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1026,6 +1026,13 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeMaxBackends();
 
+	/*
+	 * Now that loadable modules have had their chance to request additional
+	 * shared memory, determine the value of any runtime-computed GUCs that
+	 * depend on the amount of shared memory required.
+	 */
+	InitializeShmemGUCs();
+
 	/*
 	 * Set up shared memory and semaphores.
 	 */
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 64bc16fa848..1a408ad77e5 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -313,3 +313,25 @@ CreateSharedMemoryAndSemaphores(void)
 	if (shmem_startup_hook)
 		shmem_startup_hook();
 }
+
+/*
+ * InitializeShmemGUCs
+ *
+ * This function initializes runtime-computed GUCs related to the amount of
+ * shared memory required for the current configuration.
+ */
+void
+InitializeShmemGUCs(void)
+{
+	char		buf[64];
+	Size		size_b;
+	Size		size_mb;
+
+	/*
+	 * Calculate the shared memory size and round up to the nearest megabyte.
+	 */
+	size_b = CalculateShmemSize(NULL);
+	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
+	sprintf(buf, "%lu", size_mb);
+	SetConfigOption("shared_memory_size", buf, PGC_INTERNAL, PGC_S_OVERRIDE);
+}
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 467b0fd6fe7..c339acf0670 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -664,6 +664,7 @@ static int	max_index_keys;
 static int	max_identifier_length;
 static int	block_size;
 static int	segment_size;
+static int	shared_memory_size_mb;
 static int	wal_block_size;
 static bool data_checksums;
 static bool integer_datetimes;
@@ -2337,6 +2338,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"shared_memory_size", PGC_INTERNAL, RESOURCES_MEM,
+			gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
+		},
+		&shared_memory_size_mb,
+		0, 0, INT_MAX,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
-- 
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