From: Andres Freund Date: Fri, 10 Jun 2016 22:31:11 +0000 (-0700) Subject: Change default of backend_flush_after GUC to 0 (disabled). X-Git-Tag: REL9_6_BETA2~53 X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=4bc0f165cb4fbd660648c0153485b3d6f55d80ea;p=postgresql 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 --- diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 4f93e7062d..e0e5a1edc6 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 0, which disables controlled writeback, - and 2MB. The default is 128Kb on - Linux, 0 elsewhere. (Non-default values of - BLCKSZ change the default and maximum.) + and 2MB. The default is 0 (i.e. no + flush control). (Non-default values of BLCKSZ + change the maximum.) diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c index cf3eb1a4f1..9b02111a83 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 3ef2a9761c..8260e371bc 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 #------------------------------------------------------------------------------ diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h index 38b602724e..3d5dea7efb 100644 --- a/src/include/storage/bufmgr.h +++ b/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 */