0005-Allow-to-use-multiple-shared-memory-mapping-20250918.patch

From 63fe27340656c52b13f4eecebd9e73d24efe5e33 Mon Sep 17 00:00:00 2001
From: Dmitrii Dolgov <9erthalion6@gmail.com>
Date: Fri, 28 Feb 2025 19:54:47 +0100
Subject: [PATCH 05/16] Allow to use multiple shared memory mappings

Currently all the work with shared memory is done via a single anonymous
memory mapping, which limits ways how the shared memory could be organized.

Introduce possibility to allocate multiple shared memory mappings, where
a single mapping is associated with a specified shared memory segment.
There is only fixed amount of available segments, currently only one
main shared memory segment is allocated. A new shared memory API is
introduces, extended with a segment as a new parameter. As a path of
least resistance, the original API is kept in place, utilizing the main
shared memory segment.
---
 src/backend/port/posix_sema.c     |   4 +-
 src/backend/port/sysv_sema.c      |   4 +-
 src/backend/port/sysv_shmem.c     | 138 +++++++++++++++++++---------
 src/backend/port/win32_sema.c     |   2 +-
 src/backend/storage/ipc/ipc.c     |   4 +-
 src/backend/storage/ipc/ipci.c    |  63 +++++++------
 src/backend/storage/ipc/shmem.c   | 148 +++++++++++++++++++++---------
 src/backend/storage/lmgr/lwlock.c |  15 ++-
 src/include/storage/ipc.h         |   2 +-
 src/include/storage/pg_sema.h     |   2 +-
 src/include/storage/pg_shmem.h    |  18 ++++
 src/include/storage/shmem.h       |  11 +++
 12 files changed, 283 insertions(+), 128 deletions(-)

diff --git a/src/backend/port/posix_sema.c b/src/backend/port/posix_sema.c
index 269c7460817..401e1113fa1 100644
--- a/src/backend/port/posix_sema.c
+++ b/src/backend/port/posix_sema.c
@@ -193,7 +193,7 @@ PGSemaphoreShmemSize(int maxSemas)
  * we don't have to expose the counters to other processes.)
  */
 void
-PGReserveSemaphores(int maxSemas)
+PGReserveSemaphores(int maxSemas, int shmem_segment)
 {
 	struct stat statbuf;
 
@@ -220,7 +220,7 @@ PGReserveSemaphores(int maxSemas)
 	 * ShmemAlloc() won't be ready yet.
 	 */
 	sharedSemas = (PGSemaphore)
-		ShmemAllocUnlocked(PGSemaphoreShmemSize(maxSemas));
+		ShmemAllocUnlockedInSegment(PGSemaphoreShmemSize(maxSemas), shmem_segment);
 #endif
 
 	numSems = 0;
diff --git a/src/backend/port/sysv_sema.c b/src/backend/port/sysv_sema.c
index 6ac83ea1a82..7bb363989c4 100644
--- a/src/backend/port/sysv_sema.c
+++ b/src/backend/port/sysv_sema.c
@@ -327,7 +327,7 @@ PGSemaphoreShmemSize(int maxSemas)
  * have clobbered.)
  */
 void
-PGReserveSemaphores(int maxSemas)
+PGReserveSemaphores(int maxSemas, int shmem_segment)
 {
 	struct stat statbuf;
 
@@ -348,7 +348,7 @@ PGReserveSemaphores(int maxSemas)
 	 * ShmemAlloc() won't be ready yet.
 	 */
 	sharedSemas = (PGSemaphore)
-		ShmemAllocUnlocked(PGSemaphoreShmemSize(maxSemas));
+		ShmemAllocUnlockedInSegment(PGSemaphoreShmemSize(maxSemas), shmem_segment);
 	numSharedSemas = 0;
 	maxSharedSemas = maxSemas;
 
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 197926d44f6..56af0231d24 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -94,8 +94,19 @@ typedef enum
 unsigned long UsedShmemSegID = 0;
 void	   *UsedShmemSegAddr = NULL;
 
-static Size AnonymousShmemSize;
-static void *AnonymousShmem = NULL;
+typedef struct AnonymousMapping
+{
+	int shmem_segment;
+	Size shmem_size; 			/* Size of the 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 */
+} AnonymousMapping;
+
+static AnonymousMapping Mappings[ANON_MAPPINGS];
+
+/* Keeps track of used mapping segments */
+static int next_free_segment = 0;
 
 static void *InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size);
 static void IpcMemoryDetach(int status, Datum shmaddr);
@@ -104,6 +115,28 @@ static IpcMemoryState PGSharedMemoryAttach(IpcMemoryId shmId,
 										   void *attachAt,
 										   PGShmemHeader **addr);
 
+static const char*
+MappingName(int shmem_segment)
+{
+	switch (shmem_segment)
+	{
+		case MAIN_SHMEM_SEGMENT:
+			return "main";
+		default:
+			return "unknown";
+	}
+}
+
+static void
+DebugMappings()
+{
+	for(int i = 0; i < next_free_segment; i++)
+	{
+		AnonymousMapping m = Mappings[i];
+		elog(DEBUG1, "Mapping[%s]: addr %p, size %zu",
+			 MappingName(i), m.shmem, m.shmem_size);
+	}
+}
 
