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    SubjectRe: 2.6.0-test8/test9 io scheduler needs tuning?


    Michael Frank wrote:

    >On Tuesday 28 October 2003 07:50, Nick Piggin wrote:
    >
    >>cliff white wrote:
    >>
    >>
    >>>On Tue, 28 Oct 2003 05:52:45 +0800
    >>>Michael Frank <mhf@linuxmail.org> wrote:
    >>>
    >>>
    >>>
    >>>>To my surprise 2.6 - which used to do better then 2.4 - does no longer
    >>>>handle these test that well.
    >>>>
    >>>>Generally, IDE IO throughput is _very_ uneven and IO _stops_ at times with the
    >>>>system cpu load very high (and the disk LED off).
    >>>>
    >>>>IMHO the CPU scheduling is OK but the IO scheduling acts up here.
    >>>>
    >>>>The test system is a 2.4GHz P4 with 512M RAM and a 55MB/s udma IDE harddisk.
    >>>>
    >>>>The tests load the system to loadavg > 30. IO should be about 20MB/s on avg.
    >>>>
    >>>>Enclosed are vmstat -1 logs for 2.6-test9-Vanilla, followed by 2.6-test8-Vanilla
    >>>>(-mm1 behaves similar), 2.4.22-Vanilla and 2.4.21+swsusp all compiled wo preempt.
    >>>>
    >>>>IO on 2.6 stops now for seconds at a time. -test8 is worse than -test9
    >>>>
    >>>>
    >>>We see the same delta at OSDL. Try repeating your tests with 'elevator=deadline'
    >>>to confirm.
    >>>For example, on the 8-cpu platform:
    >>>STP id Kernel Name MaxJPM Change Options
    >>>281669 linux-2.6.0-test8 7014.42 0.0
    >>>281671 linux-2.6.0-test8 8294.94 +18.26% elevator=deadline
    >>>
    >>>The -mm kernels don't show this big delta. We also do not see this delta on
    >>>smaller machines
    >>>
    >>>
    >>I'm working with Randy to fix this. Attached is what I have so far. See how
    >>you go with it.
    >>
    >>
    >>
    >
    >This has been done without running a kernel compile, by $ ti-tests/ti stat ub17 ddw 4 5000
    >
    >Seems to be more even on average but still drops IO too low and then gets overloaded.
    >
    >By "too low" I mean io bo less than 10000.
    >
    >By overloaded I mean io bo goes much above 40000. The disk can do maybe 60000.
    >
    >When io bo is much above 40000, cpu scheduling is being impaired as indicated
    >by vmstat stopping output for a second or so...
    >

    The bi / bo fields in vmstat aren't exactly what the disk is doing, rather
    the requests the queue is taking. The interesting thing is how your final
    measurement compares with 2.4.

    I think 2.4 has quite a lot more free requests by default (512 vs 128 for
    2.6). This might also be causing Nigel's writeout problems perhaps. Try
    echo 512 > /sys/block/xxx/queue/nr_requests


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