BFQ cgroup weights range
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From: Michal Koutný <mkoutny@suse.com>
Date: 2021-08-24 10:56:26
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