 /*
  *	InternalIpcMemoryCreate(memKey, size)
@@ -591,14 +624,13 @@ check_huge_page_size(int *newval, void **extra, GucSource source)
 /*
  * Creates an anonymous mmap()ed shared memory segment.
  *
- * Pass the requested size in *size.  This function will modify *size to the
- * actual size of the allocation, if it ends up allocating a segment that is
- * larger than requested.
+ * This function will modify mapping size to the actual size of the allocation,
+ * if it ends up allocating a segment that is larger than requested.
  */
-static void *
-CreateAnonymousSegment(Size *size)
+static void
+CreateAnonymousSegment(AnonymousMapping *mapping)
 {
-	Size		allocsize = *size;
+	Size		allocsize = mapping->shmem_size;
 	void	   *ptr = MAP_FAILED;
 	int			mmap_errno = 0;
 
@@ -623,8 +655,11 @@ CreateAnonymousSegment(Size *size)
 				   PG_MMAP_FLAGS | mmap_flags, -1, 0);
 		mmap_errno = errno;
 		if (huge_pages == HUGE_PAGES_TRY && ptr == MAP_FAILED)
-			elog(DEBUG1, "mmap(%zu) with MAP_HUGETLB failed, huge pages disabled: %m",
-				 allocsize);
+		{
+			DebugMappings();
+			elog(DEBUG1, "segment[%s]: mmap(%zu) with MAP_HUGETLB failed, huge pages disabled: %m",
+				 MappingName(mapping->shmem_segment), allocsize);
+		}
 	}
 #endif
 
@@ -642,7 +677,7 @@ CreateAnonymousSegment(Size *size)
 		 * Use the original size, not the rounded-up value, when falling back
 		 * to non-huge pages.
 		 */
-		allocsize = *size;
+		allocsize = mapping->shmem_size;
 		ptr = mmap(NULL, allocsize, PROT_READ | PROT_WRITE,
 				   PG_MMAP_FLAGS, -1, 0);
 		mmap_errno = errno;
@@ -651,8 +686,10 @@ CreateAnonymousSegment(Size *size)
 	if (ptr == MAP_FAILED)
 	{
 		errno = mmap_errno;
+		DebugMappings();
 		ereport(FATAL,
-				(errmsg("could not map anonymous shared memory: %m"),
+				(errmsg("segment[%s]: could not map anonymous shared memory: %m",
+						MappingName(mapping->shmem_segment)),
 				 (mmap_errno == ENOMEM) ?
 				 errhint("This error usually means that PostgreSQL's request "
 						 "for a shared memory segment exceeded available memory, "
@@ -663,8 +700,8 @@ CreateAnonymousSegment(Size *size)
 						 allocsize) : 0));
 	}
 
-	*size = allocsize;
-	return ptr;
+	mapping->shmem = ptr;
+	mapping->shmem_size = allocsize;
 }
 
 /*
@@ -674,13 +711,18 @@ CreateAnonymousSegment(Size *size)
 static void
 AnonymousShmemDetach(int status, Datum arg)
 {
-	/* Release anonymous shared memory block, if any. */
-	if (AnonymousShmem != NULL)
+	for(int i = 0; i < next_free_segment; i++)
 	{
-		if (munmap(AnonymousShmem, AnonymousShmemSize) < 0)
-			elog(LOG, "munmap(%p, %zu) failed: %m",
-				 AnonymousShmem, AnonymousShmemSize);
-		AnonymousShmem = NULL;
+		AnonymousMapping m = Mappings[i];
+
+		/* Release anonymous shared memory block, if any. */
+		if (m.shmem != NULL)
+		{
+			if (munmap(m.shmem, m.shmem_size) < 0)
+				elog(LOG, "munmap(%p, %zu) failed: %m",
+					 m.shmem, m.shmem_size);
+			m.shmem = NULL;
+		}
 	}
 }
 
@@ -705,6 +747,7 @@ PGSharedMemoryCreate(Size size,
 	PGShmemHeader *hdr;
 	struct stat statbuf;
 	Size		sysvsize;
+	AnonymousMapping *mapping = &Mappings[next_free_segment];
 
 	/*
 	 * We use the data directory's ID info (inode and device numbers) to
@@ -733,11 +776,15 @@ PGSharedMemoryCreate(Size size,
 
