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Add transparent block-level memory accounting
Adds accounting of memory allocated in a memory context. Compared to various ad hoc solutions, the main advantage is that the accounting is transparent and does not require direct control over allocations (this matters for use cases where the allocations happen in user code, like for example aggregate states allocated in a transition functions). To reduce overhead, the accounting happens at the block level (not for individual chunks) and only the context immediately owning the block is updated. When inquiring about amount of memory allocated in a context, we have to recursively walk all children contexts. This "lazy" accounting works well for cases with relatively small number of contexts in the relevant subtree and/or with infrequent inquiries. Author: Jeff Davis Reivewed-by: Tomas Vondra, Melanie Plageman, Soumyadeep Chakraborty Discussion: https://www.postgresql.org/message-id/flat/027a129b8525601c6a680d27ce3a7172dab61aab.camel@j-davis.com
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‎src/backend/utils/mmgr/README

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Original file line numberDiff line numberDiff line change
@@ -23,6 +23,10 @@ The basic operations on a memory context are:
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* reset a context (free all memory allocated in the context, but not the
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context object itself)
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* inquire about the total amount of memory allocated to the context
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(the raw memory from which the context allocates chunks; not the
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chunks themselves)
29+
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Given a chunk of memory previously allocated from a context, one can
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free it or reallocate it larger or smaller (corresponding to standard C
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library's free() and realloc() routines). These operations return memory
@@ -452,3 +456,33 @@ returns the memory when reset/deleted).
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These memory contexts were initially developed for ReorderBuffer, but
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may be useful elsewhere as long as the allocation patterns match.
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Memory Accounting
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-----------------
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One of the basic memory context operations is determining the amount of
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memory used in the context (and it's children). We have multiple places
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that implement their own ad hoc memory accounting, and this is meant to
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provide a unified approach. Ad hoc accounting solutions work for places
468+
with tight control over the allocations or when it's easy to determine
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sizes of allocated chunks (e.g. places that only work with tuples).
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The accounting built into the memory contexts is transparent and works
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transparently for all allocations as long as they end up in the right
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memory context subtree.
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Consider for example aggregate functions - the aggregate state is often
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represented by an arbitrary structure, allocated from the transition
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function, so the ad hoc accounting is unlikely to work. The built-in
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accounting will however handle such cases just fine.
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To minimize overhead, the accounting is done at the block level, not for
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individual allocation chunks.
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The accounting is lazy - after a block is allocated (or freed), only the
484+
context owning that block is updated. This means that when inquiring
485+
about the memory usage in a given context, we have to walk all children
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contexts recursively. This means the memory accounting is not intended
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for cases with too many memory contexts (in the relevant subtree).
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‎src/backend/utils/mmgr/aset.c

Lines changed: 38 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -458,6 +458,9 @@ AllocSetContextCreateInternal(MemoryContext parent,
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parent,
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name);
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((MemoryContext) set)->mem_allocated =
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set->keeper->endptr - ((char *) set);
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461464
return (MemoryContext) set;
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}
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}
@@ -546,6 +549,8 @@ AllocSetContextCreateInternal(MemoryContext parent,
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parent,
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name);
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((MemoryContext) set)->mem_allocated = firstBlockSize;
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return (MemoryContext) set;
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}
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@@ -566,6 +571,7 @@ AllocSetReset(MemoryContext context)
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{
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AllocSet set = (AllocSet) context;
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AllocBlock block;
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Size keepersize = set->keeper->endptr - ((char *) set);
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AssertArg(AllocSetIsValid(set));
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@@ -604,6 +610,8 @@ AllocSetReset(MemoryContext context)
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else
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{
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/* Normal case, release the block */
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context->mem_allocated -= block->endptr - ((char*) block);
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607615
#ifdef CLOBBER_FREED_MEMORY
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wipe_mem(block, block->freeptr - ((char *) block));
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#endif
@@ -612,6 +620,8 @@ AllocSetReset(MemoryContext context)
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block = next;
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}
614622

