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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.
This commit is contained in:
Tom Lane 2014-03-20 12:58:13 -04:00
parent a4c8f14364
commit af930e606a
2 changed files with 13 additions and 21 deletions
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33
src/backend/optimizer/path/costsize.c
View File

@ -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);
}

1
src/backend/utils/misc/guc-file.l
View File

@ -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);