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    SubjectRe: [PATCH 01/10] Documentation
    Balbir Singh wrote:
    > * Vivek Goyal <vgoyal@redhat.com> [2009-03-11 21:56:46]:
    >
    >> +
    >> + lv0 lv1
    >> + / \ / \
    >> + sda sdb sdc
    >> +
    >> +Also consider following cgroup hierarchy
    >> +
    >> + root
    >> + / \
    >> + A B
    >> + / \ / \
    >> + T1 T2 T3 T4
    >> +
    >> +A and B are two cgroups and T1, T2, T3 and T4 are tasks with-in those cgroups.
    >> +Assuming T1, T2, T3 and T4 are doing IO on lv0 and lv1. These tasks should
    >> +get their fair share of bandwidth on disks sda, sdb and sdc. There is no
    >> +IO control on intermediate logical block nodes (lv0, lv1).
    >> +
    >> +So if tasks T1 and T2 are doing IO on lv0 and T3 and T4 are doing IO on lv1
    >> +only, there will not be any contetion for resources between group A and B if
    >> +IO is going to sda or sdc. But if actual IO gets translated to disk sdb, then
    >> +IO scheduler associated with the sdb will distribute disk bandwidth to
    >> +group A and B proportionate to their weight.
    >
    > What if we have partitions sda1, sda2 and sda3 instead of sda, sdb and
    > sdc?

    The bandwidth controlling is device basis, so with sda1, sda2 and sda3 instead,
    they will contending on sda.