 	/* Room for a header? */
 	Assert(size > MAXALIGN(sizeof(PGShmemHeader)));
+	mapping->shmem_size = size;
+	mapping->shmem_segment = next_free_segment;
 
 	if (shared_memory_type == SHMEM_TYPE_MMAP)
 	{
-		AnonymousShmem = CreateAnonymousSegment(&size);
-		AnonymousShmemSize = 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);
@@ -760,7 +807,7 @@ PGSharedMemoryCreate(Size size,
 	 * loop simultaneously.  (CreateDataDirLockFile() does not entirely ensure
 	 * that, but prefer fixing it over coping here.)
 	 */
-	NextShmemSegID = statbuf.st_ino;
+	NextShmemSegID = statbuf.st_ino + next_free_segment;
 
 	for (;;)
 	{
@@ -852,13 +899,13 @@ PGSharedMemoryCreate(Size size,
 	/*
 	 * Initialize space allocation status for segment.
 	 */
-	hdr->totalsize = size;
+	hdr->totalsize = mapping->shmem_size;
 	hdr->freeoffset = MAXALIGN(sizeof(PGShmemHeader));
 	*shim = hdr;
 
 	/* Save info for possible future use */
-	UsedShmemSegAddr = memAddress;
-	UsedShmemSegID = (unsigned long) NextShmemSegID;
+	mapping->seg_addr = memAddress;
+	mapping->seg_id = (unsigned long) NextShmemSegID;
 
 	/*
 	 * If AnonymousShmem is NULL here, then we're not using anonymous shared
@@ -866,10 +913,10 @@ PGSharedMemoryCreate(Size size,
 	 * block. Otherwise, the System V shared memory block is only a shim, and
 	 * we must return a pointer to the real block.
 	 */
-	if (AnonymousShmem == NULL)
+	if (mapping->shmem == NULL)
 		return hdr;
-	memcpy(AnonymousShmem, hdr, sizeof(PGShmemHeader));
-	return (PGShmemHeader *) AnonymousShmem;
+	memcpy(mapping->shmem, hdr, sizeof(PGShmemHeader));
+	return (PGShmemHeader *) mapping->shmem;
 }
 
 #ifdef EXEC_BACKEND
@@ -969,23 +1016,28 @@ PGSharedMemoryNoReAttach(void)
 void
 PGSharedMemoryDetach(void)
 {
-	if (UsedShmemSegAddr != NULL)
+	for(int i = 0; i < next_free_segment; i++)
 	{
-		if ((shmdt(UsedShmemSegAddr) < 0)
+		AnonymousMapping m = Mappings[i];
+
+		if (m.seg_addr != NULL)
+		{
+			if ((shmdt(m.seg_addr) < 0)
 #if defined(EXEC_BACKEND) && defined(__CYGWIN__)
-		/* Work-around for cygipc exec bug */
-			&& shmdt(NULL) < 0
+			/* Work-around for cygipc exec bug */
+				&& shmdt(NULL) < 0
 #endif
-			)
-			elog(LOG, "shmdt(%p) failed: %m", UsedShmemSegAddr);
-		UsedShmemSegAddr = NULL;
-	}
+				)
+				elog(LOG, "shmdt(%p) failed: %m", m.seg_addr);
+			m.seg_addr = NULL;
+		}
 
-	if (AnonymousShmem != NULL)
-	{
-		if (munmap(AnonymousShmem, AnonymousShmemSize) < 0)
-			elog(LOG, "munmap(%p, %zu) failed: %m",
-				 AnonymousShmem, AnonymousShmemSize);
-		AnonymousShmem = NULL;
+		if (m.shmem != NULL)
+		{
+			if (munmap(m.shmem, m.shmem_size) < 0)
+				elog(LOG, "munmap(%p, %zu) failed: %m",
+					 m.shmem, m.shmem_size);
+			m.shmem = NULL;
+		}
 	}
 }
diff --git a/src/backend/port/win32_sema.c b/src/backend/port/win32_sema.c
index 5854ad1f54d..e7365ff8060 100644
--- a/src/backend/port/win32_sema.c
+++ b/src/backend/port/win32_sema.c
@@ -44,7 +44,7 @@ PGSemaphoreShmemSize(int maxSemas)
  * process exits.
  */
 void
-PGReserveSemaphores(int maxSemas)
+PGReserveSemaphores(int maxSemas, int shmem_segment)
 {
 	mySemSet = (HANDLE *) malloc(maxSemas * sizeof(HANDLE));
 	if (mySemSet == NULL)
diff --git a/src/backend/storage/ipc/ipc.c b/src/backend/storage/ipc/ipc.c
index 2704e80b3a7..1965b2d3eb4 100644
--- a/src/backend/storage/ipc/ipc.c
+++ b/src/backend/storage/ipc/ipc.c
@@ -61,6 +61,8 @@ static void proc_exit_prepare(int code);
  * but provide some additional features we need --- in particular,
  * we want to register callbacks to invoke when we are disconnecting
  * from a broken shared-memory context but not exiting the postmaster.
+ * Maximum number of such exit callbacks depends on the number of shared
+ * segments.
  *
  * Callback functions can take zero, one, or two args: the first passed
  * arg is the integer exitcode, the second is the Datum supplied when
@@ -68,7 +70,7 @@ static void proc_exit_prepare(int code);
  * ----------------------------------------------------------------
  */
 
