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.c

@@ -35,10 +35,12 @@
 
 #include "lib/ilist.h"
 #include "miscadmin.h"
+#include "port/pg_bitutils.h"
 #include "storage/dsm.h"
 #include "storage/ipc.h"
 #include "storage/lwlock.h"
 #include "storage/pg_shmem.h"
+#include "utils/freepage.h"
 #include "utils/guc.h"
 #include "utils/memutils.h"
 #include "utils/resowner_private.h"
@@ -76,6 +78,8 @@ typedef struct dsm_control_item
 {
 	dsm_handle	handle;
 	uint32		refcnt;			/* 2+ = active, 1 = moribund, 0 = gone */
+	size_t		first_page;
+	size_t		npages;
 	void	   *impl_private_pm_handle; /* only needed on Windows */
 	bool		pinned;
 } dsm_control_item;
@@ -95,10 +99,15 @@ static dsm_segment *dsm_create_descriptor(void);
 static bool dsm_control_segment_sane(dsm_control_header *control,
 									 Size mapped_size);
 static uint64 dsm_control_bytes_needed(uint32 nitems);
+static inline dsm_handle make_main_region_dsm_handle(int slot);
+static inline bool is_main_region_dsm_handle(dsm_handle handle);
 
 /* Has this backend initialized the dynamic shared memory system yet? */
 static bool dsm_init_done = false;
 
+/* Preallocated DSM space in the main shared memory region. */
+static void *dsm_main_space_begin = NULL;
+
 /*
  * List of dynamic shared memory segments used by this backend.
  *
@@ -171,7 +180,7 @@ dsm_postmaster_startup(PGShmemHeader *shim)
 	{
 		Assert(dsm_control_address == NULL);
 		Assert(dsm_control_mapped_size == 0);
-		dsm_control_handle = random();
+		dsm_control_handle = random() << 1; /* Even numbers only */
 		if (dsm_control_handle == DSM_HANDLE_INVALID)
 			continue;
 		if (dsm_impl_op(DSM_OP_CREATE, dsm_control_handle, segsize,
@@ -247,8 +256,12 @@ dsm_cleanup_using_control_segment(dsm_handle old_control_handle)
 		if (refcnt == 0)
 			continue;
 
-		/* Log debugging information. */
+		/* If it was using the main shmem area, there is nothing to do. */
 		handle = old_control->item[i].handle;
+		if (is_main_region_dsm_handle(handle))
+			continue;
+
+		/* Log debugging information. */
 		elog(DEBUG2, "cleaning up orphaned dynamic shared memory with ID %u (reference count %u)",
 			 handle, refcnt);
 
@@ -348,8 +361,11 @@ dsm_postmaster_shutdown(int code, Datum arg)
 		if (dsm_control->item[i].refcnt == 0)
 			continue;
 
-		/* Log debugging information. */
 		handle = dsm_control->item[i].handle;
+		if (is_main_region_dsm_handle(handle))
+			continue;
+
+		/* Log debugging information. */
 		elog(DEBUG2, "cleaning up orphaned dynamic shared memory with ID %u",
 			 handle);
 
@@ -418,6 +434,45 @@ dsm_set_control_handle(dsm_handle h)
 }
 #endif
 
+/*
+ * Reserve some space in the main shared memory segment for DSM segments.
+ */
+size_t
+dsm_estimate_size(void)
+{
+	return 1024 * 1024 * (size_t) min_dynamic_shared_memory;
+}
+
+/*
+ * Initialize space in the main shared memory segment for DSM segments.
+ */
+void
+dsm_shmem_init(void)
+{
+	size_t		size = dsm_estimate_size();
+	bool		found;
+
+	if (size == 0)
+		return;
+
+	dsm_main_space_begin = ShmemInitStruct("Preallocated DSM", size, &found);
+	if (!found)
+	{
+		FreePageManager *fpm = (FreePageManager *) dsm_main_space_begin;
+		size_t		first_page = 0;
+		size_t		pages;
+
+		/* Reserve space for the FreePageManager. */
+		while (first_page * FPM_PAGE_SIZE < sizeof(FreePageManager))
+			++first_page;
+
+		/* Initialize it and give it all the rest of the space. */
+		FreePageManagerInitialize(fpm, dsm_main_space_begin);
+		pages = (size / FPM_PAGE_SIZE) - first_page;
+		FreePageManagerPut(fpm, first_page, pages);
+	}
+}
+
 /*
  * Create a new dynamic shared memory segment.
  *
@@ -434,6 +489,10 @@ dsm_create(Size size, int flags)
 	dsm_segment *seg;
 	uint32		i;
 	uint32		nitems;
+	size_t		npages = 0;
+	size_t		first_page = 0;
+	FreePageManager *dsm_main_space_fpm = dsm_main_space_begin;
+	bool		using_main_dsm_region = false;
 