    >
    >> +
    >> +CFQ already has the notion of fairness and it provides differential disk
    >> +access based on priority and class of the task. Just that it is flat and
    >> +with cgroup stuff, it needs to be made hierarchical.
    >> +
    >> +Rest of the IO schedulers (noop, deadline and AS) don't have any notion
    >> +of fairness among various threads.
    >> +
    >> +One of the concerns raised with modifying IO schedulers was that we don't
    >> +want to replicate the code in all the IO schedulers. These patches share
    >> +the fair queuing code which has been moved to a common layer (elevator
    >> +layer). Hence we don't end up replicating code across IO schedulers.
    >> +
    >> +Design
    >> +======
    >> +This patchset primarily uses BFQ (Budget Fair Queuing) code to provide
    >> +fairness among different IO queues. Fabio and Paolo implemented BFQ which uses
    >> +B-WF2Q+ algorithm for fair queuing.
    >> +
    >
    > References to BFQ, please. I can search them, but having them in the
    > doc would be nice.
    >
    >> +Why BFQ?
    >> +
    >> +- Not sure if weighted round robin logic of CFQ can be easily extended for
    >> + hierarchical mode. One of the things is that we can not keep dividing
    >> + the time slice of parent group among childrens. Deeper we go in hierarchy
    >> + time slice will get smaller.
    >> +
    >> + One of the ways to implement hierarchical support could be to keep track
    >> + of virtual time and service provided to queue/group and select a queue/group
    >> + for service based on any of the various available algoriths.
    >> +
    >> + BFQ already had support for hierarchical scheduling, taking those patches
    >> + was easier.
    >> +
    >
    > Could you elaborate, when you say timeslices get smaller -
    >
    > 1. Are you referring to inability to use higher resolution time?
    > 2. Loss of throughput due to timeslice degradation?
    >
    >> +- BFQ was designed to provide tighter bounds/delay w.r.t service provided
    >> + to a queue. Delay/Jitter with BFQ is supposed to be O(1).
    >> +
    >> + Note: BFQ originally used amount of IO done (number of sectors) as notion
    >> + of service provided. IOW, it tried to provide fairness in terms of
    >> + actual IO done and not in terms of actual time disk access was
    >> + given to a queue.
    >
    > I assume by sectors you mean the kernel sector size?
    >
    >> +
    >> + This patcheset modified BFQ to provide fairness in time domain because
    >> + that's what CFQ does. So idea was try not to deviate too much from
    >> + the CFQ behavior initially.
    >> +
    >> + Providing fairness in time domain makes accounting trciky because
    >> + due to command queueing, at one time there might be multiple requests
    >> + from different queues and there is no easy way to find out how much
    >> + disk time actually was consumed by the requests of a particular
    >> + queue. More about this in comments in source code.
    >> +
    >> +So it is yet to be seen if changing to time domain still retains BFQ gurantees
    >> +or not.
    >> +
    >> +From data structure point of view, one can think of a tree per device, where
    >> +io groups and io queues are hanging and are being scheduled using B-WF2Q+
    >> +algorithm. io_queue, is end queue where requests are actually stored and
    >> +dispatched from (like cfqq).
    >> +
    >> +These io queues are primarily created by and managed by end io schedulers
    >> +depending on its semantics. For example, noop, deadline and AS ioschedulers
    >> +keep one io queues per cgroup and cfqq keeps one io queue per io_context in
    >> +a cgroup (apart from async queues).
    >> +
    >
    > I assume there is one io_context per cgroup.
    >
    >> +A request is mapped to an io group by elevator layer and which io queue it
    >> +is mapped to with in group depends on ioscheduler. Currently "current" task
    >> +is used to determine the cgroup (hence io group) of the request. Down the
    >> +line we need to make use of bio-cgroup patches to map delayed writes to
    >> +right group.
    >
    > That seem acceptable
    >
    >> +
    >> +Going back to old behavior
    >> +==========================
    >> +In new scheme of things essentially we are creating hierarchical fair
    >> +queuing logic in elevator layer and chaning IO schedulers to make use of
    >> +that logic so that end IO schedulers start supporting hierarchical scheduling.
    >> +
    >> +Elevator layer continues to support the old interfaces. So even if fair queuing
    >> +is enabled at elevator layer, one can have both new hierchical scheduler as
    >> +well as old non-hierarchical scheduler operating.
    >> +
    >> +Also noop, deadline and AS have option of enabling hierarchical scheduling.
    >> +If it is selected, fair queuing is done in hierarchical manner. If hierarchical
    >> +scheduling is disabled, noop, deadline and AS should retain their existing
    >> +behavior.
    >> +
    >> +CFQ is the only exception where one can not disable fair queuing as it is
    >> +needed for provding fairness among various threads even in non-hierarchical
    >> +mode.
    >> +
    >> +Various user visible config options
    >> +===================================
    >> +CONFIG_IOSCHED_NOOP_HIER
    >> + - Enables hierchical fair queuing in noop. Not selecting this option
    >> + leads to old behavior of noop.
    >> +
    >> +CONFIG_IOSCHED_DEADLINE_HIER
    >> + - Enables hierchical fair queuing in deadline. Not selecting this
    >> + option leads to old behavior of deadline.
    >> +
    >> +CONFIG_IOSCHED_AS_HIER
    >> + - Enables hierchical fair queuing in AS. Not selecting this option
    >> + leads to old behavior of AS.
    >> +
    >> +CONFIG_IOSCHED_CFQ_HIER
    >> + - Enables hierarchical fair queuing in CFQ. Not selecting this option
    >> + still does fair queuing among various queus but it is flat and not
    >> + hierarchical.
    >> +
    >> +Config options selected automatically
    >> +=====================================
    >> +These config options are not user visible and are selected/deselected
    >> +automatically based on IO scheduler configurations.
    >> +
    >> +CONFIG_ELV_FAIR_QUEUING
    >> + - Enables/Disables the fair queuing logic at elevator layer.
    >> +
    >> +CONFIG_GROUP_IOSCHED
    >> + - Enables/Disables hierarchical queuing and associated cgroup bits.
    >> +
    >> +TODO
    >> +====
    >> +- Lots of cleanups, testing, bug fixing, optimizations, benchmarking etc...
    >> +- Convert cgroup ioprio to notion of weight.
    >> +- Anticipatory code will need more work. It is not working properly currently
    >> + and needs more thought.
    >
    > What are the problems with the code?

    Anticipatory has its own idling logic, so what is the concerning here is how to make
    as work together with commom layer.

    --
    Regards
    Gui Jianfeng



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