-#define MAX_ON_EXITS 20
+#define MAX_ON_EXITS 40
 
 struct ONEXIT
 {
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 2fa045e6b0f..8b38e985327 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -86,7 +86,7 @@ RequestAddinShmemSpace(Size size)
  * required.
  */
 Size
-CalculateShmemSize(int *num_semaphores)
+CalculateShmemSize(int *num_semaphores, int shmem_segment)
 {
 	Size		size;
 	int			numSemas;
@@ -206,33 +206,38 @@ CreateSharedMemoryAndSemaphores(void)
 
 	Assert(!IsUnderPostmaster);
 
-	/* Compute the size of the shared-memory block */
-	size = CalculateShmemSize(&numSemas);
-	elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
-
-	/*
-	 * Create the shmem segment
-	 */
-	seghdr = PGSharedMemoryCreate(size, &shim);
-
-	/*
-	 * Make sure that huge pages are never reported as "unknown" while the
-	 * server is running.
-	 */
-	Assert(strcmp("unknown",
-				  GetConfigOption("huge_pages_status", false, false)) != 0);
-
-	InitShmemAccess(seghdr);
-
-	/*
-	 * Create semaphores
-	 */
-	PGReserveSemaphores(numSemas);
-
-	/*
-	 * Set up shared memory allocation mechanism
-	 */
-	InitShmemAllocation();
+	for(int segment = 0; segment < ANON_MAPPINGS; segment++)
+	{
+		/* Compute the size of the shared-memory block */
+		size = CalculateShmemSize(&numSemas, segment);
+		elog(DEBUG3, "invoking IpcMemoryCreate(size=%zu)", size);
+
+		/*
+		 * Create the shmem segment.
+		 *
+		 * XXX: Do multiple shims are needed, one per segment?
+		 */
+		seghdr = PGSharedMemoryCreate(size, &shim);
+
+		/*
+		 * Make sure that huge pages are never reported as "unknown" while the
+		 * server is running.
+		 */
+		Assert(strcmp("unknown",
+					  GetConfigOption("huge_pages_status", false, false)) != 0);
+
+		InitShmemAccessInSegment(seghdr, segment);
+
+		/*
+		 * Create semaphores
+		 */
+		PGReserveSemaphores(numSemas, segment);
+
+		/*
+		 * Set up shared memory allocation mechanism
+		 */
+		InitShmemAllocationInSegment(segment);
+	}
 
 	/* Initialize subsystems */
 	CreateOrAttachShmemStructs();
@@ -363,7 +368,7 @@ InitializeShmemGUCs(void)
 	/*
 	 * Calculate the shared memory size and round up to the nearest megabyte.
 	 */
-	size_b = CalculateShmemSize(&num_semas);
+	size_b = CalculateShmemSize(&num_semas, MAIN_SHMEM_SEGMENT);
 	size_mb = add_size(size_b, (1024 * 1024) - 1) / (1024 * 1024);
 	sprintf(buf, "%zu", size_mb);
 	SetConfigOption("shared_memory_size", buf,
diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c
index a0770e86796..f185ed28f95 100644
--- a/src/backend/storage/ipc/shmem.c
+++ b/src/backend/storage/ipc/shmem.c
@@ -76,19 +76,19 @@
 #include "utils/builtins.h"
 
 static void *ShmemAllocRaw(Size size, Size *allocated_size);
+static void *ShmemAllocRawInSegment(Size size, Size *allocated_size,
+								 int shmem_segment);
 
 /* shared memory global variables */
 
-static PGShmemHeader *ShmemSegHdr;	/* shared mem segment header */
+ShmemSegment Segments[ANON_MAPPINGS];
 
-static void *ShmemBase;			/* start address of shared memory */
-
-static void *ShmemEnd;			/* end+1 address of shared memory */
-
-slock_t    *ShmemLock;			/* spinlock for shared memory and LWLock
-								 * allocation */
-
-static HTAB *ShmemIndex = NULL; /* primary index hashtable for shmem */
+/*
+ * Primary index hashtable for shmem, for simplicity we use a single for all
+ * shared memory segments. There can be performance consequences of that, and
+ * an alternative option would be to have one index per shared memory segments.
+ */
+static HTAB *ShmemIndex = NULL;
 
 /* To get reliable results for NUMA inquiry we need to "touch pages" once */
 static bool firstNumaTouch = true;
@@ -101,9 +101,17 @@ Datum		pg_numa_available(PG_FUNCTION_ARGS);
 void
 InitShmemAccess(PGShmemHeader *seghdr)
 {
-	ShmemSegHdr = seghdr;
-	ShmemBase = seghdr;
-	ShmemEnd = (char *) ShmemBase + seghdr->totalsize;
+	InitShmemAccessInSegment(seghdr, MAIN_SHMEM_SEGMENT);
+}
+
+void
+InitShmemAccessInSegment(PGShmemHeader *seghdr, int shmem_segment)
+{
+	PGShmemHeader *shmhdr = (PGShmemHeader *) seghdr;
+	ShmemSegment *seg = &Segments[shmem_segment];
+	seg->ShmemSegHdr = shmhdr;
+	seg->ShmemBase = (void *) shmhdr;
+	seg->ShmemEnd = (char *) seg->ShmemBase + shmhdr->totalsize;
 }
 
