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Rethink MemoryContext creation to improve performance.
This patch makes a number of interrelated changes to reduce the overhead involved in creating/deleting memory contexts. The key ideas are: * Include the AllocSetContext header of an aset.c context in its first malloc request, rather than allocating it separately in TopMemoryContext. This means that we now always create an initial or "keeper" block in an aset, even if it never receives any allocation requests. * Create freelists in which we can save and recycle recently-destroyed asets (this idea is due to Robert Haas). * In the common case where the name of a context is a constant string, just store a pointer to it in the context header, rather than copying the string. The first change eliminates a palloc/pfree cycle per context, and also avoids bloat in TopMemoryContext, at the price that creating a context now involves a malloc/free cycle even if the context never receives any allocations. That would be a loser for some common usage patterns, but recycling short-lived contexts via the freelist eliminates that pain. Avoiding copying constant strings not only saves strlen() and strcpy() overhead, but is an essential part of the freelist optimization because it makes the context header size constant. Currently we make no attempt to use the freelist for contexts with non-constant names. (Perhaps someday we'll need to think harder about that, but in current usage, most contexts with custom names are long-lived anyway.) The freelist management in this initial commit is pretty simplistic, and we might want to refine it later --- but in common workloads that will never matter because the freelists will never get full anyway. To create a context with a non-constant name, one is now required to call AllocSetContextCreateExtended and specify the MEMCONTEXT_COPY_NAME option. AllocSetContextCreate becomes a wrapper macro, and it includes a test that will complain about non-string-literal context name parameters on gcc and similar compilers. An unfortunate side effect of making AllocSetContextCreate a macro is that one is now *required* to use the size parameter abstraction macros (ALLOCSET_DEFAULT_SIZES and friends) with it; the pre-9.6 habit of writing out individual size parameters no longer works unless you switch to AllocSetContextCreateExtended. Internally to the memory-context-related modules, the context creation APIs are simplified, removing the rather baroque original design whereby a context-type module called mcxt.c which then called back into the context-type module. That saved a bit of code duplication, but not much, and it prevented context-type modules from exercising control over the allocation of context headers. In passing, I converted the test-and-elog validation of aset size parameters into Asserts to save a few more cycles. The original thought was that callers might compute size parameters on the fly, but in practice nobody does that, so it's useless to expend cycles on checking those numbers in production builds. Also, mark the memory context method-pointer structs "const", just for cleanliness. Discussion: https://postgr.es/m/2264.1512870796@sss.pgh.pa.us
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21 files changed

+539
-341
lines changed

21 files changed

+539
-341
lines changed

‎contrib/amcheck/verify_nbtree.c

Lines changed: 1 addition & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -295,9 +295,7 @@ bt_check_every_level(Relation rel, bool readonly)
295295
/* Create context for page */
296296
state->targetcontext = AllocSetContextCreate(CurrentMemoryContext,
297297
"amcheck context",
298-
ALLOCSET_DEFAULT_MINSIZE,
299-
ALLOCSET_DEFAULT_INITSIZE,
300-
ALLOCSET_DEFAULT_MAXSIZE);
298+
ALLOCSET_DEFAULT_SIZES);
301299
state->checkstrategy = GetAccessStrategy(BAS_BULKREAD);
302300

303301
/* Get true root block from meta-page */

‎src/backend/access/transam/xact.c

Lines changed: 6 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -997,11 +997,12 @@ AtStart_Memory(void)
997997
*/
998998
if (TransactionAbortContext == NULL)
999999
TransactionAbortContext =
1000-
AllocSetContextCreate(TopMemoryContext,
1001-
"TransactionAbortContext",
1002-
32 * 1024,
1003-
32 * 1024,
1004-
32 * 1024);
1000+
AllocSetContextCreateExtended(TopMemoryContext,
1001+
"TransactionAbortContext",
1002+
0,
1003+
32 * 1024,
1004+
32 * 1024,
1005+
32 * 1024);
10051006

10061007
/*
10071008
* We shouldn't have a transaction context already.

‎src/backend/catalog/partition.c

Lines changed: 4 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -520,9 +520,10 @@ RelationBuildPartitionDesc(Relation rel)
520520
}
521521

522522
/* Now build the actual relcache partition descriptor */
523-
rel->rd_pdcxt = AllocSetContextCreate(CacheMemoryContext,
524-
RelationGetRelationName(rel),
525-
ALLOCSET_DEFAULT_SIZES);
523+
rel->rd_pdcxt = AllocSetContextCreateExtended(CacheMemoryContext,
524+
RelationGetRelationName(rel),
525+
MEMCONTEXT_COPY_NAME,
526+
ALLOCSET_DEFAULT_SIZES);
526527
oldcxt = MemoryContextSwitchTo(rel->rd_pdcxt);
527528

528529
result = (PartitionDescData *) palloc0(sizeof(PartitionDescData));

