Refactor CalculateShmemSize()

This function calls many functions which return the amount of shared
memory required for different shared memory data structures. Up until
now, the returned total of these sizes was used to create a single
shared memory segment. But starting the previous patch, we create
multiple shared memory segments each of which contain one shared memory
structure related to shared buffers and one main memory segment
containing rest of the structures. Since CalculateShmemSize() is called
for every shared memory segment, and its return value is added to the
memory required for all the shared memory segments, we end up allocating
more memory than required.

Instead, CalculateShmemSize() is called only once. Each of its callees
are expected to a. return the size required from the main segment b. add
sizes to the AnonymousMappings corresponding to the other memory
segments.

For individual modules to add memory to their respective
AnonymousMappings, we need to know the different mappings upfront. Hence
ANON_MAPPINGS replaces next_free_segment.

TODOs:

1. This change however requires that the AnonymousMappings array and
   macros defining identifiers of each of the segments be
platform-independent.  This patch doesn't achieve that goal for all the
platforms for example windows. We need to fix that.

2. If postgres is invoked with -C shared_memory_size, it reports 0.
   That's because it report the GUC values before share memory sizes are
set in AnonymousMappings. Fix that too.

3.  Eliminate this assymetry in CalculateShmemSize().  See TODO in
    prologue of CalculateShmemSize().

4. This is one way to avoid requesting more memory in each segment. But
   there may be other ways to design CalculateShmemSize(). Need to think
and implement it better.

Author: Ashutosh Bapat

Author: ashutosh-bapat

src/backend/port/sysv_shmem.c

@@ -94,21 +94,7 @@ typedef enum
 unsigned long UsedShmemSegID = 0;
 void	   *UsedShmemSegAddr = NULL;
 
-typedef struct AnonymousMapping
-{
-	int shmem_segment;
-	Size shmem_size; 			/* Size of the actually used memory */
-	Size shmem_reserved; 		/* Size of the reserved mapping */
-	Pointer shmem; 				/* Pointer to the start of the mapped memory */
-	Pointer seg_addr; 			/* SysV shared memory for the header */
-	unsigned long seg_id; 		/* IPC key */
-	int segment_fd; 			/* fd for the backing anon file */
-} AnonymousMapping;
-
-static AnonymousMapping Mappings[ANON_MAPPINGS];
-
-/* Keeps track of used mapping segments */
-static int next_free_segment = 0;
+AnonymousMapping Mappings[ANON_MAPPINGS];
 
 /*
  * Anonymous mapping layout we use looks like this:
@@ -168,7 +154,7 @@ static IpcMemoryState PGSharedMemoryAttach(IpcMemoryId shmId,
 										   void *attachAt,
 										   PGShmemHeader **addr);
 
-static const char*
+const char*
 MappingName(int shmem_segment)
 {
 	switch (shmem_segment)
@@ -193,7 +179,7 @@ MappingName(int shmem_segment)
 static void
 DebugMappings()
 {
-	for(int i = 0; i < next_free_segment; i++)
+	for(int i = 0; i < ANON_MAPPINGS; i++)
 	{
 		AnonymousMapping m = Mappings[i];
 		elog(DEBUG1, "Mapping[%s]: addr %p, size %zu",
@@ -851,9 +837,6 @@ PrepareHugePages()
 {
 	void	   *ptr = MAP_FAILED;
 
-	/* Reset to handle reinitialization */
-	next_free_segment = 0;
-
 	/* Complain if hugepages demanded but we can't possibly support them */
 #if !defined(MAP_HUGETLB)
 	if (huge_pages == HUGE_PAGES_ON)
@@ -876,8 +859,7 @@ PrepareHugePages()
 		 */
 		for(int segment = 0; segment < ANON_MAPPINGS; segment++)
 		{
-			int	numSemas;
-			Size segment_size = CalculateShmemSize(&numSemas, segment);
+			Size segment_size = Mappings[segment].shmem_req_size;
 
 			if (segment_size % hugepagesize != 0)
 				segment_size += hugepagesize - (segment_size % hugepagesize);
@@ -909,7 +891,7 @@ PrepareHugePages()
 static void
 AnonymousShmemDetach(int status, Datum arg)
 {
-	for(int i = 0; i < next_free_segment; i++)
+	for(int i = 0; i < ANON_MAPPINGS; i++)
 	{
 		AnonymousMapping m = Mappings[i];
 
