block, bfq: avoid selecting a queue w/o budget

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To boost throughput on devices with internal queueing and in scenarios
where device idling is not strictly needed, bfq immediately starts
serving a new bfq_queue if the in-service bfq_queue remains without
pending I/O, even if new I/O may arrive soon for the latter queue. Then,
if such I/O actually arrives soon, bfq preempts the new in-service
bfq_queue so as to give the previous queue a chance to go on being
served (in case the previous queue should actually be the one to be
served, according to its timestamps).

However, the in-service bfq_queue, say Q, may also be without further
budget when it remains also pending I/O. Since bfq changes budgets
dynamically to fit the needs of bfq_queues, this happens more often than
one may expect. If this happens, then there is no point in trying to go
on serving Q when new I/O arrives for it soon: Q would be expired
immediately after being selected for service. This would only cause
useless overhead. This commit avoids such a useless selection.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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This commit is contained in:

Paolo Valente 2019-01-29 12:06:26 +01:00 • committed by Jens Axboe

parent 20cd32450b

commit 218cb897be

1 changed files with 9 additions and 1 deletions

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block/bfq-iosched.c

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