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    SubjectRe: [PATCH 15/20] blkio: Take care of preemptions across groups
    Date
    Vivek Goyal <vgoyal@redhat.com> writes:

    > It will not as we traverse up the hierarchy and look for the ioprio class
    > of the group entity.
    >
    > So if you got following configuration where G1 and G2 are two groups. G1
    > is prio class RT and G2 is prio class BE, then any queue in G1 will
    > preempt any queue in G2 as at highest level, G1 and G2 are different class
    > altogether.
    >
    > root
    > / \
    > G1 G2
    >
    > Normal cfqq preemption checks will not catch this. So if G2 has some BE
    > cfqq running, and some BE queue gets backlogged in G1, this new queue wil
    > not preempt the queue in G2 and it should have.
    >
    > That's why preemption checks at group level.
    >
    > Secondly if G1 and G2 are of ioprioclass BE and all the jobs in G1 are of
    > RT nature, they will not preempt the queues in G2, hence providing
    > isolation.

    Thanks for the explanation, Vivek. I somehow missed that we were
    checking the class of the group entity and not the cfqq's entity.

    Cheers,
    Jeff


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