From 4403e4e467c365b4189e3e3d3ad35cf67b8c36ed Mon Sep 17 00:00:00 2001 From: Angelo Ruocco Date: Wed, 20 Dec 2017 12:38:34 +0100 Subject: block, bfq: remove superfluous check in queue-merging setup When two or more processes do I/O in a way that the their requests are sequential in respect to one another, BFQ merges the bfq_queues associated with the processes. This way the overall I/O pattern becomes sequential, and thus there is a boost in througput. These cooperating processes usually start or restart to do I/O shortly after each other. So, in order to avoid merging non-cooperating processes, BFQ ensures that none of these queues has been in weight raising for too long. In this respect, from commit "block, bfq-sq, bfq-mq: let a queue be merged only shortly after being created", BFQ checks whether any queue (and not only weight-raised ones) is doing I/O continuously from too long to be merged. This new additional check makes the first one useless: a queue doing I/O from long enough, if being weight-raised, is also a queue in weight raising for too long to be merged. Accordingly, this commit removes the first check. Signed-off-by: Angelo Ruocco Signed-off-by: Paolo Valente Signed-off-by: Jens Axboe --- block/bfq-iosched.c | 36 +++++------------------------------- 1 file changed, 5 insertions(+), 31 deletions(-) (limited to 'block/bfq-iosched.c') diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index 7066d90f09df..9625550b2f85 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -1990,20 +1990,6 @@ static bool bfq_may_be_close_cooperator(struct bfq_queue *bfqq, return true; } -/* - * If this function returns true, then bfqq cannot be merged. The idea - * is that true cooperation happens very early after processes start - * to do I/O. Usually, late cooperations are just accidental false - * positives. In case bfqq is weight-raised, such false positives - * would evidently degrade latency guarantees for bfqq. - */ -static bool wr_from_too_long(struct bfq_queue *bfqq) -{ - return bfqq->wr_coeff > 1 && - time_is_before_jiffies(bfqq->last_wr_start_finish + - msecs_to_jiffies(100)); -} - /* * Attempt to schedule a merge of bfqq with the currently in-service * queue or with a close queue among the scheduled queues. Return @@ -2017,11 +2003,6 @@ static bool wr_from_too_long(struct bfq_queue *bfqq) * to maintain. Besides, in such a critical condition as an out of memory, * the benefits of queue merging may be little relevant, or even negligible. * - * Weight-raised queues can be merged only if their weight-raising - * period has just started. In fact cooperating processes are usually - * started together. Thus, with this filter we avoid false positives - * that would jeopardize low-latency guarantees. - * * WARNING: queue merging may impair fairness among non-weight raised * queues, for at least two reasons: 1) the original weight of a * merged queue may change during the merged state, 2) even being the @@ -2052,9 +2033,7 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq, if (bfqq->new_bfqq) return bfqq->new_bfqq; - if (!io_struct || - wr_from_too_long(bfqq) || - unlikely(bfqq == &bfqd->oom_bfqq)) + if (!io_struct || unlikely(bfqq == &bfqd->oom_bfqq)) return NULL; /* If there is only one backlogged queue, don't search. */ @@ -2063,12 +2042,9 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq, in_service_bfqq = bfqd->in_service_queue; - if (!in_service_bfqq || in_service_bfqq == bfqq - || wr_from_too_long(in_service_bfqq) || - unlikely(in_service_bfqq == &bfqd->oom_bfqq)) - goto check_scheduled; - - if (bfq_rq_close_to_sector(io_struct, request, bfqd->last_position) && + if (in_service_bfqq && in_service_bfqq != bfqq && + likely(in_service_bfqq != &bfqd->oom_bfqq) && + bfq_rq_close_to_sector(io_struct, request, bfqd->last_position) && bfqq->entity.parent == in_service_bfqq->entity.parent && bfq_may_be_close_cooperator(bfqq, in_service_bfqq)) { new_bfqq = bfq_setup_merge(bfqq, in_service_bfqq); @@ -2080,12 +2056,10 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq, * queues. The only thing we need is that the bio/request is not * NULL, as we need it to establish whether a cooperator exists. */ -check_scheduled: new_bfqq = bfq_find_close_cooperator(bfqd, bfqq, bfq_io_struct_pos(io_struct, request)); - if (new_bfqq && !wr_from_too_long(new_bfqq) && - likely(new_bfqq != &bfqd->oom_bfqq) && + if (new_bfqq && likely(new_bfqq != &bfqd->oom_bfqq) && bfq_may_be_close_cooperator(bfqq, new_bfqq)) return bfq_setup_merge(bfqq, new_bfqq); -- cgit v1.2.3