Function GetAccessStrategy

src/backend/storage/buffer/freelist.c:547–594

* GetAccessStrategy -- create a BufferAccessStrategy object * * The object is allocated in the current memory context. */

 * The object is allocated in the current memory context.
 */
BufferAccessStrategy
GetAccessStrategy(BufferAccessStrategyType btype)
{
	BufferAccessStrategy strategy;
	int			ring_size;

	/*
	 * Select ring size to use.  See buffer/README for rationales.
	 *
	 * Note: if you change the ring size for BAS_BULKREAD, see also
	 * SYNC_SCAN_REPORT_INTERVAL in access/heap/syncscan.c.
	 */
	switch (btype)
	{
		case BAS_NORMAL:
			/* if someone asks for NORMAL, just give 'em a "default" object */
			return NULL;

		case BAS_BULKREAD:
			ring_size = 256 * 1024 / BLCKSZ;
			break;
		case BAS_BULKWRITE:
			ring_size = 16 * 1024 * 1024 / BLCKSZ;
			break;
		case BAS_VACUUM:
			ring_size = 256 * 1024 / BLCKSZ;
			break;

		default:
			elog(ERROR, "unrecognized buffer access strategy: %d",
				 (int) btype);
			return NULL;		/* keep compiler quiet */
	}

	/* Make sure ring isn't an undue fraction of shared buffers */
	ring_size = Min(NBuffers / 8, ring_size);

	/* Allocate the object and initialize all elements to zeroes */
	strategy = (BufferAccessStrategy)
		palloc0(offsetof(BufferAccessStrategyData, buffers) +
				ring_size * sizeof(Buffer));

	/* Set fields that don't start out zero */
	strategy->btype = btype;
	strategy->ring_size = ring_size;

	return strategy;
}

/*
 * FreeAccessStrategy -- release a BufferAccessStrategy object