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Assert(context->mem_allocated == keepersize);
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/* Reset block size allocation sequence, too */
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set->nextBlockSize = set->initBlockSize;
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}
@@ -628,6 +638,7 @@ AllocSetDelete(MemoryContext context)
628638
{
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AllocSet set = (AllocSet) context;
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AllocBlock block = set->blocks;
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Size keepersize = set->keeper->endptr - ((char *) set);
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632643
AssertArg(AllocSetIsValid(set));
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@@ -683,6 +694,9 @@ AllocSetDelete(MemoryContext context)
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{
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AllocBlock next = block->next;
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if (block != set->keeper)
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context->mem_allocated -= block->endptr - ((char *) block);
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#ifdef CLOBBER_FREED_MEMORY
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wipe_mem(block, block->freeptr - ((char *) block));
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#endif
@@ -693,6 +707,8 @@ AllocSetDelete(MemoryContext context)
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block = next;
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}
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Assert(context->mem_allocated == keepersize);
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/* Finally, free the context header, including the keeper block */
697713
free(set);
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}
@@ -733,6 +749,9 @@ AllocSetAlloc(MemoryContext context, Size size)
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block = (AllocBlock) malloc(blksize);
734750
if (block == NULL)
735751
return NULL;
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context->mem_allocated += blksize;
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736755
block->aset = set;
737756
block->freeptr = block->endptr = ((char *) block) + blksize;
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@@ -928,6 +947,8 @@ AllocSetAlloc(MemoryContext context, Size size)
928947
if (block == NULL)
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return NULL;
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context->mem_allocated += blksize;
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931952
block->aset = set;
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block->freeptr = ((char *) block) + ALLOC_BLOCKHDRSZ;
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block->endptr = ((char *) block) + blksize;
@@ -1028,6 +1049,9 @@ AllocSetFree(MemoryContext context, void *pointer)
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set->blocks = block->next;
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if (block->next)
10301051
block->next->prev = block->prev;
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context->mem_allocated -= block->endptr - ((char*) block);
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#ifdef CLOBBER_FREED_MEMORY
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wipe_mem(block, block->freeptr - ((char *) block));
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#endif
@@ -1144,6 +1168,7 @@ AllocSetRealloc(MemoryContext context, void *pointer, Size size)
11441168
AllocBlock block = (AllocBlock) (((char *) chunk) - ALLOC_BLOCKHDRSZ);
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Size chksize;
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Size blksize;
1171+
Size oldblksize;
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/*
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* Try to verify that we have a sane block pointer: it should
@@ -1159,13 +1184,18 @@ AllocSetRealloc(MemoryContext context, void *pointer, Size size)
11591184
/* Do the realloc */
11601185
chksize = MAXALIGN(size);
11611186
blksize = chksize + ALLOC_BLOCKHDRSZ + ALLOC_CHUNKHDRSZ;
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oldblksize = block->endptr - ((char *)block);
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block = (AllocBlock) realloc(block, blksize);
11631190
if (block == NULL)
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{
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/* Disallow external access to private part of chunk header. */
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VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);
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return NULL;
11681195
}
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context->mem_allocated += blksize - oldblksize;
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block->freeptr = block->endptr = ((char *) block) + blksize;
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/* Update pointers since block has likely been moved */
@@ -1383,6 +1413,7 @@ AllocSetCheck(MemoryContext context)
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const char *name = set->header.name;
13841414
AllocBlock prevblock;
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AllocBlock block;
1416+
int64 total_allocated = 0;
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13871418
for (prevblock = NULL, block = set->blocks;
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block != NULL;
@@ -1393,6 +1424,11 @@ AllocSetCheck(MemoryContext context)
13931424
long blk_data = 0;
13941425
long nchunks = 0;
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1427+
if (set->keeper == block)
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total_allocated += block->endptr - ((char *) set);
1429+
else
1430+
total_allocated += block->endptr - ((char *) block);
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13961432
/*
13971433
* Empty block - empty can be keeper-block only
13981434
*/
@@ -1479,6 +1515,8 @@ AllocSetCheck(MemoryContext context)
14791515
elog(WARNING, "problem in alloc set %s: found inconsistent memory block %p",
14801516
name, block);
14811517
}
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1519+
Assert(total_allocated == context->mem_allocated);
14821520
}
14831521

14841522
#endif /* MEMORY_CONTEXT_CHECKING */

‎src/backend/utils/mmgr/generation.c

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@@ -297,6 +297,8 @@ GenerationReset(MemoryContext context)
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298298
dlist_delete(miter.cur);
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300+
context->mem_allocated -= block->blksize;
301+
300302
#ifdef CLOBBER_FREED_MEMORY
301303
wipe_mem(block, block->blksize);
302304
#endif
@@ -352,6 +354,8 @@ GenerationAlloc(MemoryContext context, Size size)
352354
if (block == NULL)
353355
return NULL;
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context->mem_allocated += blksize;
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355359
/* block with a single (used) chunk */
356360
block->blksize = blksize;
357361
block->nchunks = 1;
@@ -407,6 +411,8 @@ GenerationAlloc(MemoryContext context, Size size)
407411
if (block == NULL)
408412
return NULL;
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414+
context->mem_allocated += blksize;
415+
410416
block->blksize = blksize;
411417
block->nchunks = 0;
412418
block->nfree = 0;
@@ -522,6 +528,7 @@ GenerationFree(MemoryContext context, void *pointer)
522528
if (set->block == block)
523529
set->block = NULL;
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531+
context->mem_allocated -= block->blksize;
525532
free(block);
526533
}
527534

@@ -746,6 +753,7 @@ GenerationCheck(MemoryContext context)
746753
GenerationContext *gen = (GenerationContext *) context;
747754
const char *name = context->name;
748755
dlist_iter iter;
756+
int64 total_allocated = 0;
749757

750758
/* walk all blocks in this context */
751759
dlist_foreach(iter, &gen->blocks)
@@ -755,6 +763,8 @@ GenerationCheck(MemoryContext context)
755763
nchunks;
756764
char *ptr;
757765

