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    Subject2.4.13-pre5-aa1 O_DIRECT drastic HIGHMEM performance hit
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    We're looking into setting up an HD video editing solution with the
    following hardware:

    dual 1GHz P3, 2GB RAM
    Serverworks HeSL based board
    dual U160 Adaptec SCSI card
    5 68gb Fujitsu drives on each SCSI channel

    The 10 drives are software raid0 striped with a 68k chunk size. For an
    initial benchmark I got a copy of lmdd and modified it to open files
    using O_DIRECT (also hacked it to work on W2K with FILE_FLAG_NO_BUFFER).
    Here are the average results in MB/sec for 2.4.13-pre5-aa1 streaming
    chunks of data 4MB at a time to RAM where they are immediately
    discarded:

    w r
    no O_DIRECT 136 132
    O_DIRECT 111 96

    I experimented around a bit and discovered that increasing
    KIO_MAX_ATOMIC_IO from 512 to 4096 gave a significant performance boost
    for the O_DIRECT read case:
    w r
    no O_DIRECT 117 124
    O_DIRECT 111 165


    Next I recompiled without HIGHMEM support and was quite suprised
    (numbers are with KIO_MAX_ATOMIC_IO=4096)

    w r
    no O_DIRECT 138 125
    O_DIRECT 221 182

    XFS shows similar behavior: (XFS barfs if you change KIO_MAX_ATOMIC_IO
    from 512)

    w r
    O_DIRECT w/ HIGHMEM 114 146
    O_DIRECT w/o HIGHMEM 218 248


    (Incidentally, W2k reads and writes at a smooth 255 MB/sec on identical
    hardware.)

    Are we stuck with a low mem configuration or are there workarounds that
    would allow us to stick with the initial 2GB of RAM and still get ~200
    MB/sec.



    --------------------------
    Marvin Justice
    Software Developer
    BOXX Technologies, Inc.
    www.boxxtech.com
    mjustice@boxxtech.com
    (V) (512)225-6318
    (F) (512)835-0434
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