 	/* Unsafe in postmaster (and pointless in a stand-alone backend). */
 	Assert(IsUnderPostmaster);
@@ -444,20 +503,48 @@ dsm_create(Size size, int flags)
 	/* Create a new segment descriptor. */
 	seg = dsm_create_descriptor();
 
-	/* Loop until we find an unused segment identifier. */
+	/*
+	 * Lock the control segment while we try to allocate from the main shared
+	 * memory area, if configured.
+	 */
+	if (dsm_main_space_fpm)
+	{
+		npages = size / FPM_PAGE_SIZE;
+		if (size % FPM_PAGE_SIZE > 0)
+			++npages;
+
+		LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
+		if (FreePageManagerGet(dsm_main_space_fpm, npages, &first_page))
+		{
+			/* We can carve out a piece of the main shared memory segment. */
+			seg->mapped_address = (char *) dsm_main_space_begin +
+				first_page * FPM_PAGE_SIZE;
+			seg->mapped_size = npages * FPM_PAGE_SIZE;
+			using_main_dsm_region = true;
+			/* We'll choose a handle below. */
+		}
+	}
+
+	if (!using_main_dsm_region)
+	{
+		/*
+		 * We need to create a new memory segment.  Loop until we find an
+		 * unused segment identifier.
+		 */
+		if (dsm_main_space_fpm)
+			LWLockRelease(DynamicSharedMemoryControlLock);
 		for (;;)
 		{
 			Assert(seg->mapped_address == NULL && seg->mapped_size == 0);
-		seg->handle = random();
+			seg->handle = random() << 1;	/* Even numbers only */
 			if (seg->handle == DSM_HANDLE_INVALID)	/* Reserve sentinel */
 				continue;
 			if (dsm_impl_op(DSM_OP_CREATE, seg->handle, size, &seg->impl_private,
 							&seg->mapped_address, &seg->mapped_size, ERROR))
 				break;
 		}
-
-	/* Lock the control segment so we can register the new segment. */
 		LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
+	}
 
 	/* Search the control segment for an unused slot. */
 	nitems = dsm_control->nitems;
@@ -465,6 +552,14 @@ dsm_create(Size size, int flags)
 	{
 		if (dsm_control->item[i].refcnt == 0)
 		{
+			if (using_main_dsm_region)
+			{
+				seg->handle = make_main_region_dsm_handle(i);
+				dsm_control->item[i].first_page = first_page;
+				dsm_control->item[i].npages = npages;
+			}
+			else
+				Assert(!is_main_region_dsm_handle(seg->handle));
 			dsm_control->item[i].handle = seg->handle;
 			/* refcnt of 1 triggers destruction, so start at 2 */
 			dsm_control->item[i].refcnt = 2;
@@ -479,7 +574,10 @@ dsm_create(Size size, int flags)
 	/* Verify that we can support an additional mapping. */
 	if (nitems >= dsm_control->maxitems)
 	{
+		if (using_main_dsm_region)
+			FreePageManagerPut(dsm_main_space_fpm, first_page, npages);
 		LWLockRelease(DynamicSharedMemoryControlLock);
+		if (!using_main_dsm_region)
 			dsm_impl_op(DSM_OP_DESTROY, seg->handle, 0, &seg->impl_private,
 						&seg->mapped_address, &seg->mapped_size, WARNING);
 		if (seg->resowner != NULL)
@@ -495,6 +593,12 @@ dsm_create(Size size, int flags)
 	}
 
