From f20426056f2eba3f0379779f0a75722e41bc28da Mon Sep 17 00:00:00 2001 From: Khazhismel Kumykov Date: Fri, 19 Jan 2018 15:07:37 -0800 Subject: dm mpath selector: more evenly distribute ties Move the last used path to the end of the list (least preferred) so that ties are more evenly distributed. For example, in case with three paths with one that is slower than others, the remaining two would be unevenly used if they tie. This is due to the rotation not being a truely fair distribution. Illustrated: paths a, b, c, 'c' has 1 outstanding IO, a and b are 'tied' Three possible rotations: (a, b, c) -> best path 'a' (b, c, a) -> best path 'b' (c, a, b) -> best path 'a' (a, b, c) -> best path 'a' (b, c, a) -> best path 'b' (c, a, b) -> best path 'a' ... So 'a' is used 2x more than 'b', although they should be used evenly. With this change, the most recently used path is always the least preferred, removing this bias resulting in even distribution. (a, b, c) -> best path 'a' (b, c, a) -> best path 'b' (c, a, b) -> best path 'a' (c, b, a) -> best path 'b' ... Signed-off-by: Khazhismel Kumykov Reviewed-by: Martin Wilck Signed-off-by: Mike Snitzer --- drivers/md/dm-queue-length.c | 6 +++--- drivers/md/dm-service-time.c | 6 +++--- 2 files changed, 6 insertions(+), 6 deletions(-) (limited to 'drivers/md') diff --git a/drivers/md/dm-queue-length.c b/drivers/md/dm-queue-length.c index 23f178641794..969c4f1a3633 100644 --- a/drivers/md/dm-queue-length.c +++ b/drivers/md/dm-queue-length.c @@ -195,9 +195,6 @@ static struct dm_path *ql_select_path(struct path_selector *ps, size_t nr_bytes) if (list_empty(&s->valid_paths)) goto out; - /* Change preferred (first in list) path to evenly balance. */ - list_move_tail(s->valid_paths.next, &s->valid_paths); - list_for_each_entry(pi, &s->valid_paths, list) { if (!best || (atomic_read(&pi->qlen) < atomic_read(&best->qlen))) @@ -210,6 +207,9 @@ static struct dm_path *ql_select_path(struct path_selector *ps, size_t nr_bytes) if (!best) goto out; + /* Move most recently used to least preferred to evenly balance. */ + list_move_tail(&best->list, &s->valid_paths); + ret = best->path; out: spin_unlock_irqrestore(&s->lock, flags); diff --git a/drivers/md/dm-service-time.c b/drivers/md/dm-service-time.c index 7b8642045c55..f006a9005593 100644 --- a/drivers/md/dm-service-time.c +++ b/drivers/md/dm-service-time.c @@ -282,9 +282,6 @@ static struct dm_path *st_select_path(struct path_selector *ps, size_t nr_bytes) if (list_empty(&s->valid_paths)) goto out; - /* Change preferred (first in list) path to evenly balance. */ - list_move_tail(s->valid_paths.next, &s->valid_paths); - list_for_each_entry(pi, &s->valid_paths, list) if (!best || (st_compare_load(pi, best, nr_bytes) < 0)) best = pi; @@ -292,6 +289,9 @@ static struct dm_path *st_select_path(struct path_selector *ps, size_t nr_bytes) if (!best) goto out; + /* Move most recently used to least preferred to evenly balance. */ + list_move_tail(&best->list, &s->valid_paths); + ret = best->path; out: spin_unlock_irqrestore(&s->lock, flags); -- cgit v1.2.3