‎src/backend/commands/subscriptioncmds.c

Lines changed: 1 addition & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -259,9 +259,7 @@ publicationListToArray(List *publist)
259259
/* Create memory context for temporary allocations. */
260260
memcxt = AllocSetContextCreate(CurrentMemoryContext,
261261
"publicationListToArray to array",
262-
ALLOCSET_DEFAULT_MINSIZE,
263-
ALLOCSET_DEFAULT_INITSIZE,
264-
ALLOCSET_DEFAULT_MAXSIZE);
262+
ALLOCSET_DEFAULT_SIZES);
265263
oldcxt = MemoryContextSwitchTo(memcxt);
266264

267265
datums = (Datum *) palloc(sizeof(Datum) * list_length(publist));

‎src/backend/lib/knapsack.c

Lines changed: 1 addition & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -57,9 +57,7 @@ DiscreteKnapsack(int max_weight, int num_items,
5757
{
5858
MemoryContext local_ctx = AllocSetContextCreate(CurrentMemoryContext,
5959
"Knapsack",
60-
ALLOCSET_SMALL_MINSIZE,
61-
ALLOCSET_SMALL_INITSIZE,
62-
ALLOCSET_SMALL_MAXSIZE);
60+
ALLOCSET_SMALL_SIZES);
6361
MemoryContext oldctx = MemoryContextSwitchTo(local_ctx);
6462
double *values;
6563
Bitmapset **sets;

‎src/backend/replication/logical/launcher.c

Lines changed: 1 addition & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -925,9 +925,7 @@ ApplyLauncherMain(Datum main_arg)
925925
/* Use temporary context for the database list and worker info. */
926926
subctx = AllocSetContextCreate(TopMemoryContext,
927927
"Logical Replication Launcher sublist",
928-
ALLOCSET_DEFAULT_MINSIZE,
929-
ALLOCSET_DEFAULT_INITSIZE,
930-
ALLOCSET_DEFAULT_MAXSIZE);
928+
ALLOCSET_DEFAULT_SIZES);
931929
oldctx = MemoryContextSwitchTo(subctx);
932930

933931
/* search for subscriptions to start or stop. */

‎src/backend/replication/logical/reorderbuffer.c

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -237,16 +237,19 @@ ReorderBufferAllocate(void)
237237

238238
buffer->change_context = SlabContextCreate(new_ctx,
239239
"Change",
240+
0,
240241
SLAB_DEFAULT_BLOCK_SIZE,
241242
sizeof(ReorderBufferChange));
242243

243244
buffer->txn_context = SlabContextCreate(new_ctx,
244245
"TXN",
246+
0,
245247
SLAB_DEFAULT_BLOCK_SIZE,
246248
sizeof(ReorderBufferTXN));
247249

248250
buffer->tup_context = GenerationContextCreate(new_ctx,
249251
"Tuples",
252+
0,
250253
SLAB_LARGE_BLOCK_SIZE);
251254

252255
hash_ctl.keysize = sizeof(TransactionId);

‎src/backend/replication/pgoutput/pgoutput.c

Lines changed: 1 addition & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -152,9 +152,7 @@ pgoutput_startup(LogicalDecodingContext *ctx, OutputPluginOptions *opt,
152152
/* Create our memory context for private allocations. */
153153
data->context = AllocSetContextCreate(ctx->context,
154154
"logical replication output context",
155-
ALLOCSET_DEFAULT_MINSIZE,
156-
ALLOCSET_DEFAULT_INITSIZE,
157-
ALLOCSET_DEFAULT_MAXSIZE);
155+
ALLOCSET_DEFAULT_SIZES);
158156

159157
ctx->output_plugin_private = data;
160158

‎src/backend/utils/cache/relcache.c

Lines changed: 17 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -669,9 +669,10 @@ RelationBuildRuleLock(Relation relation)
669669
/*
670670
* Make the private context. Assume it'll not contain much data.
671671
*/
672-
rulescxt = AllocSetContextCreate(CacheMemoryContext,
673-
RelationGetRelationName(relation),
674-
ALLOCSET_SMALL_SIZES);
672+
rulescxt = AllocSetContextCreateExtended(CacheMemoryContext,
673+
RelationGetRelationName(relation),
674+
MEMCONTEXT_COPY_NAME,
675+
ALLOCSET_SMALL_SIZES);
675676
relation->rd_rulescxt = rulescxt;
676677

677678
/*
@@ -984,9 +985,10 @@ RelationBuildPartitionKey(Relation relation)
984985
ReleaseSysCache(tuple);
985986

