On 2016-06-09 14:37:31 -0700, Andres Freund wrote:
> I'm writing a patch right now, planning to post it later today, commit
> it tomorrow.
Attached.
>From d86fc0c966efe544b1926652196059539966b137 Mon Sep 17 00:00:00 2001
From: Andres Freund <and...@anarazel.de>
Date: Thu, 9 Jun 2016 17:15:42 -0700
Subject: [PATCH] Change default of backend_flush_after GUC to 0 (disabled).
While beneficial, both for performance 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
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=ybh2ovefashk...@mail.gmail.com
---
doc/src/sgml/config.sgml | 6 +++---
src/backend/utils/misc/guc.c | 3 +--
src/backend/utils/misc/postgresql.conf.sample | 3 +--
3 files changed, 5 insertions(+), 7 deletions(-)
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 4f93e70..e0e5a1e 100644
--- a/doc/src/sgml/config.sgml
+++ b/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>
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index cf3eb1a..9b02111 100644
--- a/src/backend/utils/misc/guc.c
+++ b/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
},
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 3ef2a97..8260e37 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/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
#------------------------------------------------------------------------------
--
2.8.1
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