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From: "Michal Koutný" <mkoutny@suse.com>
To: Paolo Valente <paolo.valente@linaro.org>, Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org, "Jan Kara" <jack@suse.cz>,
	"Michal Koutný" <mkoutny@suse.com>
Subject: BFQ cgroup weights range
Date: Tue, 24 Aug 2021 12:56:26 +0200	[thread overview]
Message-ID: <20210824105626.GA11367@blackbody.suse.cz> (raw)

Hello.

The default weight for proportional IO control associated with a cgroup
is 100 [1]. The minimum allowed weight is 1 [2] and the maximum weight
1000 [3].  This is a bit inconsistent with general cgroup weight
semantic where def/min == max/def (i.e. symmetric adjustments to both
sides) [4].

1) Is there a reason why the maximum allowed weight is (only) 1000?
   (E.g. it won't be possible to ensure 10^4 ratio of proportional
   control but 10^3 is achievable.)
2) Is the default value 100 special or absolute in a sense? (I suspect
   it is, the unchangeable weight of root cgroup members. Therefore two
   siblings with equal ratios 10:100 and 100:1000 would behave same only
   when there's no interfering IO from the root.)

Thanks,
Michal

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/bfq-cgroup.c?h=v5.14-rc7#n513
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/bfq-iosched.h?h=v5.14-rc7#n18
[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/bfq-iosched.h?h=v5.14-rc7#n19
[4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/admin-guide/cgroup-v2.rst?h=v5.14-rc7#n602

             reply	other threads:[~2021-08-24 10:56 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-24 10:56 Michal Koutný [this message]
2021-08-24 12:51 ` BFQ cgroup weights range Paolo Valente
2021-08-26 13:12   ` Michal Koutný
2021-08-26 17:25     ` Tejun Heo
2021-08-27  8:33       ` Paolo Valente

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