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

[Nick Piggin]

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@xxxxxxxxxxxxx> 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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