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    Subject[PATCHSET 1][PATCH 0/6] Filesystem AIO read/write

    Currently native linux AIO is properly supported (in the sense of
    actually being asynchronous) only for files opened with O_DIRECT.
    While this suffices for a major (and most visible) user of AIO, i.e. databases,
    other types of users like Samba require AIO support for regular file IO.
    Also, for glibc POSIX AIO to be able to switch to using native AIO instead
    of the current simulation using threads, it needs/expects asynchronous
    behaviour for both O_DIRECT and buffered file AIO.

    This patchset implements changes to make filesystem AIO read
    and write asynchronous for the non O_DIRECT case. This is mainly
    relevant in the case of reads of uncached or partially cached files, and
    O_SYNC writes.

    Instead of translating regular IO to [AIO + wait], it translates AIO
    to [regular IO - blocking + retries]. The intent of implementing it
    this way is to avoid modifying or slowing down normal usage, by keeping
    it pretty much the way it is without AIO, while avoiding code duplication.
    Instead we make AIO vs regular IO checks inside io_schedule(), i.e. at
    the blocking points. The low-level unit of distinction is a wait queue
    entry, which in the AIO case is contained in an iocb and in the
    synchronous IO case is associated with the calling task.

    The core idea is that is we complete as much IO as we can in a non-blocking
    fashion, and then continue the remaining part of the transfer again when
    woken up asynchronously via a wait queue callback when pages are ready ...
    thus each iteration progresses through more of the request until it is
    completed. The interesting part here is that owing largely to the idempotence
    in the way radix-tree page cache traveral happens, every iteration is simply
    a smaller read/write. Almost all of the iocb manipulation and advancement
    in the AIO case happens in the high level AIO code, and rather than in
    regular VFS/filesystem paths.

    The following is a sampling of comparative aio-stress results with the
    patches (each run starts with uncached files):

    ---------------------------------------------

    aio-stress throughput comparisons (in MB/s):

    file size 1GB, record size 64KB, depth 64, ios per iteration 8
    max io_submit 8, buffer alignment set to 4KB
    4 way Pentium III SMP box, Adaptec AIC-7896/7 Ultra2 SCSI, 40 MB/s
    Filesystem: ext2

    ----------------------------------------------------------------------------
    Buffered (non O_DIRECT)
    Vanilla Patched O_DIRECT
    ----------------------------------------------------------------------------
    Vanilla Patched
    Random-Read 10.08 23.91 18.91, 18.98
    Random-O_SYNC-Write 8.86 15.84 16.51, 16.53
    Sequential-Read 31.49 33.00 31.86, 31.79
    Sequential-O_SYNC-Write 8.68 32.60 31.45, 32.44
    Random-Write 31.09 (19.65) 30.90 (19.65)
    Sequential-Write 30.84 (28.94) 30.09 (28.39)

    ----------------------------------------------------------------------------

    Regards
    Suparna

    --
    Suparna Bhattacharya (suparna@in.ibm.com)
    Linux Technology Center
    IBM Software Lab, India

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