766+
total_allocated += block->blksize;
767+
758768
/*
759769
* nfree > nchunks is surely wrong, and we don't expect to see
760770
* equality either, because such a block should have gotten freed.
@@ -833,6 +843,8 @@ GenerationCheck(MemoryContext context)
833843
elog(WARNING, "problem in Generation %s: number of free chunks %d in block %p does not match header %d",
834844
name, nfree, block, block->nfree);
835845
}
846+
847+
Assert(total_allocated == context->mem_allocated);
836848
}
837849

838850
#endif /* MEMORY_CONTEXT_CHECKING */

‎src/backend/utils/mmgr/mcxt.c

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@@ -462,6 +462,30 @@ MemoryContextIsEmpty(MemoryContext context)
462462
return context->methods->is_empty(context);
463463
}
464464

465+
/*
466+
* Find the memory allocated to blocks for this memory context. If recurse is
467+
* true, also include children.
468+
*/
469+
int64
470+
MemoryContextMemAllocated(MemoryContext context, bool recurse)
471+
{
472+
int64 total = context->mem_allocated;
473+
474+
AssertArg(MemoryContextIsValid(context));
475+
476+
if (recurse)
477+
{
478+
MemoryContext child = context->firstchild;
479+
480+
for (child = context->firstchild;
481+
child != NULL;
482+
child = child->nextchild)
483+
total += MemoryContextMemAllocated(child, true);
484+
}
485+
486+
return total;
487+
}
488+
465489
/*
466490
* MemoryContextStats
467491
* Print statistics about the named context and all its descendants.
@@ -736,6 +760,7 @@ MemoryContextCreate(MemoryContext node,
736760
node->methods = methods;
737761
node->parent = parent;
738762
node->firstchild = NULL;
763+
node->mem_allocated = 0;
739764
node->prevchild = NULL;
740765
node->name = name;
741766
node->ident = NULL;

‎src/backend/utils/mmgr/slab.c

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@@ -305,12 +305,14 @@ SlabReset(MemoryContext context)
305305
#endif
306306
free(block);
307307
slab->nblocks--;
308+
context->mem_allocated -= slab->blockSize;
308309
}
309310
}
310311

311312
slab->minFreeChunks = 0;
312313

313314
Assert(slab->nblocks == 0);
315+
Assert(context->mem_allocated == 0);
314316
}
315317

316318
/*
@@ -388,6 +390,7 @@ SlabAlloc(MemoryContext context, Size size)
388390

389391
slab->minFreeChunks = slab->chunksPerBlock;
390392
slab->nblocks += 1;
393+
context->mem_allocated += slab->blockSize;
391394
}
392395

393396
/* grab the block from the freelist (even the new block is there) */
@@ -480,6 +483,9 @@ SlabAlloc(MemoryContext context, Size size)
480483
#endif
481484

482485
SlabAllocInfo(slab, chunk);
486+
487+
Assert(slab->nblocks * slab->blockSize == context->mem_allocated);
488+
483489
return SlabChunkGetPointer(chunk);
484490
}
485491

@@ -555,11 +561,13 @@ SlabFree(MemoryContext context, void *pointer)
555561
{
556562
free(block);
557563
slab->nblocks--;
564+
context->mem_allocated -= slab->blockSize;
558565
}
559566
else
560567
dlist_push_head(&slab->freelist[block->nfree], &block->node);
561568

562569
Assert(slab->nblocks >= 0);
570+
Assert(slab->nblocks * slab->blockSize == context->mem_allocated);
563571
}
564572

565573
/*
@@ -782,6 +790,8 @@ SlabCheck(MemoryContext context)
782790
name, block->nfree, block, nfree);
783791
}
784792
}
793+
794+
Assert(slab->nblocks * slab->blockSize == context->mem_allocated);
785795
}
786796

787797
#endif /* MEMORY_CONTEXT_CHECKING */

‎src/include/nodes/memnodes.h

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Original file line numberDiff line numberDiff line change
@@ -79,6 +79,7 @@ typedef struct MemoryContextData
7979
/* these two fields are placed here to minimize alignment wastage: */
8080
bool isReset; /* T = no space alloced since last reset */
8181
bool allowInCritSection; /* allow palloc in critical section */
82+
int64 mem_allocated; /* track memory allocated for this context */
8283
const MemoryContextMethods *methods; /* virtual function table */
8384
MemoryContext parent; /* NULL if no parent (toplevel context) */
8485
MemoryContext firstchild; /* head of linked list of children */

‎src/include/utils/memutils.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -82,6 +82,7 @@ extern void MemoryContextSetParent(MemoryContext context,
8282
extern Size GetMemoryChunkSpace(void *pointer);
8383
extern MemoryContext MemoryContextGetParent(MemoryContext context);
8484
extern bool MemoryContextIsEmpty(MemoryContext context);
85+
extern int64 MemoryContextMemAllocated(MemoryContext context, bool recurse);
8586
extern void MemoryContextStats(MemoryContext context);
8687
extern void MemoryContextStatsDetail(MemoryContext context, int max_children);
8788
extern void MemoryContextAllowInCriticalSection(MemoryContext context,

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