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    SubjectRe: [PATCH] cfq: Take whether cfq group is changed into account when choosing service tree
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    Hi Gui,
    On Mon, Dec 14, 2009 at 10:54 AM, Gui Jianfeng
    <guijianfeng@cn.fujitsu.com> wrote:
    > Corrado Zoccolo wrote:
    >> Hi,
    > Currently, IIUC, only the workload that didn't use up its slice will be saved, and only
    > such workloads are restoring when a group is resumed. So sometimes, we'll still get the
    > previous serving_type and workload_expires. Am i missing something?
    You are right. cfq_choose_cfqg should set the workload as expired if
    !cfqg->saved_workload_slice (just set cfqd->workload_expires = jiffies
    - 1), so the workload will be chosen again as the lowest keyed one.
    Can you send a patch to fix this?
    >
    >
    >>
    >> I have one more concern, though.
    >> RT priority has now changed meaning. Before, an RT task would always
    >> have priority access to the disk. Now, a BE task in a different group,
    >> with lower weight, can steal the disk from the RT task.
    >> A way to preserve the old meaning is to consider wheter a group has RT
    >> tasks inside when sorting groups tree, and putting those groups at the
    >> front.
    >> Usually, RT tasks will be put in the root group, and this (if
    >> group_isolation=0) will automatically make sure that also the noidle
    >> workload gets serviced quickly after RT tasks release the disk. We
    >> could even enforce that, with group_isolation=0, all RT tasks are put
    >> in the root group.
    >>
    >> The rationale behind this suggestion is that groups are for user
    >> processes, while RT is system wide, since it is only root that can
    >> grant it.
    >
    >  I agree, and one more thing, currently we can't see fairness between different
    >  idle tasks in different groups. Because we only allow idle cfqq dispatch one request
    >  for its dispatch round even if it's the only task in the cgroup, group always loose it
    >  share. So whether we can rely on group_isolation, when group_isolation == 1 we provide
    >  isolation for idle tasks.
    Agreed.

    Thanks,
    Corrado

    >
    > Thanks
    > Gui
    >
    >
    >
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