Rename huge_tlb_pages to huge_pages, and improve docs.

Christian Kruse

doc/src/sgml/config.sgml

@@ -1166,35 +1166,31 @@ include 'filename'
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-huge-tlb-pages" xreflabel="huge_tlb_pages">
-      <term><varname>huge_tlb_pages</varname> (<type>enum</type>)</term>
+     <varlistentry id="guc-huge-pages" xreflabel="huge_pages">
+      <term><varname>huge_pages</varname> (<type>enum</type>)</term>
       <indexterm>
-       <primary><varname>huge_tlb_pages</> configuration parameter</primary>
+       <primary><varname>huge_pages</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
-        Enables/disables the use of huge TLB pages. Valid values are
+        Enables/disables the use of huge memory pages. Valid values are
         <literal>try</literal> (the default), <literal>on</literal>,
         and <literal>off</literal>.
        </para>
 
        <para>
-        At present, this feature is supported only on Linux. The setting
-        is ignored on other systems.
+        At present, this feature is supported only on Linux. The setting is
+        ignored on other systems when set to <literal>try</literal>.
        </para>
 
        <para>
-        The use of huge TLB pages results in smaller page tables and
-        less CPU time spent on memory management, increasing performance. For
-        more details, see
-        <ulink url="https://wiki.debian.org/Hugepages">the Debian wiki</ulink>.
-        Remember that you will need at least shared_buffers / huge page size +
-        1 huge TLB pages. So for example for a system with 6GB shared buffers
-        and a hugepage size of 2kb of you will need at least 3156 huge pages.
+        The use of huge pages results in smaller page tables and less CPU time
+        spent on memory management, increasing performance. For more details,
+        see <xref linkend="linux-huge-pages">.
        </para>
 
        <para>
-        With <varname>huge_tlb_pages</varname> set to <literal>try</literal>,
+        With <varname>huge_pages</varname> set to <literal>try</literal>,
         the server will try to use huge pages, but fall back to using
         normal allocation if that fails. With <literal>on</literal>, failure
         to use huge pages will prevent the server from starting up. With

doc/src/sgml/runtime.sgml

@@ -1307,6 +1307,57 @@ echo -1000 > /proc/self/oom_score_adj
    </para>
    </note>
   </sect2>
+
+  <sect2 id="linux-huge-pages">
+   <title>Linux huge pages</title>
+
+   <para>
+    Using huge pages reduces overhead when using large contiguous chunks of
+    memory, like <productname>PostgreSQL</productname> does. To enable this
+    feature in <productname>PostgreSQL</productname> you need a kernel
+    with <varname>CONFIG_HUGETLBFS=y</varname> and
+    <varname>CONFIG_HUGETLB_PAGE=y</varname>. You also have to tune the system
+    setting <varname>vm.nr_hugepages</varname>. To estimate the number of
+    necessary huge pages start <productname>PostgreSQL</productname> without
+    huge pages enabled and check the <varname>VmPeak</varname> value from the
+    proc filesystem:
+<programlisting>
+$ <userinput>head -1 /path/to/data/directory/postmaster.pid</userinput>
+4170
+$ <userinput>grep ^VmPeak /proc/4170/status</userinput>
+VmPeak:  6490428 kB
+</programlisting>
+     <literal>6490428</literal> / <literal>2048</literal>
+     (<varname>PAGE_SIZE</varname> is <literal>2MB</literal> in this case) are
+     roughly <literal>3169.154</literal> huge pages, so you will need at
+     least <literal>3170</literal> huge pages:
+<programlisting>
+$ <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
+</programlisting>
+    Sometimes the kernel is not able to allocate the desired number of huge
+    pages, so it might be necessary to repeat that command or to reboot. Don't
+    forget to add an entry to <filename>/etc/sysctl.conf</filename> to persist
+    this setting through reboots.
+   </para>
+
+   <para>
+    The default behavior for huge pages in
+    <productname>PostgreSQL</productname> is to use them when possible and
+    to fallback to normal pages when failing. To enforce the use of huge
+    pages, you can set
+    <link linkend="guc-huge-pages"><varname>huge_pages</varname></link>
+    to <literal>on</literal>. Note that in this case
+    <productname>PostgreSQL</productname> will fail to start if not enough huge
+    pages are available.
+   </para>
+
+   <para>
+    For a detailed description of the <productname>Linux</productname> huge
+    pages feature have a look
+    at <ulink url="https://www.kernel.org/doc/Documentation/vm/hugetlbpage.txt">https://www.kernel.org/doc/Documentation/vm/hugetlbpage.txt</ulink>.
+   </para>
+
+  </sect2>
  </sect1>
 
 

src/backend/port/sysv_shmem.c

@@ -333,12 +333,12 @@ CreateAnonymousSegment(Size *size)
 	int			mmap_errno = 0;
 
 #ifndef MAP_HUGETLB
-	if (huge_tlb_pages == HUGE_TLB_ON)
+	if (huge_pages == HUGE_PAGES_ON)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("huge TLB pages not supported on this platform")));
 #else
-	if (huge_tlb_pages == HUGE_TLB_ON || huge_tlb_pages == HUGE_TLB_TRY)
+	if (huge_pages == HUGE_PAGES_ON || huge_pages == HUGE_PAGES_TRY)
 	{
 		/*
 		 * Round up the request size to a suitable large value.
@@ -364,13 +364,13 @@ CreateAnonymousSegment(Size *size)
 		ptr = mmap(NULL, allocsize, PROT_READ | PROT_WRITE,
 				   PG_MMAP_FLAGS | MAP_HUGETLB, -1, 0);
 		mmap_errno = errno;
-		if (huge_tlb_pages == HUGE_TLB_TRY && ptr == MAP_FAILED)
+		if (huge_pages == HUGE_PAGES_TRY && ptr == MAP_FAILED)
 			elog(DEBUG1, "mmap with MAP_HUGETLB failed, huge pages disabled: %m");
 	}
 #endif
 
