Commit b155aa0e authored by Joe Thornber's avatar Joe Thornber Committed by Mike Snitzer
Browse files

dm cache policy mq: tweak algorithm that decides when to promote a block 



Rather than maintaining a separate promote_threshold variable that we
periodically update we now use the hit count of the oldest clean
block.  Also add a fudge factor to discourage demoting dirty blocks.

With some tests this has a sizeable difference, because the old code
was too eager to demote blocks.  For example, device-mapper-test-suite's
git_extract_cache_quick test goes from taking 190 seconds, to 142
(linear on spindle takes 250).

Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
parent 41abc4e1

Documentation/device-mapper/cache-policies.txt

+3 −3
Original line number Diff line number Diff line
@@ -58,9 +58,9 @@ since spindles tend to have good bandwidth. The io_tracker counts 
contiguous I/Os to try to spot when the io is in one of these sequential

modes.



Internally the mq policy maintains a promotion threshold variable.  If

the hit count of a block not in the cache goes above this threshold it

gets promoted to the cache.  The read, write and discard promote adjustment

Internally the mq policy determines a promotion threshold.  If the hit

count of a block not in the cache goes above this threshold it gets

promoted to the cache.  The read, write and discard promote adjustment

tunables allow you to tweak the promotion threshold by adding a small

value based on the io type.  They default to 4, 8 and 1 respectively.

If you're trying to quickly warm a new cache device you may wish to

drivers/md/dm-cache-policy-mq.c

+50 −25
Original line number Diff line number Diff line
@@ -181,24 +181,30 @@ static void queue_shift_down(struct queue *q) 
 * Gives us the oldest entry of the lowest popoulated level.  If the first

 * level is emptied then we shift down one level.

 */

static struct list_head *queue_pop(struct queue *q)

static struct list_head *queue_peek(struct queue *q)

{

	unsigned level;

	struct list_head *r;



	for (level = 0; level < NR_QUEUE_LEVELS; level++)

		if (!list_empty(q->qs + level)) {

			r = q->qs[level].next;

		if (!list_empty(q->qs + level))

			return q->qs[level].next;



	return NULL;

}



static struct list_head *queue_pop(struct queue *q)

{

	struct list_head *r = queue_peek(q);



	if (r) {

		list_del(r);



		/* have we just emptied the bottom level? */

			if (level == 0 && list_empty(q->qs))

		if (list_empty(q->qs))

			queue_shift_down(q);



			return r;

	}



	return NULL;

	return r;

}



static struct list_head *list_pop(struct list_head *lh)
@@ -383,13 +389,6 @@ struct mq_policy { 
	unsigned generation;

	unsigned generation_period; /* in lookups (will probably change) */



	/*

	 * Entries in the pre_cache whose hit count passes the promotion

	 * threshold move to the cache proper.  Working out the correct

	 * value for the promotion_threshold is crucial to this policy.

	 */

	unsigned promote_threshold;



	unsigned discard_promote_adjustment;

	unsigned read_promote_adjustment;

	unsigned write_promote_adjustment;
@@ -406,6 +405,7 @@ struct mq_policy { 
#define DEFAULT_DISCARD_PROMOTE_ADJUSTMENT 1

#define DEFAULT_READ_PROMOTE_ADJUSTMENT 4

#define DEFAULT_WRITE_PROMOTE_ADJUSTMENT 8

#define DISCOURAGE_DEMOTING_DIRTY_THRESHOLD 128



/*----------------------------------------------------------------*/
@@ -518,6 +518,12 @@ static struct entry *pop(struct mq_policy *mq, struct queue *q) 
	return e;

}



static struct entry *peek(struct queue *q)

{

	struct list_head *h = queue_peek(q);

	return h ? container_of(h, struct entry, list) : NULL;

}



/*

 * Has this entry already been updated?

 */
@@ -570,10 +576,6 @@ static void check_generation(struct mq_policy *mq) 
					break;

			}

		}



		mq->promote_threshold = nr ? total / nr : 1;

		if (mq->promote_threshold * nr < total)

			mq->promote_threshold++;

	}

}
@@ -640,6 +642,30 @@ static int demote_cblock(struct mq_policy *mq, dm_oblock_t *oblock) 
	return 0;

}



/*

 * Entries in the pre_cache whose hit count passes the promotion

 * threshold move to the cache proper.  Working out the correct

 * value for the promotion_threshold is crucial to this policy.

 */

static unsigned promote_threshold(struct mq_policy *mq)

{

	struct entry *e;



	if (any_free_cblocks(mq))

		return 0;



	e = peek(&mq->cache_clean);

	if (e)

		return e->hit_count;



	e = peek(&mq->cache_dirty);

	if (e)

		return e->hit_count + DISCOURAGE_DEMOTING_DIRTY_THRESHOLD;



	/* This should never happen */

	return 0;

}



/*

 * We modify the basic promotion_threshold depending on the specific io.

 *
@@ -653,7 +679,7 @@ static unsigned adjusted_promote_threshold(struct mq_policy *mq, 
					   bool discarded_oblock, int data_dir)

{

	if (data_dir == READ)

		return mq->promote_threshold + mq->read_promote_adjustment;

		return promote_threshold(mq) + mq->read_promote_adjustment;



	if (discarded_oblock && (any_free_cblocks(mq) || any_clean_cblocks(mq))) {

		/*
@@ -663,7 +689,7 @@ static unsigned adjusted_promote_threshold(struct mq_policy *mq, 
		return mq->discard_promote_adjustment;

	}



	return mq->promote_threshold + mq->write_promote_adjustment;

	return promote_threshold(mq) + mq->write_promote_adjustment;

}



static bool should_promote(struct mq_policy *mq, struct entry *e,
@@ -1230,7 +1256,6 @@ static struct dm_cache_policy *mq_create(dm_cblock_t cache_size, 
	mq->tick = 0;

	mq->hit_count = 0;

	mq->generation = 0;

	mq->promote_threshold = 0;

	mq->discard_promote_adjustment = DEFAULT_DISCARD_PROMOTE_ADJUSTMENT;

	mq->read_promote_adjustment = DEFAULT_READ_PROMOTE_ADJUSTMENT;

	mq->write_promote_adjustment = DEFAULT_WRITE_PROMOTE_ADJUSTMENT;

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