986987
/* Success --- now copy to the cache memory */
987-
partkeycxt = AllocSetContextCreate(CacheMemoryContext,
988-
RelationGetRelationName(relation),
989-
ALLOCSET_SMALL_SIZES);
988+
partkeycxt = AllocSetContextCreateExtended(CacheMemoryContext,
989+
RelationGetRelationName(relation),
990+
MEMCONTEXT_COPY_NAME,
991+
ALLOCSET_SMALL_SIZES);
990992
relation->rd_partkeycxt = partkeycxt;
991993
oldcxt = MemoryContextSwitchTo(relation->rd_partkeycxt);
992994
relation->rd_partkey = copy_partition_key(key);
@@ -1566,9 +1568,10 @@ RelationInitIndexAccessInfo(Relation relation)
15661568
* a context, and not just a couple of pallocs, is so that we won't leak
15671569
* any subsidiary info attached to fmgr lookup records.
15681570
*/
1569-
indexcxt = AllocSetContextCreate(CacheMemoryContext,
1570-
RelationGetRelationName(relation),
1571-
ALLOCSET_SMALL_SIZES);
1571+
indexcxt = AllocSetContextCreateExtended(CacheMemoryContext,
1572+
RelationGetRelationName(relation),
1573+
MEMCONTEXT_COPY_NAME,
1574+
ALLOCSET_SMALL_SIZES);
15721575
relation->rd_indexcxt = indexcxt;
15731576

15741577
/*
@@ -5537,9 +5540,11 @@ load_relcache_init_file(bool shared)
55375540
* prepare index info context --- parameters should match
55385541
* RelationInitIndexAccessInfo
55395542
*/
5540-
indexcxt = AllocSetContextCreate(CacheMemoryContext,
5541-
RelationGetRelationName(rel),
5542-
ALLOCSET_SMALL_SIZES);
5543+
indexcxt =
5544+
AllocSetContextCreateExtended(CacheMemoryContext,
5545+
RelationGetRelationName(rel),
5546+
MEMCONTEXT_COPY_NAME,
5547+
ALLOCSET_SMALL_SIZES);
55435548
rel->rd_indexcxt = indexcxt;
55445549

55455550
/*

‎src/backend/utils/cache/ts_cache.c

Lines changed: 4 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -294,9 +294,10 @@ lookup_ts_dictionary_cache(Oid dictId)
294294
Assert(!found); /* it wasn't there a moment ago */
295295

296296
/* Create private memory context the first time through */
297-
saveCtx = AllocSetContextCreate(CacheMemoryContext,
298-
NameStr(dict->dictname),
299-
ALLOCSET_SMALL_SIZES);
297+
saveCtx = AllocSetContextCreateExtended(CacheMemoryContext,
298+
NameStr(dict->dictname),
299+
MEMCONTEXT_COPY_NAME,
300+
ALLOCSET_SMALL_SIZES);
300301
}
301302
else
302303
{

‎src/backend/utils/hash/dynahash.c

Lines changed: 5 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -340,9 +340,11 @@ hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
340340
CurrentDynaHashCxt = info->hcxt;
341341
else
342342
CurrentDynaHashCxt = TopMemoryContext;
343-
CurrentDynaHashCxt = AllocSetContextCreate(CurrentDynaHashCxt,
344-
tabname,
345-
ALLOCSET_DEFAULT_SIZES);
343+
CurrentDynaHashCxt =
344+
AllocSetContextCreateExtended(CurrentDynaHashCxt,
345+
tabname,
346+
MEMCONTEXT_COPY_NAME,
347+
ALLOCSET_DEFAULT_SIZES);
346348
}
347349

348350
/* Initialize the hash header, plus a copy of the table name */

‎src/backend/utils/mmgr/README

Lines changed: 5 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -177,8 +177,7 @@ every other context is a direct or indirect child of this one. Allocating
177177
here is essentially the same as "malloc", because this context will never
178178
be reset or deleted. This is for stuff that should live forever, or for
179179
stuff that the controlling module will take care of deleting at the
180-
appropriate time. An example is fd.c's tables of open files, as well as
181-
the context management nodes for memory contexts themselves. Avoid
180+
appropriate time. An example is fd.c's tables of open files. Avoid
182181
allocating stuff here unless really necessary, and especially avoid
183182
running with CurrentMemoryContext pointing here.
184183

@@ -420,11 +419,10 @@ a maximum block size. Selecting smaller values can prevent wastage of
420419
space in contexts that aren't expected to hold very much (an example
421420
is the relcache's per-relation contexts).
422421

423-
Also, it is possible to specify a minimum context size. If this
424-
value is greater than zero then a block of that size will be grabbed
425-
immediately upon context creation, and cleared but not released during
426-
context resets. This feature is needed for ErrorContext (see above),
427-
but will most likely not be used for other contexts.
422+
Also, it is possible to specify a minimum context size, in case for some
423+
reason that should be different from the initial size for additional
424+
blocks. An aset.c context will always contain at least one block,
425+
of size minContextSize if that is specified, otherwise initBlockSize.
428426

429427
We expect that per-tuple contexts will be reset frequently and typically
430428
will not allocate very much space per tuple cycle. To make this usage

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