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    SubjectRe: [PATCH] readahead:add blk_run_backing_dev
    On Fri, 22 May 2009 10:33:23 +0800
    Wu Fengguang <> wrote:

    > > I tested above patch, and I got same performance number.
    > > I wonder why if (PageUptodate(page)) check is there...
    > Thanks! This is an interesting micro timing behavior that
    > demands some research work. The above check is to confirm if it's
    > the PageUptodate() case that makes the difference. So why that case
    > happens so frequently so as to impact the performance? Will it also
    > happen in NFS?
    > The problem is readahead IO pipeline is not running smoothly, which is
    > undesirable and not well understood for now.

    The patch causes a remarkably large performance increase. A 9%
    reduction in time for a linear read? I'd be surprised if the workload
    even consumed 9% of a CPU, so where on earth has the kernel gone to?

    Have you been able to reproduce this in your testing?


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