Reduce the default value of vacuum_cost_page_miss.

When commit f425b60 introduced cost based vacuum delays back in 2004,
the defaults reflected then-current trends in hardware, as well as
certain historical limitations in PostgreSQL.  There have been enormous
improvements in both areas since that time.  The cost limit GUC defaults
finally became much more representative of current trends following
commit cbccac3, which decreased autovacuum_vacuum_cost_delay's default
by 10x for PostgreSQL 12 (it went from 20ms to only 2ms).

The relative costs have shifted too.  This should also be accounted for
by the defaults.  More specifically, the relative importance of avoiding
dirtying pages within VACUUM has greatly increased, primarily due to
main memory capacity scaling and trends in flash storage.  Within
Postgres itself, improvements like sequential access during index
vacuuming (at least in nbtree and GiST indexes) have also been
contributing factors.

To reflect all this, decrease the default of vacuum_cost_page_miss to 2.
Since the default of vacuum_cost_page_dirty remains 20, dirtying a page
is now considered 10x "costlier" than a page miss by default.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WzmLPFnkWT8xMjmcsm7YS3+_Qi3iRWAb2+_Bc8UhVyHfuA@mail.gmail.com

Author: petergeoghegan

doc/src/sgml/config.sgml

@@ -2116,7 +2116,7 @@ include_dir 'conf.d'
          The estimated cost for vacuuming a buffer that has to be read from
          disk.  This represents the effort to lock the buffer pool,
          lookup the shared hash table, read the desired block in from
-         the disk and scan its content. The default value is 10.
+         the disk and scan its content. The default value is 2.
         </para>
        </listitem>
       </varlistentry>

src/backend/utils/init/globals.c

@@ -137,7 +137,7 @@ int			max_parallel_workers = 8;
 int			MaxBackends = 0;
 
 int			VacuumCostPageHit = 1;	/* GUC parameters for vacuum */
-int			VacuumCostPageMiss = 10;
+int			VacuumCostPageMiss = 2;
 int			VacuumCostPageDirty = 20;
 int			VacuumCostLimit = 200;
 double		VacuumCostDelay = 0;

src/backend/utils/misc/guc.c

@@ -2413,7 +2413,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&VacuumCostPageMiss,
-		10, 0, 10000,
+		2, 0, 10000,
 		NULL, NULL, NULL
 	},
 

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

@@ -164,7 +164,7 @@
 
 #vacuum_cost_delay = 0			# 0-100 milliseconds (0 disables)
 #vacuum_cost_page_hit = 1		# 0-10000 credits
-#vacuum_cost_page_miss = 10		# 0-10000 credits
+#vacuum_cost_page_miss = 2		# 0-10000 credits
 #vacuum_cost_page_dirty = 20		# 0-10000 credits
 #vacuum_cost_limit = 200		# 1-10000 credits