 /*
@@ -114,7 +122,13 @@ InitShmemAccess(PGShmemHeader *seghdr)
 void
 InitShmemAllocation(void)
 {
-	PGShmemHeader *shmhdr = ShmemSegHdr;
+	InitShmemAllocationInSegment(MAIN_SHMEM_SEGMENT);
+}
+
+void
+InitShmemAllocationInSegment(int shmem_segment)
+{
+	PGShmemHeader *shmhdr = Segments[shmem_segment].ShmemSegHdr;
 	char	   *aligned;
 
 	Assert(shmhdr != NULL);
@@ -123,9 +137,9 @@ InitShmemAllocation(void)
 	 * Initialize the spinlock used by ShmemAlloc.  We must use
 	 * ShmemAllocUnlocked, since obviously ShmemAlloc can't be called yet.
 	 */
-	ShmemLock = (slock_t *) ShmemAllocUnlocked(sizeof(slock_t));
+	Segments[shmem_segment].ShmemLock = (slock_t *) ShmemAllocUnlockedInSegment(sizeof(slock_t), shmem_segment);
 
-	SpinLockInit(ShmemLock);
+	SpinLockInit(Segments[shmem_segment].ShmemLock);
 
 	/*
 	 * Allocations after this point should go through ShmemAlloc, which
@@ -150,11 +164,17 @@ InitShmemAllocation(void)
  */
 void *
 ShmemAlloc(Size size)
+{
+	return ShmemAllocInSegment(size, MAIN_SHMEM_SEGMENT);
+}
+
+void *
+ShmemAllocInSegment(Size size, int shmem_segment)
 {
 	void	   *newSpace;
 	Size		allocated_size;
 
-	newSpace = ShmemAllocRaw(size, &allocated_size);
+	newSpace = ShmemAllocRawInSegment(size, &allocated_size, shmem_segment);
 	if (!newSpace)
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
@@ -184,6 +204,12 @@ ShmemAllocNoError(Size size)
  */
 static void *
 ShmemAllocRaw(Size size, Size *allocated_size)
+{
+	return ShmemAllocRawInSegment(size, allocated_size, MAIN_SHMEM_SEGMENT);
+}
+
+static void *
+ShmemAllocRawInSegment(Size size, Size *allocated_size, int shmem_segment)
 {
 	Size		newStart;
 	Size		newFree;
@@ -203,22 +229,22 @@ ShmemAllocRaw(Size size, Size *allocated_size)
 	size = CACHELINEALIGN(size);
 	*allocated_size = size;
 
-	Assert(ShmemSegHdr != NULL);
+	Assert(Segments[shmem_segment].ShmemSegHdr != NULL);
 
-	SpinLockAcquire(ShmemLock);
+	SpinLockAcquire(Segments[shmem_segment].ShmemLock);
 
-	newStart = ShmemSegHdr->freeoffset;
+	newStart = Segments[shmem_segment].ShmemSegHdr->freeoffset;
 
 	newFree = newStart + size;
-	if (newFree <= ShmemSegHdr->totalsize)
+	if (newFree <= Segments[shmem_segment].ShmemSegHdr->totalsize)
 	{
-		newSpace = (char *) ShmemBase + newStart;
-		ShmemSegHdr->freeoffset = newFree;
+		newSpace = (char *) Segments[shmem_segment].ShmemBase + newStart;
+		Segments[shmem_segment].ShmemSegHdr->freeoffset = newFree;
 	}
 	else
 		newSpace = NULL;
 
-	SpinLockRelease(ShmemLock);
+	SpinLockRelease(Segments[shmem_segment].ShmemLock);
 
 	/* note this assert is okay with newSpace == NULL */
 	Assert(newSpace == (void *) CACHELINEALIGN(newSpace));
@@ -236,6 +262,12 @@ ShmemAllocRaw(Size size, Size *allocated_size)
  */
 void *
 ShmemAllocUnlocked(Size size)
+{
+	return ShmemAllocUnlockedInSegment(size, MAIN_SHMEM_SEGMENT);
+}
+
+void *
+ShmemAllocUnlockedInSegment(Size size, int shmem_segment)
 {
 	Size		newStart;
 	Size		newFree;
@@ -246,19 +278,19 @@ ShmemAllocUnlocked(Size size)
 	 */
 	size = MAXALIGN(size);
 
-	Assert(ShmemSegHdr != NULL);
+	Assert(Segments[shmem_segment].ShmemSegHdr != NULL);
 
-	newStart = ShmemSegHdr->freeoffset;
+	newStart = Segments[shmem_segment].ShmemSegHdr->freeoffset;
 
 	newFree = newStart + size;
-	if (newFree > ShmemSegHdr->totalsize)
+	if (newFree > Segments[shmem_segment].ShmemSegHdr->totalsize)
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
 				 errmsg("out of shared memory (%zu bytes requested)",
 						size)));
-	ShmemSegHdr->freeoffset = newFree;
+	Segments[shmem_segment].ShmemSegHdr->freeoffset = newFree;
 
