Messages in this thread |  | | Subject | Re: [PATCH BUGFIX] block, bfq: fix asymmetric scenarios detection | From | Jens Axboe <> | Date | Thu, 25 Oct 2018 08:18:29 -0600 |
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On 10/24/18 11:13 AM, Paolo Valente wrote: > From: Federico Motta <federico@willer.it> > > Since commit 2d29c9f89fcd ("block, bfq: improve asymmetric scenarios > detection"), a scenario is defined asymmetric when one of the > following conditions holds: > - active bfq_queues have different weights > - one or more group of entities (bfq_queue or other groups of entities) > are active > bfq grants fairness and low latency also in such asymmetric scenarios, > by plugging the dispatching of I/O if the bfq_queue in service happens > to be temporarily idle. This plugging may lower throughput, so it is > important to do it only when strictly needed. > > By mystake, in commit '2d29c9f89fcd' ("block, bfq: improve asymmetric > scenarios detection") the num_active_groups counter was firstly > incremented and subsequently decremented at any entity (group or > bfq_queue) weight change. > > This is useless, because only transitions from active to inactive and > vice versa matter for that counter. Unfortunately this is also > incorrect in the following case: the entity at issue is a bfq_queue > and it is under weight raising. In fact in this case there is a > spurious increment of the num_active_groups counter. > > This spurious increment may cause scenarios to be wrongly detected as > asymmetric, thus causing useless plugging and loss of throughput. > > This commit fixes this issue by simply removing the above useless and > wrong increments and decrements.
Applied for 4.20, thanks.
-- Jens Axboe
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