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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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