Date: Thu, 23 Mar 2023 20:50:50 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, sfrost@snowman.net,
	alvherre@alvh.no-ip.org, andres@anarazel.de,
	nathandbossart@gmail.com, jchampion@timescale.com,
	john.naylor@enterprisedb.com, masao.fujii@oss.nttdata.com,
	noriyoshi.shinoda@hpe.com, pgsql-hackers@postgresql.org,
	rjuju123@gmail.com, sawada.mshk@gmail.com, tgl@sss.pgh.pa.us,
	thomas.munro@gmail.com
Subject: Re: Improve logging when using Huge Pages
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On Thu, Mar 23, 2023 at 05:25:46PM +0900, Kyotaro Horiguchi wrote:
> Wouldn't storing the value in the shared memory itself work? Though,
> that space does become almost dead for the server's lifetime...

I'm sure it's possible, but it's also not worth writing a special
implementation just to handle huge_pages_active, which is better written
in 30 lines than in 300 lines.

If we needed to avoid using a GUC, maybe it'd work to add
huge_pages_active to PGShmemHeader.  But "avoid using gucs at all costs"
isn't the goal, and using a GUC parallels all the similar, and related
things without needing to allocate extra bits of shared_memory.

On Thu, Mar 23, 2023 at 07:23:28AM +0900, Michael Paquier wrote:
> On Wed, Mar 22, 2023 at 05:18:28PM -0500, Justin Pryzby wrote:
> > Wow, good point.  I think to make it work we'd need put
> > huge_pages_active into BackendParameters and handle it in
> > save_backend_variables().  If so, that'd be strong argument for using a
> > GUC, which already has all the necessary infrastructure for exposing the
> > server's state.
> 
> I am afraid so, duplicating an existing infrastructure for a need like
> the one of this thread is not really appealing.

This goes back to using a GUC, and:

 - removes GUC_RUNTIME_COMPUTED, since that causes a useless error when
   using -C if the server is running, rather than successfully printing
   "unknown".
 - I also renamed it from huge_pages_active = {true,false,unknown} to
   huge_pages_STATUS = {on,off,unknown}.  This parallels huge_pages,
   which is documented to accept values on/off/try.  Also, true/false
   isn't how bools are displayed.
 - I realized that the rename suggested implementing it as an enum GUC,
   and re-using the existing HUGE_PAGES_{ON,OFF} values (and adding an
   "UNKNOWN").  Maybe this also avoids Stephen's earlier objection to
   using a string ?

I mis-used cirrusci to check that the GUC does work correctly under
windows.

If there's continuing aversions to using a GUC, please say so, and try
to suggest an alternate implementation you think would be justified.
It'd need to work under windows with EXEC_BACKEND.

-- 
Justin

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From 951d481b74ac978482b9d5794b437f741518f3a9 Mon Sep 17 00:00:00 2001
From: Justin Pryzby <pryzbyj@telsasoft.com>
Date: Mon, 23 Jan 2023 18:33:51 -0600
Subject: [PATCH v6] add GUC: huge_pages_active

This is useful to show the current state of huge pages when
huge_pages=try.  The effective status is not otherwise visible without
OS level tools like gdb or /proc/N/smaps.

https://www.postgresql.org/message-id/flat/TU4PR8401MB1152EBB0D271F827E2E37A01EECC9@TU4PR8401MB1152.NAMPRD84.PROD.OUTLOOK.COM
---
 doc/src/sgml/config.sgml            | 17 ++++++++++++++++-
 src/backend/port/sysv_shmem.c       |  3 +++
 src/backend/port/win32_shmem.c      |  4 ++++
 src/backend/utils/misc/guc_tables.c | 20 ++++++++++++++++++++
 src/include/storage/pg_shmem.h      |  5 +++--
 5 files changed, 46 insertions(+), 3 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 510ff88fc5e..88998238645 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1708,7 +1708,8 @@ include_dir 'conf.d'
         server will try to request huge pages, but fall back to the default if
         that fails. With <literal>on</literal>, failure to request huge pages
         will prevent the server from starting up. With <literal>off</literal>,
-        huge pages will not be requested.
+        huge pages will not be requested.  The actual state of huge pages is
+        indicated by the server variable <xref linkend="guc-huge-pages-status"/>.
        </para>
 