-	newSpace = (char *) ShmemBase + newStart;
+	newSpace = (char *) Segments[shmem_segment].ShmemBase + newStart;
 
 	Assert(newSpace == (void *) MAXALIGN(newSpace));
 
@@ -273,7 +305,13 @@ ShmemAllocUnlocked(Size size)
 bool
 ShmemAddrIsValid(const void *addr)
 {
-	return (addr >= ShmemBase) && (addr < ShmemEnd);
+	return ShmemAddrIsValidInSegment(addr, MAIN_SHMEM_SEGMENT);
+}
+
+bool
+ShmemAddrIsValidInSegment(const void *addr, int shmem_segment)
+{
+	return (addr >= Segments[shmem_segment].ShmemBase) && (addr < Segments[shmem_segment].ShmemEnd);
 }
 
 /*
@@ -334,6 +372,18 @@ ShmemInitHash(const char *name,		/* table string name for shmem index */
 			  int64 max_size,	/* max size of the table */
 			  HASHCTL *infoP,	/* info about key and bucket size */
 			  int hash_flags)	/* info about infoP */
+{
+	return ShmemInitHashInSegment(name, init_size, max_size, infoP, hash_flags,
+							   MAIN_SHMEM_SEGMENT);
+}
+
+HTAB *
+ShmemInitHashInSegment(const char *name,		/* table string name for shmem index */
+			  long init_size,		/* initial table size */
+			  long max_size,		/* max size of the table */
+			  HASHCTL *infoP,		/* info about key and bucket size */
+			  int hash_flags,		/* info about infoP */
+			  int shmem_segment) 	/* in which segment to keep the table */
 {
 	bool		found;
 	void	   *location;
@@ -350,9 +400,9 @@ ShmemInitHash(const char *name,		/* table string name for shmem index */
 	hash_flags |= HASH_SHARED_MEM | HASH_ALLOC | HASH_DIRSIZE;
 
 	/* look it up in the shmem index */
-	location = ShmemInitStruct(name,
+	location = ShmemInitStructInSegment(name,
 							   hash_get_shared_size(infoP, hash_flags),
-							   &found);
+							   &found, shmem_segment);
 
 	/*
 	 * if it already exists, attach to it rather than allocate and initialize
@@ -385,6 +435,13 @@ ShmemInitHash(const char *name,		/* table string name for shmem index */
  */
 void *
 ShmemInitStruct(const char *name, Size size, bool *foundPtr)
+{
+	return ShmemInitStructInSegment(name, size, foundPtr, MAIN_SHMEM_SEGMENT);
+}
+
+void *
+ShmemInitStructInSegment(const char *name, Size size, bool *foundPtr,
+					  int shmem_segment)
 {
 	ShmemIndexEnt *result;
 	void	   *structPtr;
@@ -393,7 +450,7 @@ ShmemInitStruct(const char *name, Size size, bool *foundPtr)
 
 	if (!ShmemIndex)
 	{
-		PGShmemHeader *shmemseghdr = ShmemSegHdr;
+		PGShmemHeader *shmemseghdr = Segments[shmem_segment].ShmemSegHdr;
 
 		/* Must be trying to create/attach to ShmemIndex itself */
 		Assert(strcmp(name, "ShmemIndex") == 0);
@@ -416,7 +473,7 @@ ShmemInitStruct(const char *name, Size size, bool *foundPtr)
 			 * process can be accessing shared memory yet.
 			 */
 			Assert(shmemseghdr->index == NULL);
-			structPtr = ShmemAlloc(size);
+			structPtr = ShmemAllocInSegment(size, shmem_segment);
 			shmemseghdr->index = structPtr;
 			*foundPtr = false;
 		}
@@ -433,8 +490,8 @@ ShmemInitStruct(const char *name, Size size, bool *foundPtr)
 		LWLockRelease(ShmemIndexLock);
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
-				 errmsg("could not create ShmemIndex entry for data structure \"%s\"",
-						name)));
+				 errmsg("could not create ShmemIndex entry for data structure \"%s\" in segment %d",
+						name, shmem_segment)));
 	}
 
 	if (*foundPtr)
@@ -459,7 +516,7 @@ ShmemInitStruct(const char *name, Size size, bool *foundPtr)
 		Size		allocated_size;
 
 		/* It isn't in the table yet. allocate and initialize it */
-		structPtr = ShmemAllocRaw(size, &allocated_size);
+		structPtr = ShmemAllocRawInSegment(size, &allocated_size, shmem_segment);
 		if (structPtr == NULL)
 		{
 			/* out of memory; remove the failed ShmemIndex entry */
@@ -478,14 +535,13 @@ ShmemInitStruct(const char *name, Size size, bool *foundPtr)
 
 	LWLockRelease(ShmemIndexLock);
 
-	Assert(ShmemAddrIsValid(structPtr));
+	Assert(ShmemAddrIsValidInSegment(structPtr, shmem_segment));
 
 	Assert(structPtr == (void *) CACHELINEALIGN(structPtr));
 
 	return structPtr;
 }
 
-
 /*
  * Add two Size values, checking for overflow
  */
@@ -542,10 +598,11 @@ pg_get_shmem_allocations(PG_FUNCTION_ARGS)
 
