From af930e606a3217db3909029c6c3f8d003ba70920 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Thu, 20 Mar 2014 12:58:13 -0400
Subject: [PATCH] Again fix initialization of auto-tuned effective_cache_size.
The previous method was overly complex and underly correct; in particular,
by assigning the default value with PGC_S_OVERRIDE, it prevented later
attempts to change the setting in postgresql.conf, as noted by Jeff Janes.
We should just assign the default value with source PGC_S_DYNAMIC_DEFAULT,
which will have the desired priority relative to the boot_val as well as
user-set values.
There is still a gap in this method: if there's an explicit assignment of
effective_cache_size = -1 in the postgresql.conf file, and that assignment
appears before shared_buffers is assigned, the code will substitute 4 times
the bootstrap default for shared_buffers, and that value will then persist
(since it will have source PGC_S_FILE). I don't see any very nice way
to avoid that though, and it's not a case to be expected in practice.
The existing comments in guc-file.l look forward to a redesign of the
DYNAMIC_DEFAULT mechanism; if that ever happens, we should consider this
case as one of the things we'd like to improve.
---
src/backend/optimizer/path/costsize.c | 33 ++++++++++-----------------
src/backend/utils/misc/guc-file.l | 1 +
2 files changed, 13 insertions(+), 21 deletions(-)
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 9bca96849d3..326794acb85 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -4107,16 +4107,7 @@ check_effective_cache_size(int *newval, void **extra, GucSource source)
if (*newval <= 0)
{
/*
- * If we haven't yet changed the initial default of -1, just let it
- * be. We'll fix it later on during GUC initialization, when
- * set_default_effective_cache_size is called. (If we try to do it
- * immediately, we may not be looking at the final value of NBuffers.)
- */
- if (effective_cache_size == -1)
- return true;
-
- /*
- * Otherwise, substitute the auto-tune value, being wary of overflow.
+ * Substitute the auto-tune value, being wary of overflow.
*/
if (NBuffers < INT_MAX / 4)
*newval = NBuffers * 4;
@@ -4130,22 +4121,22 @@ check_effective_cache_size(int *newval, void **extra, GucSource source)
}
/*
- * initialize effective_cache_size at the end of GUC startup
+ * Initialize effective_cache_size at the end of GUC startup, or when
+ * a setting in postgresql.conf is removed.
+ *
+ * Note: check_effective_cache_size() will have been called when the boot_val
+ * was installed, but we will not have known the final value of NBuffers at
+ * that time, which is why this has to be called at the end of GUC startup.
*/
void
set_default_effective_cache_size(void)
{
/*
- * If the value of effective_cache_size is still -1 (or zero), replace it
- * with the auto-tune value.
+ * We let check_effective_cache_size() compute the actual setting. Note
+ * that this call is a no-op if the user has supplied a setting (since
+ * that will have a higher priority than PGC_S_DYNAMIC_DEFAULT).
*/
- if (effective_cache_size <= 0)
- {
- /* disable the short-circuit in check_effective_cache_size */
- effective_cache_size = 0;
- /* and let check_effective_cache_size() compute the setting */
- SetConfigOption("effective_cache_size", "-1",
- PGC_POSTMASTER, PGC_S_OVERRIDE);
- }
+ SetConfigOption("effective_cache_size", "-1",
+ PGC_POSTMASTER, PGC_S_DYNAMIC_DEFAULT);
Assert(effective_cache_size > 0);
}
diff --git a/src/backend/utils/misc/guc-file.l b/src/backend/utils/misc/guc-file.l
index 5d830c79d01..ee380405215 100644
--- a/src/backend/utils/misc/guc-file.l
+++ b/src/backend/utils/misc/guc-file.l
@@ -298,6 +298,7 @@ ProcessConfigFile(GucContext context)
{
InitializeGUCOptionsFromEnvironment();
pg_timezone_abbrev_initialize();
+ set_default_effective_cache_size();
/* this selects SQL_ASCII in processes not connected to a database */
SetConfigOption("client_encoding", GetDatabaseEncodingName(),
PGC_BACKEND, PGC_S_DYNAMIC_DEFAULT);
--
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