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    SubjectRe: [PATCH] io-controller: Add io group reference handling for request
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    Andrea Righi <righi.andrea@gmail.com> wrote:
    > On Wed, May 27, 2009 at 03:56:31PM +0900, Ryo Tsuruta wrote:
    > > > I think that only putting the hook in try_to_unmap() doesn't work
    > > > correctly, because IOs will be charged to reclaiming processes or
    > > > kswapd. These IOs should be charged to processes which cause memory
    > > > pressure.
    > >
    > > Consider the following case:
    > >
    > > (1) There are two processes Proc-A and Proc-B.
    > > (2) Proc-A maps a large file into many pages by mmap() and writes
    > > many data to the file.
    > > (3) After (2), Proc-B try to get a page, but there are no available
    > > pages because Proc-A has used them.
    > > (4) kernel starts to reclaim pages, call try_to_unmap() to unmap
    > > a page which is owned by Proc-A, then blkio_cgroup_set_owner()
    > > sets Proc-B's ID on the page because the task's context is Proc-B.
    > > (5) After (4), kernel writes the page out to a disk. This IO is
    > > charged to Proc-B.
    > >
    > > In the above case, I think that the IO should be charged to a Proc-A,
    > > because the IO is caused by Proc-A's memory pressure.
    > > I think we should consider in the case without memory and swap
    > > isolation.
    >
    > mmmh.. even if they're strictly related I think we're mixing two
    > different problems in this way: memory pressure control and IO control.
    >
    > It seems you're proposing something like the badness() for OOM
    > conditions to charge swap IO depending on how bad is a cgroup in terms
    > of memory consumption. I don't think this is the right way to proceed,
    > also because we already have the memory and swap control.

    cgroups support multiple hierarchy and it allows to have different
    divisions of tasks among hierarchy like below:

    PIDs
    mem+swap /hier1/grp1/tasks <= 1, 2, 3, 4
    blkio /hier2/grp2/tasks <= 1, 2
    grp3/tasks <= 3, 4

    Don't we need to consider this case?

    Thanks,
    Ryo Tsuruta


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