Add huge_page_size setting for use on Linux.

This allows the huge page size to be set explicitly.  The default is 0,
meaning it will use the system default, as before.

Author: Odin Ugedal <odin@ugedal.com>
Discussion: https://postgr.es/m/20200608154639.20254-1-odin%40ugedal.com

src/backend/port/sysv_shmem.c

@@ -32,6 +32,7 @@
 #endif
 
 #include "miscadmin.h"
+#include "port/pg_bitutils.h"
 #include "portability/mem.h"
 #include "storage/dsm.h"
 #include "storage/fd.h"
@@ -448,7 +449,7 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
 #ifdef MAP_HUGETLB
 
 /*
- * Identify the huge page size to use.
+ * Identify the huge page size to use, and compute the related mmap flags.
  *
  * Some Linux kernel versions have a bug causing mmap() to fail on requests
  * that are not a multiple of the hugepage size.  Versions without that bug
@@ -464,25 +465,13 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
  * hugepage sizes, we might want to think about more invasive strategies,
  * such as increasing shared_buffers to absorb the extra space.
  *
- * Returns the (real or assumed) page size into *hugepagesize,
+ * Returns the (real, assumed or config provided) page size into *hugepagesize,
  * and the hugepage-related mmap flags to use into *mmap_flags.
- *
- * Currently *mmap_flags is always just MAP_HUGETLB.  Someday, on systems
- * that support it, we might OR in additional bits to specify a particular
- * non-default huge page size.
  */
 static void
 GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 {
-	/*
-	 * If we fail to find out the system's default huge page size, assume it
-	 * is 2MB.  This will work fine when the actual size is less.  If it's
-	 * more, we might get mmap() or munmap() failures due to unaligned
-	 * requests; but at this writing, there are no reports of any non-Linux
-	 * systems being picky about that.
-	 */
-	*hugepagesize = 2 * 1024 * 1024;
-	*mmap_flags = MAP_HUGETLB;
+	Size		default_hugepagesize = 0;
 
 	/*
 	 * System-dependent code to find out the default huge page size.
@@ -491,6 +480,7 @@ GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 	 * nnnn kB".  Ignore any failures, falling back to the preset default.
 	 */
 #ifdef __linux__
+
 	{
 		FILE	   *fp = AllocateFile("/proc/meminfo", "r");
 		char		buf[128];
@@ -505,7 +495,7 @@ GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 				{
 					if (ch == 'k')
 					{
-						*hugepagesize = sz * (Size) 1024;
+						default_hugepagesize = sz * (Size) 1024;
 						break;
 					}
 					/* We could accept other units besides kB, if needed */
@@ -515,6 +505,44 @@ GetHugePageSize(Size *hugepagesize, int *mmap_flags)
 		}
 	}
 #endif							/* __linux__ */
+
+	if (huge_page_size != 0)
+	{
+		/* If huge page size is requested explicitly, use that. */
+		*hugepagesize = (Size) huge_page_size * 1024;
+	}
+	else if (default_hugepagesize != 0)
+	{
+		/* Otherwise use the system default, if we have it. */
+		*hugepagesize = default_hugepagesize;
+	}
+	else
+	{
+		/*
+		 * If we fail to find out the system's default huge page size, or no
+		 * huge page size is requested explicitly, assume it is 2MB. This will
+		 * work fine when the actual size is less.  If it's more, we might get
+		 * mmap() or munmap() failures due to unaligned requests; but at this
+		 * writing, there are no reports of any non-Linux systems being picky
+		 * about that.
+		 */
+		*hugepagesize = 2 * 1024 * 1024;
+	}
+
+	*mmap_flags = MAP_HUGETLB;
+
+	/*
+	 * On recent enough Linux, also include the explicit page size, if
+	 * necessary.
+	 */
+#if defined(MAP_HUGE_MASK) && defined(MAP_HUGE_SHIFT)
+	if (*hugepagesize != default_hugepagesize)
+	{
+		int			shift = pg_ceil_log2_64(*hugepagesize);
+
+		*mmap_flags |= (shift & MAP_HUGE_MASK) << MAP_HUGE_SHIFT;
+	}
+#endif
 }
 
