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    SubjectRe: [PATCH] deadline io scheduler
    Andrew Morton wrote:
    >
    > I'll test scsi now.
    >

    aic7xxx, Fujitsu "MAF3364L SUN36G" (36G SCA-2)


    Maximum number of TCQ tags=253

    fifo_batch time cat kernel/*.c (seconds)
    64 58
    32 54
    16 20
    8 58
    4 1:15
    2 53

    Maximum number of TCQ tags=4

    fifo_batch time cat kernel/*.c (seconds)
    64 53
    32 39
    16 33
    8 21
    4 22
    2 36
    1 22


    Maximum number of TCQ tags = 0:

    fifo_batch time cat kernel/*.c (seconds)
    64 22
    32 10.3
    16 10.5
    8 5.5
    4 3.2
    2 1.9

    I selected fifo_batch=16 and altered writes_starved and read_expires
    again. They made no appreciable difference.

    From this I can only conclude that my poor little read was stuck
    in the disk for ages while TCQ busily allowed new incoming writes
    to bypass already-sent reads.

    A dreadful misdesign. Unless we can control this with barriers,
    and if Fujutsu is typical, TCQ is just uncontrollable. I, for
    one, would not turn it on in a pink fit.
    -
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