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Commitaf930e6

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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 laterattempts 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 asuser-set values.There is still a gap in this method: if there's an explicit assignment ofeffective_cache_size = -1 in the postgresql.conf file, and that assignmentappears before shared_buffers is assigned, the code will substitute 4 timesthe 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 wayto 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 theDYNAMIC_DEFAULT mechanism; if that ever happens, we should consider thiscase as one of the things we'd like to improve.
1 parenta4c8f14 commitaf930e6

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2 files changed

+13
-21
lines changed

2 files changed

+13
-21
lines changed

‎src/backend/optimizer/path/costsize.c

Lines changed: 12 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -4107,16 +4107,7 @@ check_effective_cache_size(int *newval, void **extra, GucSource source)
41074107
if (*newval <=0)
41084108
{
41094109
/*
4110-
* If we haven't yet changed the initial default of -1, just let it
4111-
* be.We'll fix it later on during GUC initialization, when
4112-
* set_default_effective_cache_size is called.(If we try to do it
4113-
* immediately, we may not be looking at the final value of NBuffers.)
4114-
*/
4115-
if (effective_cache_size==-1)
4116-
return true;
4117-
4118-
/*
4119-
* Otherwise, substitute the auto-tune value, being wary of overflow.
4110+
* Substitute the auto-tune value, being wary of overflow.
41204111
*/
41214112
if (NBuffers<INT_MAX /4)
41224113
*newval=NBuffers*4;
@@ -4130,22 +4121,22 @@ check_effective_cache_size(int *newval, void **extra, GucSource source)
41304121
}
41314122

41324123
/*
4133-
* initialize effective_cache_size at the end of GUC startup
4124+
* Initialize effective_cache_size at the end of GUC startup, or when
4125+
* a setting in postgresql.conf is removed.
4126+
*
4127+
* Note: check_effective_cache_size() will have been called when the boot_val
4128+
* was installed, but we will not have known the final value of NBuffers at
4129+
* that time, which is why this has to be called at the end of GUC startup.
41344130
*/
41354131
void
41364132
set_default_effective_cache_size(void)
41374133
{
41384134
/*
4139-
* If the value of effective_cache_size is still -1 (or zero), replace it
4140-
* with the auto-tune value.
4135+
* We let check_effective_cache_size() compute the actual setting.Note
4136+
* that this call is a no-op if the user has supplied a setting (since
4137+
* that will have a higher priority than PGC_S_DYNAMIC_DEFAULT).
41414138
*/
4142-
if (effective_cache_size <=0)
4143-
{
4144-
/* disable the short-circuit in check_effective_cache_size */
4145-
effective_cache_size=0;
4146-
/* and let check_effective_cache_size() compute the setting */
4147-
SetConfigOption("effective_cache_size","-1",
4148-
PGC_POSTMASTER,PGC_S_OVERRIDE);
4149-
}
4139+
SetConfigOption("effective_cache_size","-1",
4140+
PGC_POSTMASTER,PGC_S_DYNAMIC_DEFAULT);
41504141
Assert(effective_cache_size>0);
41514142
}

‎src/backend/utils/misc/guc-file.l

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -298,6 +298,7 @@ ProcessConfigFile(GucContext context)
298298
{
299299
InitializeGUCOptionsFromEnvironment();
300300
pg_timezone_abbrev_initialize();
301+
set_default_effective_cache_size();
301302
/* this selects SQL_ASCII in processes not connected to a database */
302303
SetConfigOption("client_encoding",GetDatabaseEncodingName(),
303304
PGC_BACKEND, PGC_S_DYNAMIC_DEFAULT);

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