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>
diff --git a/drivers/md/dm-cache-policy-mq.c b/drivers/md/dm-cache-policy-mq.c
index 0e385e4..334d098 100644
--- a/drivers/md/dm-cache-policy-mq.c
+++ b/drivers/md/dm-cache-policy-mq.c
@@ -181,26 +181,32 @@
* 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;
- list_del(r);
-
- /* have we just emptied the bottom level? */
- if (level == 0 && list_empty(q->qs))
- queue_shift_down(q);
-
- return r;
- }
+ 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 (list_empty(q->qs))
+ queue_shift_down(q);
+ }
+
+ return r;
+}
+
static struct list_head *list_pop(struct list_head *lh)
{
struct list_head *r = lh->next;
@@ -383,13 +389,6 @@
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 @@
#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 @@
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 @@
break;
}
}
-
- mq->promote_threshold = nr ? total / nr : 1;
- if (mq->promote_threshold * nr < total)
- mq->promote_threshold++;
}
}
@@ -641,6 +643,30 @@
}
/*
+ * 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.
*
* If the origin block has been discarded then there's no cost to copy it
@@ -653,7 +679,7 @@
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 @@
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 @@
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;