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

    On Sat, Dec 19, 2009 at 10:25:47PM +0800, Steve Rago wrote:
    > On Sat, 2009-12-19 at 20:20 +0800, Wu Fengguang wrote:
    > >
    > > On Thu, Dec 17, 2009 at 04:17:57PM +0800, Peter Zijlstra wrote:
    > > > On Wed, 2009-12-16 at 21:03 -0500, Steve Rago wrote:
    > > > > Eager Writeback for NFS Clients
    > > > > -------------------------------
    > > > > Prevent applications that write large sequential streams of data (like backup, for example)
    > > > > from entering into a memory pressure state, which degrades performance by falling back to
    > > > > synchronous operations (both synchronous writes and additional commits).
    > >
    > > What exactly is the "memory pressure state" condition? What's the
    > > code to do the "synchronous writes and additional commits" and maybe
    > > how they are triggered?
    > Memory pressure occurs when most of the client pages have been dirtied
    > by an application (think backup server writing multi-gigabyte files that
    > exceed the size of main memory). The system works harder to be able to
    > free dirty pages so that they can be reused. For a local file system,
    > this means writing the pages to disk. For NFS, however, the writes
    > leave the pages in an "unstable" state until the server responds to a
    > commit request. Generally speaking, commit processing is far more
    > expensive than write processing on the server; both are done with the
    > inode locked, but since the commit takes so long, all writes are
    > blocked, which stalls the pipeline.

    Let me try reiterate the problem with code, please correct me if I'm

    1) normal fs sets I_DIRTY_DATASYNC when extending i_size, however NFS
    will set the flag for any pages written -- why this trick? To
    guarantee the call of nfs_commit_inode()? Which unfortunately turns
    almost every server side NFS write into sync writes..

    nfs_writepage ----[short time later]---> nfs_writeback_release*

    if (I_DIRTY_SYNC || I_DIRTY_DATASYNC) <---- so this will be true for most time

    2) NFS commit stops pipeline because it sleep&wait inside i_mutex,
    which blocks all other NFSDs trying to write/writeback the inode.

    take i_mutex
    drop i_mutex

    If filemap_fdatawait() can be moved out of i_mutex (or just remove
    the lock), we solve the root problem:

    [take i_mutex]
    filemap_fdatawrite => can also be blocked, but less a problem
    [drop i_mutex]

    Maybe it's a dumb question, but what's the purpose of i_mutex here?
    For correctness or to prevent livelock? I can imagine some livelock
    problem here (current implementation can easily wait for extra
    pages), however not too hard to fix.

    The proposed patch essentially takes two actions in nfs_file_write()
    - to start writeback when the per-file nr_dirty goes high
    without committing
    - to throttle dirtying when the per-file nr_writeback goes high
    I guess this effectively prevents pdflush from kicking in with
    its bad committing behavior

    In general it's reasonable to keep NFS per-file nr_dirty low, however
    questionable to do per-file nr_writeback throttling. This does not
    work well with the global limits - eg. when there are many dirty
    files, the summed-up nr_writeback will still grow out of control.
    And it's more likely to impact user visible responsiveness than
    a global limit. But my opinion can be biased -- me have a patch to
    do global NFS nr_writeback limit ;)


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