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Commit0bc0036

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Rework MultiXactId cache code
The original performs too poorly; in some scenarios it shows way toohigh while profiling. Try to make it a bit smarter to avoid excessivecosst. In particular, make it have a maximum size, and have entries besorted in LRU order; once the max size is reached, evict the oldestentry to avoid it from growing too large.Per complaint from Andres Freund in connection with new tuple freezingcode.
1 parent0cfc67b commit0bc0036

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+42
-10
lines changed

1 file changed

+42
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lines changed

‎src/backend/access/transam/multixact.c

Lines changed: 42 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -72,6 +72,7 @@
7272
#include"catalog/pg_type.h"
7373
#include"commands/dbcommands.h"
7474
#include"funcapi.h"
75+
#include"lib/ilist.h"
7576
#include"miscadmin.h"
7677
#include"pg_trace.h"
7778
#include"postmaster/autovacuum.h"
@@ -261,13 +262,15 @@ static MultiXactId *OldestVisibleMXactId;
261262
*/
262263
typedefstructmXactCacheEnt
263264
{
264-
structmXactCacheEnt*next;
265265
MultiXactIdmulti;
266266
intnmembers;
267+
dlist_nodenode;
267268
MultiXactMembermembers[FLEXIBLE_ARRAY_MEMBER];
268269
}mXactCacheEnt;
269270

270-
staticmXactCacheEnt*MXactCache=NULL;
271+
#defineMAX_CACHE_ENTRIES256
272+
staticdlist_headMXactCache=DLIST_STATIC_INIT(MXactCache);
273+
staticintMXactCacheMembers=0;
271274
staticMemoryContextMXactContext=NULL;
272275

273276
#ifdefMULTIXACT_DEBUG
@@ -1301,16 +1304,18 @@ mxactMemberComparator(const void *arg1, const void *arg2)
13011304
staticMultiXactId
13021305
mXactCacheGetBySet(intnmembers,MultiXactMember*members)
13031306
{
1304-
mXactCacheEnt*entry;
1307+
dlist_iteriter;
13051308

13061309
debug_elog3(DEBUG2,"CacheGet: looking for %s",
13071310
mxid_to_string(InvalidMultiXactId,nmembers,members));
13081311

13091312
/* sort the array so comparison is easy */
13101313
qsort(members,nmembers,sizeof(MultiXactMember),mxactMemberComparator);
13111314

1312-
for (entry=MXactCache;entry!=NULL;entry=entry->next)
1315+
dlist_foreach(iter,&MXactCache)
13131316
{
1317+
mXactCacheEnt*entry=dlist_container(mXactCacheEnt,node,iter.cur);
1318+
13141319
if (entry->nmembers!=nmembers)
13151320
continue;
13161321

@@ -1321,6 +1326,7 @@ mXactCacheGetBySet(int nmembers, MultiXactMember *members)
13211326
if (memcmp(members,entry->members,nmembers*sizeof(MultiXactMember))==0)
13221327
{
13231328
debug_elog3(DEBUG2,"CacheGet: found %u",entry->multi);
1329+
dlist_move_head(&MXactCache,iter.cur);
13241330
returnentry->multi;
13251331
}
13261332
}
@@ -1340,12 +1346,14 @@ mXactCacheGetBySet(int nmembers, MultiXactMember *members)
13401346
staticint
13411347
mXactCacheGetById(MultiXactIdmulti,MultiXactMember**members)
13421348
{
1343-
mXactCacheEnt*entry;
1349+
dlist_iteriter;
13441350

13451351
debug_elog3(DEBUG2,"CacheGet: looking for %u",multi);
13461352

1347-
for (entry=MXactCache;entry!=NULL;entry=entry->next)
1353+
dlist_foreach(iter,&MXactCache)
13481354
{
1355+
mXactCacheEnt*entry=dlist_container(mXactCacheEnt,node,iter.cur);
1356+
13491357
if (entry->multi==multi)
13501358
{
13511359
MultiXactMember*ptr;
@@ -1361,6 +1369,14 @@ mXactCacheGetById(MultiXactId multi, MultiXactMember **members)
13611369
mxid_to_string(multi,
13621370
entry->nmembers,
13631371
entry->members));
1372+
1373+
/*
1374+
* Note we modify the list while not using a modifyable iterator.
1375+
* This is acceptable only because we exit the iteration
1376+
* immediately afterwards.
1377+
*/
1378+
dlist_move_head(&MXactCache,iter.cur);
1379+
13641380
returnentry->nmembers;
13651381
}
13661382
}
@@ -1404,8 +1420,22 @@ mXactCachePut(MultiXactId multi, int nmembers, MultiXactMember *members)
14041420
/* mXactCacheGetBySet assumes the entries are sorted, so sort them */
14051421
qsort(entry->members,nmembers,sizeof(MultiXactMember),mxactMemberComparator);
14061422

1407-
entry->next=MXactCache;
1408-
MXactCache=entry;
1423+
dlist_push_head(&MXactCache,&entry->node);
1424+
if (MXactCacheMembers++ >=MAX_CACHE_ENTRIES)
1425+
{
1426+
dlist_node*node;
1427+
mXactCacheEnt*entry;
1428+
1429+
node=dlist_tail_node(&MXactCache);
1430+
dlist_delete(node);
1431+
MXactCacheMembers--;
1432+
1433+
entry=dlist_container(mXactCacheEnt,node,node);
1434+
debug_elog3(DEBUG2,"CachePut: pruning cached multi %u",
1435+
entry->multi);
1436+
1437+
pfree(entry);
1438+
}
14091439
}
14101440

14111441
staticchar*
@@ -1480,7 +1510,8 @@ AtEOXact_MultiXact(void)
14801510
* a child of TopTransactionContext, we needn't delete it explicitly.
14811511
*/
14821512
MXactContext=NULL;
1483-
MXactCache=NULL;
1513+
dlist_init(&MXactCache);
1514+
MXactCacheMembers=0;
14841515
}
14851516

14861517
/*
@@ -1546,7 +1577,8 @@ PostPrepare_MultiXact(TransactionId xid)
15461577
* Discard the local MultiXactId cache like in AtEOX_MultiXact
15471578
*/
15481579
MXactContext=NULL;
1549-
MXactCache=NULL;
1580+
dlist_init(&MXactCache);
1581+
MXactCacheMembers=0;
15501582
}
15511583

15521584
/*

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