 	/* output all allocated entries */
 	memset(nulls, 0, sizeof(nulls));
+	/* XXX: take all shared memory segments into account. */
 	while ((ent = (ShmemIndexEnt *) hash_seq_search(&hstat)) != NULL)
 	{
 		values[0] = CStringGetTextDatum(ent->key);
-		values[1] = Int64GetDatum((char *) ent->location - (char *) ShmemSegHdr);
+		values[1] = Int64GetDatum((char *) ent->location - (char *) Segments[MAIN_SHMEM_SEGMENT].ShmemSegHdr);
 		values[2] = Int64GetDatum(ent->size);
 		values[3] = Int64GetDatum(ent->allocated_size);
 		named_allocated += ent->allocated_size;
@@ -557,15 +614,15 @@ pg_get_shmem_allocations(PG_FUNCTION_ARGS)
 	/* output shared memory allocated but not counted via the shmem index */
 	values[0] = CStringGetTextDatum("<anonymous>");
 	nulls[1] = true;
-	values[2] = Int64GetDatum(ShmemSegHdr->freeoffset - named_allocated);
+	values[2] = Int64GetDatum(Segments[MAIN_SHMEM_SEGMENT].ShmemSegHdr->freeoffset - named_allocated);
 	values[3] = values[2];
 	tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
 
 	/* output as-of-yet unused shared memory */
 	nulls[0] = true;
-	values[1] = Int64GetDatum(ShmemSegHdr->freeoffset);
+	values[1] = Int64GetDatum(Segments[MAIN_SHMEM_SEGMENT].ShmemSegHdr->freeoffset);
 	nulls[1] = false;
-	values[2] = Int64GetDatum(ShmemSegHdr->totalsize - ShmemSegHdr->freeoffset);
+	values[2] = Int64GetDatum(Segments[MAIN_SHMEM_SEGMENT].ShmemSegHdr->totalsize - Segments[MAIN_SHMEM_SEGMENT].ShmemSegHdr->freeoffset);
 	values[3] = values[2];
 	tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
 
@@ -630,7 +687,12 @@ pg_get_shmem_allocations_numa(PG_FUNCTION_ARGS)
 	 * this is not very likely, and moreover we have more entries, each of
 	 * them using only fraction of the total pages.
 	 */
-	shm_total_page_count = (ShmemSegHdr->totalsize / os_page_size) + 1;
+	for(int segment = 0; segment < ANON_MAPPINGS; segment++)
+	{
+		PGShmemHeader *shmhdr = Segments[segment].ShmemSegHdr;
+		shm_total_page_count += (shmhdr->totalsize / os_page_size) + 1;
+	}
+
 	page_ptrs = palloc0(sizeof(void *) * shm_total_page_count);
 	pages_status = palloc(sizeof(int) * shm_total_page_count);
 
diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index 46c82c63ca5..93792a83af9 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -80,6 +80,8 @@
 #include "pg_trace.h"
 #include "pgstat.h"
 #include "port/pg_bitutils.h"
+#include "postmaster/postmaster.h"
+#include "storage/pg_shmem.h"
 #include "storage/proc.h"
 #include "storage/proclist.h"
 #include "storage/procnumber.h"
@@ -594,12 +596,15 @@ LWLockNewTrancheId(const char *name)
 	/*
 	 * We use the ShmemLock spinlock to protect LWLockCounter and
 	 * LWLockTrancheNames.
+	 * 
+	 * XXX: Looks like this is the only use of Segments outside of shmem.c,
+	 * it's maybe worth it to reshape this part to hide Segments structure.
 	 */
-	SpinLockAcquire(ShmemLock);
+	SpinLockAcquire(Segments[MAIN_SHMEM_SEGMENT].ShmemLock);
 
 	if (*LWLockCounter - LWTRANCHE_FIRST_USER_DEFINED >= MAX_NAMED_TRANCHES)
 	{
-		SpinLockRelease(ShmemLock);
+		SpinLockRelease(Segments[MAIN_SHMEM_SEGMENT].ShmemLock);
 		ereport(ERROR,
 				(errmsg("maximum number of tranches already registered"),
 				 errdetail("No more than %d tranches may be registered.",
@@ -610,7 +615,7 @@ LWLockNewTrancheId(const char *name)
 	LocalLWLockCounter = *LWLockCounter;
 	strlcpy(LWLockTrancheNames[result - LWTRANCHE_FIRST_USER_DEFINED], name, NAMEDATALEN);
 
