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Commit 1de0c4cd authored by Arianna Avanzini's avatar Arianna Avanzini Committed by Jens Axboe
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block, bfq: reduce idling only in symmetric scenarios

A seeky queue (i..e, a queue containing random requests) is assigned a
very small device-idling slice, for throughput issues. Unfortunately,
given the process associated with a seeky queue, this behavior causes
the following problem: if the process, say P, performs sync I/O and
has a higher weight than some other processes doing I/O and associated
with non-seeky queues, then BFQ may fail to guarantee to P its
reserved share of the throughput. The reason is that idling is key
for providing service guarantees to processes doing sync I/O [1].

This commit addresses this issue by allowing the device-idling slice
to be reduced for a seeky queue only if the scenario happens to be
symmetric, i.e., if all the queues are to receive the same share of
the throughput.

[1] P. Valente, A. Avanzini, "Evolution of the BFQ Storage I/O
    Scheduler", Proceedings of the First Workshop on Mobile System
    Technologies (MST-2015), May 2015.
    http://algogroup.unimore.it/people/paolo/disk_sched/mst-2015.pdf



Signed-off-by: default avatarArianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: default avatarRiccardo Pizzetti <riccardo.pizzetti@gmail.com>
Signed-off-by: default avatarSamuele Zecchini <samuele.zecchini92@gmail.com>
Signed-off-by: default avatarPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
parent 36eca894
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