pg_prewarm: Add automatic prewarm feature.

Periodically while the server is running, and at shutdown, write out a
list of blocks in shared buffers.  When the server reaches consistency
-- unfortunatey, we can't do it before that point without breaking
things -- reload those blocks into any still-unused shared buffers.

Mithun Cy and Robert Haas, reviewed and tested by Beena Emerson,
Amit Kapila, Jim Nasby, and Rafia Sabih.

Discussion: http://postgr.es/m/CAD__OugubOs1Vy7kgF6xTjmEqTR4CrGAv8w+ZbaY_+MZeitukw@mail.gmail.com

contrib/file_fdw/data/list1.csv

+1,foo
+1,bar

contrib/file_fdw/data/list2.bad

+2,baz
+1,qux

contrib/file_fdw/data/list2.csv

+2,baz
+2,qux

contrib/pg_prewarm/Makefile

 # contrib/pg_prewarm/Makefile
 
 MODULE_big = pg_prewarm
-OBJS = pg_prewarm.o $(WIN32RES)
+OBJS = pg_prewarm.o autoprewarm.o $(WIN32RES)
 
 EXTENSION = pg_prewarm
-DATA = pg_prewarm--1.1.sql pg_prewarm--1.0--1.1.sql
+DATA = pg_prewarm--1.1--1.2.sql pg_prewarm--1.1.sql pg_prewarm--1.0--1.1.sql
 PGFILEDESC = "pg_prewarm - preload relation data into system buffer cache"
 
 ifdef USE_PGXS

contrib/pg_prewarm/pg_prewarm--1.1--1.2.sql

+/* contrib/pg_prewarm/pg_prewarm--1.1--1.2.sql */
+
+-- complain if script is sourced in psql, rather than via ALTER EXTENSION
+\echo Use "ALTER EXTENSION pg_prewarm UPDATE TO '1.2'" to load this file. \quit
+
+CREATE FUNCTION autoprewarm_start_worker()
+RETURNS VOID STRICT
+AS 'MODULE_PATHNAME', 'autoprewarm_start_worker'
+LANGUAGE C;
+
+CREATE FUNCTION autoprewarm_dump_now()
+RETURNS pg_catalog.int8 STRICT
+AS 'MODULE_PATHNAME', 'autoprewarm_dump_now'
+LANGUAGE C;

contrib/pg_prewarm/pg_prewarm.control

 # pg_prewarm extension
 comment = 'prewarm relation data'
-default_version = '1.1'
+default_version = '1.2'
 module_pathname = '$libdir/pg_prewarm'
 relocatable = true

doc/src/sgml/pgprewarm.sgml

@@ -10,7 +10,13 @@
  <para>
   The <filename>pg_prewarm</filename> module provides a convenient way
   to load relation data into either the operating system buffer cache
-  or the <productname>PostgreSQL</productname> buffer cache.
+  or the <productname>PostgreSQL</productname> buffer cache.  Prewarming
+  can be performed manually using the <filename>pg_prewarm</> function,
+  or can be performed automatically by including <literal>pg_prewarm</> in
+  <xref linkend="guc-shared-preload-libraries">.  In the latter case, the
+  system will run a background worker which periodically records the contents
+  of shared buffers in a file called <filename>autoprewarm.blocks</> and
+  will, using 2 background workers, reload those same blocks after a restart.
  </para>
 
  <sect2>
@@ -55,6 +61,67 @@ pg_prewarm(regclass, mode text default 'buffer', fork text default 'main',
    cache. For these reasons, prewarming is typically most useful at startup,
    when caches are largely empty.
   </para>
+
+<synopsis>
+autoprewarm_start_worker() RETURNS void
+</synopsis>
+
+  <para>
+   Launch the main autoprewarm worker.  This will normally happen
+   automatically, but is useful if automatic prewarm was not configured at
+   server startup time and you wish to start up the worker at a later time.
+  </para>
+
+<synopsis>
+autoprewarm_dump_now() RETURNS int8
+</synopsis>
+
+  <para>
+   Update <filename>autoprewarm.blocks</> immediately.  This may be useful
+   if the autoprewarm worker is not running but you anticipate running it
+   after the next restart.  The return value is the number of records written
+   to <filename>autoprewarm.blocks</>.
+  </para>
+ </sect2>
+
+ <sect2>
+  <title>Configuration Parameters</title>
+
+ <variablelist>
+   <varlistentry>
+    <term>
+     <varname>pg_prewarm.autoprewarm</varname> (<type>boolean</type>)
+     <indexterm>
+      <primary><varname>pg_prewarm.autoprewarm</> configuration parameter</primary>
+     </indexterm>
+    </term>
+    <listitem>
+     <para>
+      Controls whether the server should run the autoprewarm worker. This is
+      on by default. This parameter can only be set at server start.
+     </para>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+
+  <variablelist>
+   <varlistentry>
+   <term>
+     <varname>pg_prewarm.autoprewarm_interval</varname> (<type>int</type>)
+     <indexterm>
+      <primary><varname>pg_prewarm.autoprewarm_interval</> configuration parameter</primary>
+     </indexterm>
+    </term>
+    <listitem>
+     <para>
+      This is the interval between updates to <literal>autoprewarm.blocks</>.
+      The default is 300 seconds. If set to 0, the file will not be
+      dumped at regular intervals, but only when the server is shut down.
+     </para>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+
  </sect2>
 
  <sect2>

src/backend/storage/buffer/freelist.c

@@ -168,6 +168,23 @@ ClockSweepTick(void)
 	return victim;
 }
 
+/*
+ * have_free_buffer -- a lockless check to see if there is a free buffer in
+ *					   buffer pool.
+ *
+ * If the result is true that will become stale once free buffers are moved out
+ * by other operations, so the caller who strictly want to use a free buffer
+ * should not call this.
+ */
+bool
+have_free_buffer()
+{
+	if (StrategyControl->firstFreeBuffer >= 0)
+		return true;
+	else
+		return false;
+}
+
 /*
  * StrategyGetBuffer
  *

src/include/storage/buf_internals.h

@@ -317,6 +317,7 @@ extern void StrategyNotifyBgWriter(int bgwprocno);
 
 extern Size StrategyShmemSize(void);
 extern void StrategyInitialize(bool init);
+extern bool have_free_buffer(void);
 
 /* buf_table.c */
 extern Size BufTableShmemSize(int size);

src/tools/pgindent/typedefs.list

@@ -138,6 +138,7 @@ AttrDefault
 AttrNumber
 AttributeOpts
 AuthRequest
+AutoPrewarmSharedState
 AutoVacOpts
 AutoVacuumShmemStruct
 AutoVacuumWorkItem
@@ -218,6 +219,7 @@ BlobInfo
 Block
 BlockId
 BlockIdData
+BlockInfoRecord
 BlockNumber
 BlockSampler
 BlockSamplerData