@@ -927,7 +909,7 @@ AnonymousShmemDetach(int status, Datum arg)
 /*
  * PGSharedMemoryCreate
  *
- * Create a shared memory segment of the given size and initialize its
+ * Create a shared memory segment for the given mapping and initialize its
  * standard header.  Also, register an on_shmem_exit callback to release
  * the storage.
  *
@@ -937,15 +919,14 @@ AnonymousShmemDetach(int status, Datum arg)
  * postmaster or backend.
  */
 PGShmemHeader *
-PGSharedMemoryCreate(Size size,
+PGSharedMemoryCreate(AnonymousMapping *mapping,
 					 PGShmemHeader **shim)
 {
 	IpcMemoryKey NextShmemSegID;
 	void	   *memAddress;
 	PGShmemHeader *hdr;
 	struct stat statbuf;
 	Size		sysvsize, total_reserved;
-	AnonymousMapping *mapping = &Mappings[next_free_segment];
 
 	/*
 	 * We use the data directory's ID info (inode and device numbers) to
@@ -965,13 +946,12 @@ PGSharedMemoryCreate(Size size,
 				 errmsg("huge pages not supported with the current \"shared_memory_type\" setting")));
 
 	/* Room for a header? */
-	Assert(size > MAXALIGN(sizeof(PGShmemHeader)));
+	Assert(mapping->shmem_req_size > MAXALIGN(sizeof(PGShmemHeader)));
 
 	/* Prepare the mapping information */
-	mapping->shmem_size = size;
-	mapping->shmem_segment = next_free_segment;
+	mapping->shmem_size = mapping->shmem_req_size;
 	total_reserved = (Size) MaxAvailableMemory * BLCKSZ;
-	mapping->shmem_reserved = total_reserved * SHMEM_RESIZE_RATIO[next_free_segment];
+	mapping->shmem_reserved = total_reserved * SHMEM_RESIZE_RATIO[mapping->shmem_segment];
 
 	/* Round up to be a multiple of BLCKSZ */
 	mapping->shmem_reserved = mapping->shmem_reserved + BLCKSZ -
@@ -982,8 +962,6 @@ PGSharedMemoryCreate(Size size,
 		/* On success, mapping data will be modified. */
 		CreateAnonymousSegment(mapping);
 
-		next_free_segment++;
-
 		/* Register on-exit routine to unmap the anonymous segment */
 		on_shmem_exit(AnonymousShmemDetach, (Datum) 0);
 
@@ -992,7 +970,7 @@ PGSharedMemoryCreate(Size size,
 	}
 	else
 	{
-		sysvsize = size;
+		sysvsize = mapping->shmem_req_size;
 
 		/* huge pages are only available with mmap */
 		SetConfigOption("huge_pages_status", "off",
@@ -1005,7 +983,7 @@ PGSharedMemoryCreate(Size size,
 	 * loop simultaneously.  (CreateDataDirLockFile() does not entirely ensure
 	 * that, but prefer fixing it over coping here.)
 	 */
-	NextShmemSegID = statbuf.st_ino + next_free_segment;
+	NextShmemSegID = statbuf.st_ino + mapping->shmem_segment;
 
 	for (;;)
 	{
@@ -1214,7 +1192,7 @@ PGSharedMemoryNoReAttach(void)
 void
 PGSharedMemoryDetach(void)
 {
-	for(int i = 0; i < next_free_segment; i++)
+	for(int i = 0; i < ANON_MAPPINGS; i++)
 	{
 		AnonymousMapping m = Mappings[i];
 

src/backend/port/win32_shmem.c

@@ -204,7 +204,7 @@ EnableLockPagesPrivilege(int elevel)
  * standard header.
  */
 PGShmemHeader *
-PGSharedMemoryCreate(Size size,
+PGSharedMemoryCreate(AnonymousMapping *mapping,
 					 PGShmemHeader **shim)
 {
 	void	   *memAddress;
@@ -216,7 +216,7 @@ PGSharedMemoryCreate(Size size,
 	DWORD		size_high;
 	DWORD		size_low;
 	SIZE_T		largePageSize = 0;
-	Size		orig_size = size;
+	Size		size = mapping->shmem_req_size;
 	DWORD		flProtect = PAGE_READWRITE;
 	DWORD		desiredAccess;
 