 	/* Enter the handle into a new array slot. */
+	if (using_main_dsm_region)
+	{
+		seg->handle = make_main_region_dsm_handle(nitems);
+		dsm_control->item[i].first_page = first_page;
+		dsm_control->item[i].npages = npages;
+	}
 	dsm_control->item[nitems].handle = seg->handle;
 	/* refcnt of 1 triggers destruction, so start at 2 */
 	dsm_control->item[nitems].refcnt = 2;
@@ -580,6 +684,12 @@ dsm_attach(dsm_handle h)
 		/* Otherwise we've found a match. */
 		dsm_control->item[i].refcnt++;
 		seg->control_slot = i;
+		if (is_main_region_dsm_handle(seg->handle))
+		{
+			seg->mapped_address = (char *) dsm_main_space_begin +
+				dsm_control->item[i].first_page * FPM_PAGE_SIZE;
+			seg->mapped_size = dsm_control->item[i].npages * FPM_PAGE_SIZE;
+		}
 		break;
 	}
 	LWLockRelease(DynamicSharedMemoryControlLock);
@@ -597,6 +707,7 @@ dsm_attach(dsm_handle h)
 	}
 
 	/* Here's where we actually try to map the segment. */
+	if (!is_main_region_dsm_handle(seg->handle))
 		dsm_impl_op(DSM_OP_ATTACH, seg->handle, 0, &seg->impl_private,
 					&seg->mapped_address, &seg->mapped_size, ERROR);
 
@@ -688,6 +799,7 @@ dsm_detach(dsm_segment *seg)
 	 */
 	if (seg->mapped_address != NULL)
 	{
+		if (!is_main_region_dsm_handle(seg->handle))
 			dsm_impl_op(DSM_OP_DETACH, seg->handle, 0, &seg->impl_private,
 						&seg->mapped_address, &seg->mapped_size, WARNING);
 		seg->impl_private = NULL;
@@ -729,10 +841,15 @@ dsm_detach(dsm_segment *seg)
 			 * other reason, the postmaster may not have any better luck than
 			 * we did.  There's not much we can do about that, though.
 			 */
-			if (dsm_impl_op(DSM_OP_DESTROY, seg->handle, 0, &seg->impl_private,
+			if (is_main_region_dsm_handle(seg->handle) ||
+				dsm_impl_op(DSM_OP_DESTROY, seg->handle, 0, &seg->impl_private,
 							&seg->mapped_address, &seg->mapped_size, WARNING))
 			{
 				LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
+				if (is_main_region_dsm_handle(seg->handle))
+					FreePageManagerPut((FreePageManager *) dsm_main_space_begin,
+									   dsm_control->item[control_slot].first_page,
+									   dsm_control->item[control_slot].npages);
 				Assert(dsm_control->item[control_slot].handle == seg->handle);
 				Assert(dsm_control->item[control_slot].refcnt == 1);
 				dsm_control->item[control_slot].refcnt = 0;
@@ -894,10 +1011,15 @@ dsm_unpin_segment(dsm_handle handle)
 		 * pass the mapped size, mapped address, and private data as NULL
 		 * here.
 		 */
-		if (dsm_impl_op(DSM_OP_DESTROY, handle, 0, &junk_impl_private,
+		if (is_main_region_dsm_handle(handle) ||
+			dsm_impl_op(DSM_OP_DESTROY, handle, 0, &junk_impl_private,
 						&junk_mapped_address, &junk_mapped_size, WARNING))
 		{
 			LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
+			if (is_main_region_dsm_handle(handle))
+				FreePageManagerPut((FreePageManager *) dsm_main_space_begin,
+								   dsm_control->item[control_slot].first_page,
+								   dsm_control->item[control_slot].npages);
 			Assert(dsm_control->item[control_slot].handle == handle);
 			Assert(dsm_control->item[control_slot].refcnt == 1);
 			dsm_control->item[control_slot].refcnt = 0;
@@ -1094,3 +1216,28 @@ dsm_control_bytes_needed(uint32 nitems)
 	return offsetof(dsm_control_header, item)
 		+ sizeof(dsm_control_item) * (uint64) nitems;
 }
+
+static inline dsm_handle
+make_main_region_dsm_handle(int slot)
+{
+	dsm_handle	handle;
+
+	/*
+	 * We need to create a handle that doesn't collide with any existing extra
+	 * segment created by dsm_impl_op(), so we'll make it odd.  It also
+	 * mustn't collide with any other main area pseudo-segment, so we'll
+	 * include the slot number in some of the bits.  We also want to make an
+	 * effort to avoid newly created and recently destroyed handles from being
+	 * confused, so we'll make the rest of the bits random.
+	 */
+	handle = 1;
+	handle |= slot << 1;
+	handle |= random() << (pg_leftmost_one_pos32(dsm_control->maxitems) + 1);
+	return handle;
+}
+
+static inline bool
+is_main_region_dsm_handle(dsm_handle handle)
+{
+	return handle & 1;
+}

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.