-	if (huge_tlb_pages == HUGE_TLB_OFF ||
-		(huge_tlb_pages == HUGE_TLB_TRY && ptr == MAP_FAILED))
+	if (huge_pages == HUGE_PAGES_OFF ||
+		(huge_pages == HUGE_PAGES_TRY && ptr == MAP_FAILED))
 	{
 		/*
 		 * use the original size, not the rounded up value, when falling
@@ -431,10 +431,10 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port)
 	Size		sysvsize;
 
 #if defined(EXEC_BACKEND) || !defined(MAP_HUGETLB)
-	if (huge_tlb_pages == HUGE_TLB_ON)
+	if (huge_pages == HUGE_PAGES_ON)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-				 errmsg("huge TLB pages not supported on this platform")));
+				 errmsg("huge pages not supported on this platform")));
 #endif
 
 	/* Room for a header? */

src/backend/port/win32_shmem.c

@@ -128,10 +128,10 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port)
 	DWORD		size_high;
 	DWORD		size_low;
 
-	if (huge_tlb_pages == HUGE_TLB_ON)
+	if (huge_pages == HUGE_PAGES_ON)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-				 errmsg("huge TLB pages not supported on this platform")));
+				 errmsg("huge pages not supported on this platform")));
 
 	/* Room for a header? */
 	Assert(size > MAXALIGN(sizeof(PGShmemHeader)));

src/backend/utils/misc/guc.c

@@ -393,16 +393,16 @@ static const struct config_enum_entry synchronous_commit_options[] = {
  * Although only "on", "off", "try" are documented, we accept all the likely
  * variants of "on" and "off".
  */
-static const struct config_enum_entry huge_tlb_options[] = {
-	{"off", HUGE_TLB_OFF, false},
-	{"on", HUGE_TLB_ON, false},
-	{"try", HUGE_TLB_TRY, false},
-	{"true", HUGE_TLB_ON, true},
-	{"false", HUGE_TLB_OFF, true},
-	{"yes", HUGE_TLB_ON, true},
-	{"no", HUGE_TLB_OFF, true},
-	{"1", HUGE_TLB_ON, true},
-	{"0", HUGE_TLB_OFF, true},
+static const struct config_enum_entry huge_pages_options[] = {
+	{"off", HUGE_PAGES_OFF, false},
+	{"on", HUGE_PAGES_ON, false},
+	{"try", HUGE_PAGES_TRY, false},
+	{"true", HUGE_PAGES_ON, true},
+	{"false", HUGE_PAGES_OFF, true},
+	{"yes", HUGE_PAGES_ON, true},
+	{"no", HUGE_PAGES_OFF, true},
+	{"1", HUGE_PAGES_ON, true},
+	{"0", HUGE_PAGES_OFF, true},
 	{NULL, 0, false}
 };
 
@@ -470,7 +470,7 @@ int			tcp_keepalives_count;
  * This really belongs in pg_shmem.c, but is defined here so that it doesn't
  * need to be duplicated in all the different implementations of pg_shmem.c.
  */
-int			huge_tlb_pages;
+int			huge_pages;
 
 /*
  * These variables are all dummies that don't do anything, except in some
@@ -3497,12 +3497,12 @@ static struct config_enum ConfigureNamesEnum[] =
 	},
 
 	{
-		{"huge_tlb_pages", PGC_POSTMASTER, RESOURCES_MEM,
-			gettext_noop("Use of huge TLB pages on Linux"),
+		{"huge_pages", PGC_POSTMASTER, RESOURCES_MEM,
+			gettext_noop("Use of huge pages on Linux"),
 			NULL
 		},
-		&huge_tlb_pages,
-		HUGE_TLB_TRY, huge_tlb_options,
+		&huge_pages,
+		HUGE_PAGES_TRY, huge_pages_options,
 		NULL, NULL, NULL
 	},
 

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

@@ -115,7 +115,7 @@
 
 #shared_buffers = 32MB			# min 128kB
 					# (change requires restart)
-#huge_tlb_pages = try			# on, off, or try
+#huge_pages = try			# on, off, or try
 					# (change requires restart)
 #temp_buffers = 8MB			# min 800kB
 #max_prepared_transactions = 0		# zero disables the feature

src/include/storage/pg_shmem.h

@@ -39,15 +39,15 @@ typedef struct PGShmemHeader	/* standard header for all Postgres shmem */
 } PGShmemHeader;
 
 /* GUC variable */
-extern int huge_tlb_pages;
+extern int huge_pages;
 
-/* Possible values for huge_tlb_pages */
+/* Possible values for huge_pages */
 typedef enum
 {
-	HUGE_TLB_OFF,
-	HUGE_TLB_ON,
-	HUGE_TLB_TRY
-} HugeTlbType;
+	HUGE_PAGES_OFF,
+	HUGE_PAGES_ON,
+	HUGE_PAGES_TRY
+} HugePagesType;
 
 #ifndef WIN32
 extern unsigned long UsedShmemSegID;