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    SubjectRe: [RESEND] [PATCH] readahead:add blk_run_backing_dev
    On Wed, Jul 15, 2009 at 02:52:27AM +0800, Vladislav Bolkhovitin wrote:
    > Wu Fengguang, on 07/13/2009 04:36 PM wrote:
    > >> Test done with XFS on both the target and the initiator. This confirms
    > >> your findings, using files instead of block devices is faster, but
    > >> only when using the io_context patch.
    > >
    > > It shows that the one really matters is the io_context patch,
    > > even when context readahead is running. I guess what happened
    > > in the tests are:
    > > - without readahead (or readahead algorithm failed to do proper
    > > sequential readaheads), the SCST processes will be submitting
    > > small but close to each other IOs. CFQ relies on the io_context
    > > patch to prevent unnecessary idling.
    > > - with proper readahead, the SCST processes will also be submitting
    > > close readahead IOs. For example, one file's 100-102MB pages is
    > > readahead by process A, while its 102-104MB pages may be
    > > readahead by process B. In this case CFQ will also idle waiting
    > > for process A to submit the next IO, but in fact that IO is being
    > > submitted by process B. So the io_context patch is still necessary
    > > even when context readahead is working fine. I guess context
    > > readahead do have the added value of possibly enlarging the IO size
    > > (however this benchmark seems to not very sensitive to IO size).
    > Looks like the truth. Although with 2MB RA I expect CFQ to do idling >10
    > times less, which should bring bigger improvement than few %%.
    > For how long CFQ idles? For HZ/125, i.e. 8 ms with HZ 250?

    Yes, 8ms by default. Note that the 8ms idle time is armed when the
    last IO from current process completes. So it would be definitely a
    waste if the cooperative process submitted the next read/readahead
    IO within this 8ms idle window (without cfq_coop.patch).


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