Preallocate some DSM space at startup.

Create an optional region in the main shared memory segment that can be
used to acquire and release "fast" DSM segments, and can benefit from
huge pages allocated at cluster startup time, if configured.  Fall back
to the existing mechanisms when that space is full.  The size is
controlled by a new GUC min_dynamic_shared_memory, defaulting to 0.

Main region DSM segments initially contain whatever garbage the memory
held last time they were used, rather than zeroes.  That change revealed
that DSA areas failed to initialize themselves correctly in memory that
wasn't zeroed first, so fix that problem.

Discussion: https://postgr.es/m/CA%2BhUKGLAE2QBv-WgGp%2BD9P_J-%3Dyne3zof9nfMaqq1h3EGHFXYQ%40mail.gmail.com

doc/src/sgml/config.sgml

@@ -1906,6 +1906,30 @@ include_dir 'conf.d'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-min-dynamic-shared-memory" xreflabel="min_dynamic_shared_memory">
+      <term><varname>min_dynamic_shared_memory</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>min_dynamic_shared_memory</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Specifies the amount of memory that should be allocated at server
+        startup time for use by parallel queries.  When this memory region is
+        insufficient or exhausted by concurrent queries, new parallel queries
+        try to allocate extra shared memory temporarily from the operating
+        system using the method configured with
+        <varname>dynamic_shared_memory_type</varname>, which may be slower due
+        to memory management overheads.  Memory that is allocated at startup
+        time with <varname>min_dynamic_shared_memory</varname> is affected by
+        the <varname>huge_pages</varname> setting on operating systems where
+        that is supported, and may be more likely to benefit from larger pages
+        on operating systems where that is managed automatically.
+        The default value is <literal>0</literal> (none).
+       </para>
+      </listitem>
+     </varlistentry>
+
      </variablelist>
      </sect2>
 

src/backend/storage/ipc/dsm_impl.c

@@ -113,6 +113,9 @@ const struct config_enum_entry dynamic_shared_memory_options[] = {
 /* Implementation selector. */
 int			dynamic_shared_memory_type;
 
+/* Amount of space reserved for DSM segments in the main area. */
+int			min_dynamic_shared_memory;
+
 /* Size of buffer to be used for zero-filling. */
 #define ZBUFFER_SIZE				8192
 

src/backend/storage/ipc/ipci.c

@@ -120,6 +120,7 @@ CreateSharedMemoryAndSemaphores(void)
 		size = add_size(size, SpinlockSemaSize());
 		size = add_size(size, hash_estimate_size(SHMEM_INDEX_SIZE,
 												 sizeof(ShmemIndexEnt)));
+		size = add_size(size, dsm_estimate_size());
 		size = add_size(size, BufferShmemSize());
 		size = add_size(size, LockShmemSize());
 		size = add_size(size, PredicateLockShmemSize());
@@ -209,6 +210,8 @@ CreateSharedMemoryAndSemaphores(void)
 	 */
 	InitShmemIndex();
 
+	dsm_shmem_init();
+
 	/*
 	 * Set up xlog, clog, and buffers
 	 */

src/backend/utils/misc/guc.c

@@ -2231,6 +2231,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"min_dynamic_shared_memory", PGC_POSTMASTER, RESOURCES_MEM,
+			gettext_noop("Amount of dynamic shared memory reserved at startup."),
+			NULL,
+			GUC_UNIT_MB
+		},
+		&min_dynamic_shared_memory,
+		0, 0, Min(INT_MAX, SIZE_MAX / 1024 / 1024),
+		NULL, NULL, NULL
+	},
+
 	/*
 	 * We sometimes multiply the number of shared buffers by two without
 	 * checking for overflow, so we mustn't allow more than INT_MAX / 2.

src/backend/utils/misc/postgresql.conf.sample

@@ -148,6 +148,7 @@
 					#   windows
 					#   mmap
 					# (change requires restart)
+#min_dynamic_shared_memory = 0MB	# (change requires restart)
 
 # - Disk -
 

src/backend/utils/mmgr/dsa.c

@@ -1223,6 +1223,7 @@ create_internal(void *place, size_t size,
 	 * space.
 	 */
 	control = (dsa_area_control *) place;
+	memset(place, 0, sizeof(*control));
 	control->segment_header.magic =
 		DSA_SEGMENT_HEADER_MAGIC ^ control_handle ^ 0;
 	control->segment_header.next = DSA_SEGMENT_INDEX_NONE;
@@ -1233,14 +1234,10 @@ create_internal(void *place, size_t size,
 	control->handle = control_handle;
 	control->max_total_segment_size = (size_t) -1;
 	control->total_segment_size = size;
-	memset(&control->segment_handles[0], 0,
-		   sizeof(dsm_handle) * DSA_MAX_SEGMENTS);
 	control->segment_handles[0] = control_handle;
 	for (i = 0; i < DSA_NUM_SEGMENT_BINS; ++i)
 		control->segment_bins[i] = DSA_SEGMENT_INDEX_NONE;
-	control->high_segment_index = 0;
 	control->refcnt = 1;
-	control->freed_segment_counter = 0;
 	control->lwlock_tranche_id = tranche_id;
 
 	/*

src/include/storage/dsm.h

@@ -29,6 +29,9 @@ extern void dsm_postmaster_startup(struct PGShmemHeader *);
 extern void dsm_backend_shutdown(void);
 extern void dsm_detach_all(void);
 
+extern size_t dsm_estimate_size(void);
+extern void dsm_shmem_init(void);
+
 #ifdef EXEC_BACKEND
 extern void dsm_set_control_handle(dsm_handle h);
 #endif

src/include/storage/dsm_impl.h

@@ -40,6 +40,7 @@
 
 /* GUC. */
 extern int	dynamic_shared_memory_type;
+extern int	min_dynamic_shared_memory;
 
 /*
  * Directory for on-disk state.