I wrote:
> Alvaro Herrera <alvhe...@2ndquadrant.com> writes:
>> Would it make sense to remove the only the few oldest entries, instead
>> of all of them? As is, I think this causes a storm of reloads every
>> once in a while, if the number of FKs in the system is large enough.
>> Maybe on a cache hit we could push the entry to the head of the list,
>> and then remove N entries from the back of the list when the threshold
>> is reached.
> (FWIW, it might take less code to put the recently-used entries at the
> back of the list. Then the loop in InvalidateConstraintCacheCallBack
> could just invalidate/delete entries if either they're targets, or
> the current list length exceeds the threshold.)
Specifically, we could change it as per attached. But this adds some
cycles to the mainline usage of fetching a valid cache entry, so I'd
want to see some evidence that there are use-cases it helps before
applying it.
regards, tom lane
diff --git a/src/backend/utils/adt/ri_triggers.c b/src/backend/utils/adt/ri_triggers.c
index 018cb99..54376ed 100644
*** a/src/backend/utils/adt/ri_triggers.c
--- b/src/backend/utils/adt/ri_triggers.c
*************** ri_LoadConstraintInfo(Oid constraintOid)
*** 2814,2820 ****
ri_InitHashTables();
/*
! * Find or create a hash entry. If we find a valid one, just return it.
*/
riinfo = (RI_ConstraintInfo *) hash_search(ri_constraint_cache,
(void *) &constraintOid,
--- 2814,2821 ----
ri_InitHashTables();
/*
! * Find or create a hash entry. If we find a valid one, just return it,
! * after pushing it to the end of the list to mark it recently used.
*/
riinfo = (RI_ConstraintInfo *) hash_search(ri_constraint_cache,
(void *) &constraintOid,
*************** ri_LoadConstraintInfo(Oid constraintOid)
*** 2822,2828 ****
--- 2823,2833 ----
if (!found)
riinfo->valid = false;
else if (riinfo->valid)
+ {
+ dlist_delete(&riinfo->valid_link);
+ dlist_push_tail(&ri_constraint_cache_valid_list, &riinfo->valid_link);
return riinfo;
+ }
/*
* Fetch the pg_constraint row so we can fill in the entry.
*************** ri_LoadConstraintInfo(Oid constraintOid)
*** 2931,2936 ****
--- 2936,2942 ----
/*
* For efficient processing of invalidation messages below, we keep a
* doubly-linked list, and a count, of all currently valid entries.
+ * The most recently used entries are at the *end* of the list.
*/
dlist_push_tail(&ri_constraint_cache_valid_list, &riinfo->valid_link);
ri_constraint_cache_valid_count++;
*************** ri_LoadConstraintInfo(Oid constraintOid)
*** 2948,2953 ****
--- 2954,2965 ----
* Invalidate any ri_constraint_cache entry associated with the syscache
* entry with the specified hash value, or all entries if hashvalue == 0.
*
+ * We also invalidate entries if there are too many valid entries. This rule
+ * avoids O(N^2) behavior in situations where a session touches many foreign
+ * keys and also does many ALTER TABLEs, such as a restore from pg_dump. When
+ * we do kill entries for that reason, they'll be the least recently used
+ * ones, since they're at the front of the list.
+ *
* Note: at the time a cache invalidation message is processed there may be
* active references to the cache. Because of this we never remove entries
* from the cache, but only mark them invalid, which is harmless to active
*************** InvalidateConstraintCacheCallBack(Datum
*** 2961,2981 ****
Assert(ri_constraint_cache != NULL);
- /*
- * If the list of currently valid entries gets excessively large, we mark
- * them all invalid so we can empty the list. This arrangement avoids
- * O(N^2) behavior in situations where a session touches many foreign keys
- * and also does many ALTER TABLEs, such as a restore from pg_dump.
- */
- if (ri_constraint_cache_valid_count > 1000)
- hashvalue = 0; /* pretend it's a cache reset */
-
dlist_foreach_modify(iter, &ri_constraint_cache_valid_list)
{
RI_ConstraintInfo *riinfo = dlist_container(RI_ConstraintInfo,
valid_link, iter.cur);
! if (hashvalue == 0 || riinfo->oidHashValue == hashvalue)
{
riinfo->valid = false;
/* Remove invalidated entries from the list, too */
--- 2973,2985 ----
Assert(ri_constraint_cache != NULL);
dlist_foreach_modify(iter, &ri_constraint_cache_valid_list)
{
RI_ConstraintInfo *riinfo = dlist_container(RI_ConstraintInfo,
valid_link, iter.cur);
! if (hashvalue == 0 || riinfo->oidHashValue == hashvalue ||
! ri_constraint_cache_valid_count > 1000)
{
riinfo->valid = false;
/* Remove invalidated entries from the list, too */
--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers
