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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: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
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@ -58,9 +58,9 @@ since spindles tend to have good bandwidth. The io_tracker counts
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contiguous I/Os to try to spot when the io is in one of these sequential
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modes.
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Internally the mq policy maintains a promotion threshold variable. If
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the hit count of a block not in the cache goes above this threshold it
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gets promoted to the cache. The read, write and discard promote adjustment
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Internally the mq policy determines a promotion threshold. If the hit
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count of a block not in the cache goes above this threshold it gets
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promoted to the cache. The read, write and discard promote adjustment
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tunables allow you to tweak the promotion threshold by adding a small
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value based on the io type. They default to 4, 8 and 1 respectively.
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If you're trying to quickly warm a new cache device you may wish to
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@ -181,26 +181,32 @@ static void queue_shift_down(struct queue *q)
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* Gives us the oldest entry of the lowest popoulated level. If the first
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* level is emptied then we shift down one level.
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*/
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static struct list_head *queue_pop(struct queue *q)
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static struct list_head *queue_peek(struct queue *q)
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{
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unsigned level;
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struct list_head *r;
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for (level = 0; level < NR_QUEUE_LEVELS; level++)
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if (!list_empty(q->qs + level)) {
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r = q->qs[level].next;
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list_del(r);
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/* have we just emptied the bottom level? */
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if (level == 0 && list_empty(q->qs))
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queue_shift_down(q);
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return r;
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}
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if (!list_empty(q->qs + level))
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return q->qs[level].next;
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return NULL;
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}
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static struct list_head *queue_pop(struct queue *q)
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{
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struct list_head *r = queue_peek(q);
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if (r) {
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list_del(r);
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/* have we just emptied the bottom level? */
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if (list_empty(q->qs))
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queue_shift_down(q);
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}
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return r;
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}
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static struct list_head *list_pop(struct list_head *lh)
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{
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struct list_head *r = lh->next;
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@ -383,13 +389,6 @@ struct mq_policy {
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unsigned generation;
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unsigned generation_period; /* in lookups (will probably change) */
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/*
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* Entries in the pre_cache whose hit count passes the promotion
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* threshold move to the cache proper. Working out the correct
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* value for the promotion_threshold is crucial to this policy.
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*/
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unsigned promote_threshold;
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unsigned discard_promote_adjustment;
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unsigned read_promote_adjustment;
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unsigned write_promote_adjustment;
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@ -406,6 +405,7 @@ struct mq_policy {
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#define DEFAULT_DISCARD_PROMOTE_ADJUSTMENT 1
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#define DEFAULT_READ_PROMOTE_ADJUSTMENT 4
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#define DEFAULT_WRITE_PROMOTE_ADJUSTMENT 8
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#define DISCOURAGE_DEMOTING_DIRTY_THRESHOLD 128
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/*----------------------------------------------------------------*/
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@ -518,6 +518,12 @@ static struct entry *pop(struct mq_policy *mq, struct queue *q)
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return e;
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}
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static struct entry *peek(struct queue *q)
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{
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struct list_head *h = queue_peek(q);
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return h ? container_of(h, struct entry, list) : NULL;
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}
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/*
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* Has this entry already been updated?
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*/
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@ -570,10 +576,6 @@ static void check_generation(struct mq_policy *mq)
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break;
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}
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}
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mq->promote_threshold = nr ? total / nr : 1;
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if (mq->promote_threshold * nr < total)
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mq->promote_threshold++;
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}
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}
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@ -640,6 +642,30 @@ static int demote_cblock(struct mq_policy *mq, dm_oblock_t *oblock)
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return 0;
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}
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/*
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* Entries in the pre_cache whose hit count passes the promotion
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* threshold move to the cache proper. Working out the correct
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* value for the promotion_threshold is crucial to this policy.
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*/
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static unsigned promote_threshold(struct mq_policy *mq)
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{
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struct entry *e;
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if (any_free_cblocks(mq))
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return 0;
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e = peek(&mq->cache_clean);
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if (e)
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return e->hit_count;
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e = peek(&mq->cache_dirty);
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if (e)
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return e->hit_count + DISCOURAGE_DEMOTING_DIRTY_THRESHOLD;
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/* This should never happen */
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return 0;
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}
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/*
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* We modify the basic promotion_threshold depending on the specific io.
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*
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@ -653,7 +679,7 @@ static unsigned adjusted_promote_threshold(struct mq_policy *mq,
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bool discarded_oblock, int data_dir)
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{
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if (data_dir == READ)
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return mq->promote_threshold + mq->read_promote_adjustment;
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return promote_threshold(mq) + mq->read_promote_adjustment;
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if (discarded_oblock && (any_free_cblocks(mq) || any_clean_cblocks(mq))) {
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/*
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@ -663,7 +689,7 @@ static unsigned adjusted_promote_threshold(struct mq_policy *mq,
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return mq->discard_promote_adjustment;
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}
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return mq->promote_threshold + mq->write_promote_adjustment;
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return promote_threshold(mq) + mq->write_promote_adjustment;
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}
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static bool should_promote(struct mq_policy *mq, struct entry *e,
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@ -1230,7 +1256,6 @@ static struct dm_cache_policy *mq_create(dm_cblock_t cache_size,
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mq->tick = 0;
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mq->hit_count = 0;
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mq->generation = 0;
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mq->promote_threshold = 0;
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mq->discard_promote_adjustment = DEFAULT_DISCARD_PROMOTE_ADJUSTMENT;
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mq->read_promote_adjustment = DEFAULT_READ_PROMOTE_ADJUSTMENT;
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mq->write_promote_adjustment = DEFAULT_WRITE_PROMOTE_ADJUSTMENT;
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