-	SpinLockRelease(ShmemLock);
+	SpinLockRelease(Segments[MAIN_SHMEM_SEGMENT].ShmemLock);
 
 	return result;
 }
@@ -732,9 +737,9 @@ GetLWTrancheName(uint16 trancheId)
 	 */
 	if (trancheId >= LocalLWLockCounter)
 	{
-		SpinLockAcquire(ShmemLock);
+		SpinLockAcquire(Segments[MAIN_SHMEM_SEGMENT].ShmemLock);
 		LocalLWLockCounter = *LWLockCounter;
-		SpinLockRelease(ShmemLock);
+		SpinLockRelease(Segments[MAIN_SHMEM_SEGMENT].ShmemLock);
 
 		if (trancheId >= LocalLWLockCounter)
 			elog(ERROR, "tranche %d is not registered", trancheId);
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index 3baf418b3d1..6ebda479ced 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -77,7 +77,7 @@ extern void check_on_shmem_exit_lists_are_empty(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
-extern Size CalculateShmemSize(int *num_semaphores);
+extern Size CalculateShmemSize(int *num_semaphores, int shmem_segment);
 extern void CreateSharedMemoryAndSemaphores(void);
 #ifdef EXEC_BACKEND
 extern void AttachSharedMemoryStructs(void);
diff --git a/src/include/storage/pg_sema.h b/src/include/storage/pg_sema.h
index fa6ca35a51f..8ae9637fcd0 100644
--- a/src/include/storage/pg_sema.h
+++ b/src/include/storage/pg_sema.h
@@ -41,7 +41,7 @@ typedef HANDLE PGSemaphore;
 extern Size PGSemaphoreShmemSize(int maxSemas);
 
 /* Module initialization (called during postmaster start or shmem reinit) */
-extern void PGReserveSemaphores(int maxSemas);
+extern void PGReserveSemaphores(int maxSemas, int shmem_segment);
 
 /* Allocate a PGSemaphore structure with initial count 1 */
 extern PGSemaphore PGSemaphoreCreate(void);
diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index 5f7d4b83a60..2348c59b5a0 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -25,6 +25,7 @@
 #define PG_SHMEM_H
 
 #include "storage/dsm_impl.h"
+#include "storage/spin.h"
 
 typedef struct PGShmemHeader	/* standard header for all Postgres shmem */
 {
@@ -41,6 +42,20 @@ typedef struct PGShmemHeader	/* standard header for all Postgres shmem */
 #endif
 } PGShmemHeader;
 
+typedef struct ShmemSegment
+{
+	PGShmemHeader *ShmemSegHdr; 	/* shared mem segment header */
+	void *ShmemBase; 				/* start address of shared memory */
+	void *ShmemEnd; 				/* end+1 address of shared memory */
+	slock_t    *ShmemLock; 			/* spinlock for shared memory and LWLock
+									 * allocation */
+} ShmemSegment;
+
+/* Number of available segments for anonymous memory mappings */
+#define ANON_MAPPINGS 1
+
+extern PGDLLIMPORT ShmemSegment Segments[ANON_MAPPINGS];
+
 /* GUC variables */
 extern PGDLLIMPORT int shared_memory_type;
 extern PGDLLIMPORT int huge_pages;
@@ -91,4 +106,7 @@ extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
 extern void PGSharedMemoryDetach(void);
 extern void GetHugePageSize(Size *hugepagesize, int *mmap_flags);
 
+/* The main segment, contains everything except buffer blocks and related data. */
+#define MAIN_SHMEM_SEGMENT 0
+
 #endif							/* PG_SHMEM_H */
diff --git a/src/include/storage/shmem.h b/src/include/storage/shmem.h
index cd683a9d2d9..910c43f54f4 100644
--- a/src/include/storage/shmem.h
+++ b/src/include/storage/shmem.h
@@ -30,15 +30,26 @@ extern PGDLLIMPORT slock_t *ShmemLock;
 typedef struct PGShmemHeader PGShmemHeader; /* avoid including
 											 * storage/pg_shmem.h here */
 extern void InitShmemAccess(PGShmemHeader *seghdr);
+extern void InitShmemAccessInSegment(struct PGShmemHeader *seghdr,
+									 int shmem_segment);
 extern void InitShmemAllocation(void);
+extern void InitShmemAllocationInSegment(int shmem_segment);
 extern void *ShmemAlloc(Size size);
+extern void *ShmemAllocInSegment(Size size, int shmem_segment);
 extern void *ShmemAllocNoError(Size size);
 extern void *ShmemAllocUnlocked(Size size);
+extern void *ShmemAllocUnlockedInSegment(Size size, int shmem_segment);
 extern bool ShmemAddrIsValid(const void *addr);
+extern bool ShmemAddrIsValidInSegment(const void *addr, int shmem_segment);
 extern void InitShmemIndex(void);
 extern HTAB *ShmemInitHash(const char *name, int64 init_size, int64 max_size,
 						   HASHCTL *infoP, int hash_flags);
+extern HTAB *ShmemInitHashInSegment(const char *name, long init_size,
+									long max_size, HASHCTL *infoP,
+									int hash_flags, int shmem_segment);
 extern void *ShmemInitStruct(const char *name, Size size, bool *foundPtr);
+extern void *ShmemInitStructInSegment(const char *name, Size size,
+									  bool *foundPtr, int shmem_segment);
 extern Size add_size(Size s1, Size s2);
 extern Size mul_size(Size s1, Size s2);
 
-- 
2.34.1