@@ -304,7 +304,7 @@ PGSharedMemoryCreate(Size size,
 				 * Use the original size, not the rounded-up value, when
 				 * falling back to non-huge pages.
 				 */
-				size = orig_size;
+				size = mapping->shmem_req_size;
 				flProtect = PAGE_READWRITE;
 				goto retry;
 			}
@@ -391,6 +391,7 @@ PGSharedMemoryCreate(Size size,
 	hdr->totalsize = size;
 	hdr->freeoffset = MAXALIGN(sizeof(PGShmemHeader));
 	hdr->dsm_control = 0;
+	mapping->shmem_size = size;
 
 	/* Save info for possible future use */
 	UsedShmemSegAddr = memAddress;

src/backend/postmaster/postmaster.c

@@ -963,13 +963,6 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	process_shmem_requests();
 
-	/*
-	 * 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();
-
 	/*
 	 * Now that modules have been loaded, we can process any custom resource
 	 * managers specified in the wal_consistency_checking GUC.
@@ -1005,6 +998,13 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	CreateSharedMemoryAndSemaphores();
 
+	/*
+	 * 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();
+
 	/*
 	 * Estimate number of openable files.  This must happen after setting up
 	 * semaphores, because on some platforms semaphores count as open files.

src/backend/storage/buffer/buf_init.c

@@ -158,48 +158,31 @@ BufferManagerShmemInit(void)
  * data.
  */
 Size
-BufferManagerShmemSize(int shmem_segment)
+BufferManagerShmemSize(void)
 {
-	Size		size = 0;
+	size_t size;
 
-	if (shmem_segment == MAIN_SHMEM_SEGMENT)
-		return size;
+	/* size of buffer descriptors, plus alignment padding */
+	size = add_size(0, mul_size(NBuffers, sizeof(BufferDescPadded)));
+	size = add_size(size, PG_CACHE_LINE_SIZE);
+	Mappings[BUFFER_DESCRIPTORS_SHMEM_SEGMENT].shmem_req_size = size;
 
-	if (shmem_segment == BUFFER_DESCRIPTORS_SHMEM_SEGMENT)
-	{
-		/* size of buffer descriptors */
-		size = add_size(size, mul_size(NBuffers, sizeof(BufferDescPadded)));
-		/* to allow aligning buffer descriptors */
-		size = add_size(size, PG_CACHE_LINE_SIZE);
-	}
+	/* size of data pages, plus alignment padding */
+	size = add_size(0, PG_IO_ALIGN_SIZE);
+	size = add_size(size, mul_size(NBuffers, BLCKSZ));
+	Mappings[BUFFERS_SHMEM_SEGMENT].shmem_req_size = size;
 
-	if (shmem_segment == BUFFERS_SHMEM_SEGMENT)
-	{
-		/* size of data pages, plus alignment padding */
-		size = add_size(size, PG_IO_ALIGN_SIZE);
-		size = add_size(size, mul_size(NBuffers, BLCKSZ));
-	}
+	/* size of stuff controlled by freelist.c */
+	Mappings[STRATEGY_SHMEM_SEGMENT].shmem_req_size = StrategyShmemSize();
 
-	if (shmem_segment == STRATEGY_SHMEM_SEGMENT)
-	{
-		/* size of stuff controlled by freelist.c */
-		size = add_size(size, StrategyShmemSize());
-	}
+	/* size of I/O condition variables, plus alignment padding */
+	size = add_size(0, mul_size(NBuffers,
+								   sizeof(ConditionVariableMinimallyPadded)));
+	size = add_size(size, PG_CACHE_LINE_SIZE);
+	Mappings[BUFFER_IOCV_SHMEM_SEGMENT].shmem_req_size = size;
 
-	if (shmem_segment == BUFFER_IOCV_SHMEM_SEGMENT)
-	{
-		/* size of I/O condition variables */
-		size = add_size(size, mul_size(NBuffers,
-									   sizeof(ConditionVariableMinimallyPadded)));
-		/* to allow aligning the above */
-		size = add_size(size, PG_CACHE_LINE_SIZE);
-	}
-
-	if (shmem_segment == CHECKPOINT_BUFFERS_SHMEM_SEGMENT)
-	{
-		/* size of checkpoint sort array in bufmgr.c */
-		size = add_size(size, mul_size(NBuffers, sizeof(CkptSortItem)));
-	}
+	/* size of checkpoint sort array in bufmgr.c */
+	Mappings[CHECKPOINT_BUFFERS_SHMEM_SEGMENT].shmem_req_size = mul_size(NBuffers, sizeof(CkptSortItem));
 
 	return size;
 }