Auto-tune effective_cache size to be 4x shared buffers

Author: bmomjian

doc/src/sgml/config.sgml

@@ -2758,7 +2758,7 @@ include 'filename'
 
        <para>
         Random access to mechanical disk storage is normally much more expensive
-        than four-times sequential access.  However, a lower default is used
+        than four times sequential access.  However, a lower default is used
         (4.0) because the majority of random accesses to disk, such as indexed
         reads, are assumed to be in cache.  The default value can be thought of
         as modeling random access as 40 times slower than sequential, while
@@ -2841,9 +2841,17 @@ include 'filename'
       <listitem>
        <para>
         Sets the planner's assumption about the effective size of the
-        disk cache that is available to a single query.  This is
-        factored into estimates of the cost of using an index; a
-        higher value makes it more likely index scans will be used, a
+        disk cache that is available to a single query.  The default
+        setting of -1 selects a size equal to four times the size of <xref
+        linkend="guc-shared-buffers">, but not less than the size of one
+        shared buffer page, typically <literal>8kB</literal>.  This value
+        can be set manually if the automatic choice is too large or too
+        small.
+       </para>
+
+       <para>
+        This value is factored into estimates of the cost of using an index;
+        a higher value makes it more likely index scans will be used, a
         lower value makes it more likely sequential scans will be
         used. When setting this parameter you should consider both
         <productname>PostgreSQL</productname>'s shared buffers and the
@@ -2855,8 +2863,10 @@ include 'filename'
         memory allocated by <productname>PostgreSQL</productname>, nor
         does it reserve kernel disk cache; it is used only for estimation
         purposes.  The system also does not assume data remains in
-        the disk cache between queries.  The default is 128 megabytes
-        (<literal>128MB</>).
+        the disk cache between queries.  The auto-tuning
+        selected by the default setting of -1 should give reasonable
+        results if this database cluster is can utilize most of the memory
+        on this server.
        </para>
       </listitem>
      </varlistentry>

src/backend/optimizer/path/costsize.c

@@ -87,6 +87,7 @@
 #include "optimizer/planmain.h"
 #include "optimizer/restrictinfo.h"
 #include "parser/parsetree.h"
+#include "utils/guc.h"
 #include "utils/lsyscache.h"
 #include "utils/selfuncs.h"
 #include "utils/spccache.h"
@@ -95,14 +96,13 @@
 
 #define LOG2(x)  (log(x) / 0.693147180559945)
 
-
 double		seq_page_cost = DEFAULT_SEQ_PAGE_COST;
 double		random_page_cost = DEFAULT_RANDOM_PAGE_COST;
 double		cpu_tuple_cost = DEFAULT_CPU_TUPLE_COST;
 double		cpu_index_tuple_cost = DEFAULT_CPU_INDEX_TUPLE_COST;
 double		cpu_operator_cost = DEFAULT_CPU_OPERATOR_COST;
 
-int			effective_cache_size = DEFAULT_EFFECTIVE_CACHE_SIZE;
+int			effective_cache_size = -1;
 
 Cost		disable_cost = 1.0e10;
 
@@ -456,6 +456,52 @@ cost_index(IndexPath *path, PlannerInfo *root, double loop_count)
 	path->path.total_cost = startup_cost + run_cost;
 }
 
+void
+set_default_effective_cache_size(void)
+{
+	/*
+	 * If the value of effective_cache_size is -1, use the preferred
+	 * auto-tune value.
+	 */
+	if (effective_cache_size == -1)
+	{
+		char		buf[32];
+
+		snprintf(buf, sizeof(buf), "%d", NBuffers * DEFAULT_EFFECTIVE_CACHE_SIZE_MULTI);
+		SetConfigOption("effective_cache_size", buf, PGC_POSTMASTER, PGC_S_OVERRIDE);
+	}
+	Assert(effective_cache_size > 0);
+}
+
+/*
+ * GUC check_hook for effective_cache_size
+ */
+bool
+check_effective_cache_size(int *newval, void **extra, GucSource source)
+{
+	/*
+	 * -1 indicates a request for auto-tune.
+	 */
+	if (*newval == -1)
+	{
+		/*
+		 * If we haven't yet changed the boot_val default of -1, just let it
+		 * be.	We'll fix it in index_pages_fetched
+		 */
+		if (effective_cache_size == -1)
+			return true;
+
+		/* Otherwise, substitute the auto-tune value */
+		*newval = NBuffers * DEFAULT_EFFECTIVE_CACHE_SIZE_MULTI;
+	}
+
+	/* set minimum? */
+	if (*newval < 1)
+		*newval = 1;
+
+	return true;
+}
+
 /*
  * index_pages_fetched
  *	  Estimate the number of pages actually fetched after accounting for

src/backend/postmaster/postmaster.c

@@ -118,6 +118,7 @@
 #include "utils/builtins.h"
 #include "utils/datetime.h"
 #include "utils/dynamic_loader.h"
+#include "utils/guc.h"
 #include "utils/memutils.h"
 #include "utils/ps_status.h"
 #include "utils/timeout.h"
@@ -4475,6 +4476,8 @@ SubPostmasterMain(int argc, char *argv[])
 	memset(&port, 0, sizeof(Port));
 	read_backend_variables(argv[2], &port);
 
+	set_default_effective_cache_size();
+
 	/*
 	 * Set reference point for stack-depth checking
 	 */

src/backend/utils/misc/guc.c

@@ -2410,8 +2410,8 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_BLOCKS,
 		},
 		&effective_cache_size,
-		DEFAULT_EFFECTIVE_CACHE_SIZE, 1, INT_MAX,
-		NULL, NULL, NULL
+		-1, -1, INT_MAX,
+		check_effective_cache_size, NULL, NULL
 	},
 
 	{

src/include/optimizer/cost.h

@@ -27,7 +27,7 @@
 #define DEFAULT_CPU_INDEX_TUPLE_COST 0.005
 #define DEFAULT_CPU_OPERATOR_COST  0.0025
 
-#define DEFAULT_EFFECTIVE_CACHE_SIZE  16384		/* measured in pages */
+#define DEFAULT_EFFECTIVE_CACHE_SIZE_MULTI 4
 
 typedef enum
 {

src/include/utils/guc.h

@@ -386,6 +386,8 @@ extern void assign_search_path(const char *newval, void *extra);
 
 /* in access/transam/xlog.c */
 extern bool check_wal_buffers(int *newval, void **extra, GucSource source);
+extern bool check_effective_cache_size(int *newval, void **extra, GucSource source);
+extern void set_default_effective_cache_size(void);
 extern void assign_xlog_sync_method(int new_sync_method, void *extra);
 
 #endif   /* GUC_H */

src/test/regress/expected/join.out

@@ -2713,6 +2713,7 @@ where thousand = (q1 + q2);
 --
 -- test placement of movable quals in a parameterized join tree
 --
+set effective_cache_size = '128MB';
 explain (costs off)
 select * from tenk1 t1 left join
   (tenk1 t2 join tenk1 t3 on t2.thousand = t3.unique2)

src/test/regress/sql/join.sql

@@ -711,6 +711,8 @@ where thousand = (q1 + q2);
 -- test placement of movable quals in a parameterized join tree
 --
 
+set effective_cache_size = '128MB';
+
 explain (costs off)
 select * from tenk1 t1 left join
   (tenk1 t2 join tenk1 t3 on t2.thousand = t3.unique2)