Change default of backend_flush_after GUC to 0 (disabled).

While beneficial, both for throughput and average/worst case latency, in
a significant number of workloads, there are other workloads in which
backend_flush_after can cause significant performance regressions in
comparison to < 9.6 releases. The regression is most likely when the hot
data set is bigger than shared buffers, but significantly smaller than
the operating system's page cache.

I personally think that the benefit of enabling backend flush control is
considerably bigger than the potential downsides, but a fair argument
can be made that not regressing is more important than improving
performance/latency. As the latter is the consensus, change the default
to 0.

The other settings introduced in 428b1d6b2 do not have the same
potential for regressions, so leave them enabled.

Benchmarks leading up to changing the default have been performed by
Mithun Cy, Ashutosh Sharma and Robert Haas.

Discussion: CAD__OuhPmc6XH=wYRm_+Q657yQE88DakN4=Ybh2oveFasHkoeA@mail.gmail.com

doc/src/sgml/config.sgml

@@ -2038,9 +2038,9 @@ include_dir 'conf.d'
          than the OS's page cache, where performance might degrade.  This
          setting may have no effect on some platforms.  The valid range is
          between <literal>0</literal>, which disables controlled writeback,
-         and <literal>2MB</literal>.  The default is <literal>128Kb</> on
-         Linux, <literal>0</> elsewhere.  (Non-default values of
-         <symbol>BLCKSZ</symbol> change the default and maximum.)
+         and <literal>2MB</literal>.  The default is <literal>0</> (i.e. no
+         flush control).  (Non-default values of <symbol>BLCKSZ</symbol>
+         change the maximum.)
         </para>
        </listitem>
       </varlistentry>

src/backend/utils/misc/guc.c

@@ -2457,8 +2457,7 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_BLOCKS
 		},
 		&backend_flush_after,
-		/* see bufmgr.h: OS dependent default */
-		DEFAULT_BACKEND_FLUSH_AFTER, 0, WRITEBACK_MAX_PENDING_FLUSHES,
+		0, 0, WRITEBACK_MAX_PENDING_FLUSHES,
 		NULL, NULL, NULL
 	},
 

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

@@ -170,8 +170,7 @@
 #max_parallel_workers_per_gather = 2	# taken from max_worker_processes
 #old_snapshot_threshold = -1		# 1min-60d; -1 disables; 0 is immediate
 									# (change requires restart)
-#backend_flush_after = 0		# 0 disables,
-					# default is 128kb on linux, 0 otherwise
+#backend_flush_after = 0		# 0 disables, default is 0
 
 
 #------------------------------------------------------------------------------

src/include/storage/bufmgr.h

@@ -60,11 +60,9 @@ extern PGDLLIMPORT int NBuffers;
 /* FIXME: Also default to on for mmap && msync(MS_ASYNC)? */
 #ifdef HAVE_SYNC_FILE_RANGE
 #define DEFAULT_CHECKPOINT_FLUSH_AFTER 32
-#define DEFAULT_BACKEND_FLUSH_AFTER 16
 #define DEFAULT_BGWRITER_FLUSH_AFTER 64
 #else
 #define DEFAULT_CHECKPOINT_FLUSH_AFTER 0
-#define DEFAULT_BACKEND_FLUSH_AFTER 0
 #define DEFAULT_BGWRITER_FLUSH_AFTER 0
 #endif   /* HAVE_SYNC_FILE_RANGE */