PG参数maintenance_work_mem解析

typedef struct ItemPointerData
{
	BlockIdData ip_blkid;
	OffsetNumber ip_posid;
}

static void
lazy_space_alloc(LVRelState *vacrel, int nworkers, BlockNumber nblocks)
{
	LVDeadTuples *dead_tuples;
	long		maxtuples;

	...

	maxtuples = compute_max_dead_tuples(nblocks, vacrel->nindexes > 0);

	dead_tuples = (LVDeadTuples *) palloc(SizeOfDeadTuples(maxtuples));
	dead_tuples->num_tuples = 0;
	dead_tuples->max_tuples = (int) maxtuples;

	vacrel->dead_tuples = dead_tuples;
}

/*
 * Return the maximum number of dead tuples we can record.
 */
static long
compute_max_dead_tuples(BlockNumber relblocks, bool hasindex)
{
	long		maxtuples;
	int			vac_work_mem = IsAutoVacuumWorkerProcess() &&
	autovacuum_work_mem != -1 ?
	autovacuum_work_mem : maintenance_work_mem;

	if (hasindex)
	{
		maxtuples = MAXDEADTUPLES(vac_work_mem * 1024L);
		maxtuples = Min(maxtuples, INT_MAX);
		maxtuples = Min(maxtuples, MAXDEADTUPLES(MaxAllocSize));

		/* curious coding here to ensure the multiplication can't overflow */
		if ((BlockNumber) (maxtuples / LAZY_ALLOC_TUPLES) > relblocks)
			maxtuples = relblocks * LAZY_ALLOC_TUPLES;

		/* stay sane if small maintenance_work_mem */
		maxtuples = Max(maxtuples, MaxHeapTuplesPerPage);
	}
	else
		maxtuples = MaxHeapTuplesPerPage;

	return maxtuples;
}

/* The dead tuple space consists of LVDeadTuples and dead tuple TIDs */
#define SizeOfDeadTuples(cnt) \
	add_size(offsetof(LVDeadTuples, itemptrs), \
			 mul_size(sizeof(ItemPointerData), cnt))
#define MAXDEADTUPLES(max_size) \
		(((max_size) - offsetof(LVDeadTuples, itemptrs)) / sizeof(ItemPointerData))

#define offsetof(type, field)	((long) &((type *)0)->field)

/* It's possible we could use a different value for this in frontend code */
#define MaxAllocSize	((Size) 0x3fffffff) /* 1 gigabyte - 1 */

/*
 * Guesstimation of number of dead tuples per page.  This is used to
 * provide an upper limit to memory allocated when vacuuming small
 * tables.
 */
#define LAZY_ALLOC_TUPLES		MaxHeapTuplesPerPage

#define MaxHeapTuplesPerPage	\
	((int) ((BLCKSZ - SizeOfPageHeaderData) / \
			(MAXALIGN(SizeofHeapTupleHeader) + sizeof(ItemIdData))))


typedef struct LVDeadTuples
{
	int			max_tuples;		/* # slots allocated in array */
	int			num_tuples;		/* current # of entries */
	/* List of TIDs of tuples we intend to delete */
	/* NB: this list is ordered by TID address */
	ItemPointerData itemptrs[FLEXIBLE_ARRAY_MEMBER];	/* array of
														 * ItemPointerData */
} LVDeadTuples;

static Tuplesortstate *
tuplesort_begin_common(int workMem, SortCoordinate coordinate,
					   bool randomAccess)
{
	Tuplesortstate *state;
	MemoryContext maincontext;
	MemoryContext sortcontext;
	MemoryContext oldcontext;

	...

	/*
	 * workMem is forced to be at least 64KB, the current minimum valid value
	 * for the work_mem GUC.  This is a defense against parallel sort callers
	 * that divide out memory among many workers in a way that leaves each
	 * with very little memory.
	 */
	state->allowedMem = Max(workMem, 64) * (int64) 1024;
	state->sortcontext = sortcontext;
	state->maincontext = maincontext;

	/*
	 * Initial size of array must be more than ALLOCSET_SEPARATE_THRESHOLD;
	 * see comments in grow_memtuples().
	 */
	state->memtupsize = INITIAL_MEMTUPSIZE;
	state->memtuples = NULL;

	/*
	 * After all of the other non-parallel-related state, we setup all of the
	 * state needed for each batch.
	 */
	tuplesort_begin_batch(state);

	/*
	 * Initialize parallel-related state based on coordination information
	 * from caller
	 */
	if (!coordinate)
	{
		/* Serial sort */
		state->shared = NULL;
		state->worker = -1;
		state->nParticipants = -1;
	}
	else if (coordinate->isWorker)
	{
		/* Parallel worker produces exactly one final run from all input */
		state->shared = coordinate->sharedsort;
		state->worker = worker_get_identifier(state);
		state->nParticipants = -1;
	}
	else
	{
		/* Parallel leader state only used for final merge */
		state->shared = coordinate->sharedsort;
		state->worker = -1;
		state->nParticipants = coordinate->nParticipants;
		Assert(state->nParticipants >= 1);
	}

	MemoryContextSwitchTo(oldcontext);

	return state;
}

/*
 * Allocate dead_items and dead_items_info (either using palloc, or in dynamic
 * shared memory). Sets both in vacrel for caller.
 *
 * Also handles parallel initialization as part of allocating dead_items in
 * DSM when required.
 */
static void
dead_items_alloc(LVRelState *vacrel, int nworkers)
{
	VacDeadItemsInfo *dead_items_info;
	int			vac_work_mem = AmAutoVacuumWorkerProcess() &&
		autovacuum_work_mem != -1 ?
		autovacuum_work_mem : maintenance_work_mem;

	...

	/*
	 * Serial VACUUM case. Allocate both dead_items and dead_items_info
	 * locally.
	 */

	dead_items_info = (VacDeadItemsInfo *) palloc(sizeof(VacDeadItemsInfo));
	dead_items_info->max_bytes = vac_work_mem * 1024L;
	dead_items_info->num_items = 0;
	vacrel->dead_items_info = dead_items_info;

	vacrel->dead_items = TidStoreCreateLocal(dead_items_info->max_bytes, true);
}