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    SubjectRe: [PATCH] improve the performance of large sequential write NFS workloads

    On Tue, 2009-12-22 at 13:25 +0100, Jan Kara wrote:
    > > I originally spent several months playing with the balance_dirty_pages
    > > algorithm. The main drawback is that it affects more than the inodes
    > > that the caller is writing and that the control of what to do is too
    > Can you be more specific here please?

    Sure; balance_dirty_pages() will schedule writeback by the flusher
    thread once the number of dirty pages exceeds dirty_background_ratio.
    The flusher thread calls writeback_inodes_wb() to flush all dirty inodes
    associated with the bdi. Similarly, the process dirtying the pages will
    call writeback_inodes_wbc() when it's bdi threshold has been exceeded.
    The first problem is that these functions process all dirty inodes with
    the same backing device, which can lead to excess (duplicate) flushing
    of the same inode. Second, there is no distinction between pages that
    need to be committed and pages that have commits pending in
    NR_UNSTABLE_NFS/BDI_RECLAIMABLE (a page that has a commit pending won't
    be cleaned any faster by sending more commits). This tends to overstate
    the amount of memory that can be cleaned, leading to additional commit
    requests. Third, these functions generate a commit for each set of
    writes they do, which might not be appropriate. For background writing,
    you'd like to delay the commit as long as possible.


    > >
    > > Part of the patch does implement a heuristic write-behind. See where
    > > nfs_wb_eager() is called.
    > I believe that if we had per-bdi dirty_background_ratio and set it low
    > for NFS's bdi, then the write-behind logic would not be needed
    > (essentially the flusher thread should submit the writes to the server
    > early).
    > Honza

    Maybe so, but you still need something to prevent the process that is
    dirtying pages from continuing, because a process can always write to
    memory faster than writing to disk/network, so the flusher won't be able
    to keep up.


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      Last update: 2009-12-22 17:23    [W:0.020 / U:14.864 seconds]
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