 #endif							/* MAP_HUGETLB */
@@ -583,7 +611,7 @@ CreateAnonymousSegment(Size *size)
 						 "(currently %zu bytes), reduce PostgreSQL's shared "
 						 "memory usage, perhaps by reducing shared_buffers or "
 						 "max_connections.",
-						 *size) : 0));
+						 allocsize) : 0));
 	}
 
 	*size = allocsize;

src/backend/utils/misc/guc.c

@@ -20,11 +20,14 @@
 #include <float.h>
 #include <math.h>
 #include <limits.h>
-#include <unistd.h>
+#ifndef WIN32
+#include <sys/mman.h>
+#endif
 #include <sys/stat.h>
 #ifdef HAVE_SYSLOG
 #include <syslog.h>
 #endif
+#include <unistd.h>
 
 #include "access/commit_ts.h"
 #include "access/gin.h"
@@ -198,6 +201,7 @@ static bool check_max_wal_senders(int *newval, void **extra, GucSource source);
 static bool check_autovacuum_work_mem(int *newval, void **extra, GucSource source);
 static bool check_effective_io_concurrency(int *newval, void **extra, GucSource source);
 static bool check_maintenance_io_concurrency(int *newval, void **extra, GucSource source);
+static bool check_huge_page_size(int *newval, void **extra, GucSource source);
 static void assign_pgstat_temp_directory(const char *newval, void *extra);
 static bool check_application_name(char **newval, void **extra, GucSource source);
 static void assign_application_name(const char *newval, void *extra);
@@ -576,6 +580,7 @@ int			ssl_renegotiation_limit;
  * need to be duplicated in all the different implementations of pg_shmem.c.
  */
 int			huge_pages;
+int			huge_page_size;
 
 /*
  * These variables are all dummies that don't do anything, except in some
@@ -3381,6 +3386,17 @@ static struct config_int ConfigureNamesInt[] =
 		NULL, assign_tcp_user_timeout, show_tcp_user_timeout
 	},
 
+	{
+		{"huge_page_size", PGC_POSTMASTER, RESOURCES_MEM,
+			gettext_noop("The size of huge page that should be requested."),
+			NULL,
+			GUC_UNIT_KB
+		},
+		&huge_page_size,
+		0, 0, INT_MAX,
+		check_huge_page_size, NULL, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, 0, 0, 0, NULL, NULL, NULL
@@ -11565,6 +11581,20 @@ check_maintenance_io_concurrency(int *newval, void **extra, GucSource source)
 	return true;
 }
 
+static bool
+check_huge_page_size(int *newval, void **extra, GucSource source)
+{
+#if !(defined(MAP_HUGE_MASK) && defined(MAP_HUGE_SHIFT))
+	/* Recent enough Linux only, for now.  See GetHugePageSize(). */
+	if (*newval != 0)
+	{
+		GUC_check_errdetail("huge_page_size must be 0 on this platform.");
+		return false;
+	}
+#endif
+	return true;
+}
+
 static void
 assign_pgstat_temp_directory(const char *newval, void *extra)
 {

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

@@ -122,6 +122,8 @@
 					# (change requires restart)
 #huge_pages = try			# on, off, or try
 					# (change requires restart)
+#huge_page_size = 0			# zero for system default
+					# (change requires restart)
 #temp_buffers = 8MB			# min 800kB
 #max_prepared_transactions = 0		# zero disables the feature
 					# (change requires restart)

src/include/storage/pg_shmem.h

@@ -44,6 +44,7 @@ typedef struct PGShmemHeader	/* standard header for all Postgres shmem */
 /* GUC variables */
 extern int	shared_memory_type;
 extern int	huge_pages;
+extern int	huge_page_size;
 
 /* Possible values for huge_pages */
 typedef enum