Hello! I was looking into why cgroup weights do not have any measurable impact on writeback throughput from different cgroups. This actually a regression from CFQ where things work more or less OK and weights have roughly the impact they should. The problem can be reproduced e.g. by running the following easy fio job in two cgroups with different weight: [writer] directory=/mnt/repro/ numjobs=1 rw=write size=8g time_based runtime=30 ramp_time=10 blocksize=1m direct=0 ioengine=sync I can observe there's no significat difference in the amount of data written from different cgroups despite their weights are in say 1:3 ratio. After some debugging I've understood the dynamics of the system. There are two issues: 1) The amount of scheduler tags needs to be significantly larger than the amount of device tags. Otherwise there are not enough requests waiting in BFQ to be dispatched to the device and thus there's nothing to schedule on. 2) Even with enough scheduler tags, writers from two cgroups eventually start contending on scheduler tag allocation. These are served on first come first served basis so writers from both cgroups feed requests into bfq with approximately the same speed. Since bfq prefers IO from heavier cgroup, that is submitted and completed faster and eventually we end up in a situation when there's no IO from the heavier cgroup in bfq and all scheduler tags are consumed by requests from the lighter cgroup. At that point bfq just dispatches lots of the IO from the lighter cgroup since there's no contender for disk throughput. As a result observed throughput for both cgroups are the same. This series fixes this problem by accounting how many scheduler tags are allocated for each cgroup and if a cgroup has more tags allocated than its fair share (based on weights) in its service tree, we heavily limit scheduler tag bitmap depth for it so that it is not be able to starve other cgroups from scheduler tags. What do people think about this? Honza
