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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 <angeloruocco90@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Angelo Ruocco 2017-12-20 12:38:34 +01:00 committed by Jens Axboe
parent 7b8fa3b900
commit 4403e4e467
1 changed files with 5 additions and 31 deletions
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36
block/bfq-iosched.c
View File

@ -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);