        <para>
@@ -10660,6 +10661,20 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-huge-pages-status" xreflabel="huge_pages_status">
+      <term><varname>huge_pages_status</varname> (<type>enum</type>)
+      <indexterm>
+       <primary><varname>huge_pages_status</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the state of huge pages in the current instance.
+        See <xref linkend="guc-huge-pages"/> for more information.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
       <term><varname>integer_datetimes</varname> (<type>boolean</type>)
       <indexterm>
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index eaba244bc9c..b6104c6ce60 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -627,6 +627,9 @@ CreateAnonymousSegment(Size *size)
 	}
 #endif
 
+	SetConfigOption("huge_pages_status", ptr == MAP_FAILED ? "off" : "on",
+					PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
+
 	if (ptr == MAP_FAILED && huge_pages != HUGE_PAGES_ON)
 	{
 		/*
diff --git a/src/backend/port/win32_shmem.c b/src/backend/port/win32_shmem.c
index 62e08074770..199f2e23a13 100644
--- a/src/backend/port/win32_shmem.c
+++ b/src/backend/port/win32_shmem.c
@@ -327,6 +327,10 @@ retry:
 			Sleep(1000);
 			continue;
 		}
+
+		SetConfigOption("huge_pages_status", (flProtect & SEC_LARGE_PAGES) ?
+						"on" : "off", PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
+
 		break;
 	}
 
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 8696d965dee..785bee948e9 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -348,6 +348,13 @@ static const struct config_enum_entry huge_pages_options[] = {
 	{NULL, 0, false}
 };
 
+static const struct config_enum_entry huge_pages_status_options[] = {
+	{"off", HUGE_PAGES_OFF, false},
+	{"on", HUGE_PAGES_ON, false},
+	{"unknown", HUGE_PAGES_UNKNOWN, false},
+	{NULL, 0, false}
+};
+
 static const struct config_enum_entry recovery_prefetch_options[] = {
 	{"off", RECOVERY_PREFETCH_OFF, false},
 	{"on", RECOVERY_PREFETCH_ON, false},
@@ -536,6 +543,8 @@ int			ssl_renegotiation_limit;
 int			huge_pages = HUGE_PAGES_TRY;
 int			huge_page_size;
 
+int			huge_pages_status = HUGE_PAGES_UNKNOWN;
+
 /*
  * These variables are all dummies that don't do anything, except in some
  * cases provide the value for SHOW to display.  The real state is elsewhere
@@ -4833,6 +4842,17 @@ struct config_enum ConfigureNamesEnum[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"huge_pages_status", PGC_INTERNAL, PRESET_OPTIONS,
+			gettext_noop("Indicates the status of huge pages."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+		},
+		&huge_pages_status,
+		HUGE_PAGES_UNKNOWN, huge_pages_status_options,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"recovery_prefetch", PGC_SIGHUP, WAL_RECOVERY,
 			gettext_noop("Prefetch referenced blocks during recovery."),
diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index 4dd05f156d5..bfda42b0b26 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -46,12 +46,13 @@ extern PGDLLIMPORT int shared_memory_type;
 extern PGDLLIMPORT int huge_pages;
 extern PGDLLIMPORT int huge_page_size;
 
-/* Possible values for huge_pages */
+/* Possible values for huge_pages/huge_pages_status */
 typedef enum
 {
 	HUGE_PAGES_OFF,
 	HUGE_PAGES_ON,
-	HUGE_PAGES_TRY
+	HUGE_PAGES_TRY,		/* only for huge_pages */
+	HUGE_PAGES_UNKNOWN	/* only for huge_pages_status */
 }			HugePagesType;
 
 /* Possible values for shared_memory_type */
-- 
2.34.1


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