Re: [PATCH] Direct I/O bio size regression

[Nick Piggin]

Al Boldi wrote:
> Jens Axboe wrote:
>
> > On Mon, Apr 24 2006, Al Boldi wrote:
> >
> > > On my system max_hw_sectors_kb is fixed at 1024, and max_sectors_kb
> > > defaults to 512, which leads to terribly fluctuating thruput.
> > >
> > > Setting max_sectors_kb = max_hw_sectors_kb makes things even worse.
> > >
> > > Tuning max_sectors_kb to ~192 only stabilizes this situation.
> >
> > That sounds pretty strange. Do you have a test case?
>
>
> I would think that, if you could get your hands on some hw that defaults to 
> the same values, you may easily see the same problem by doing this:
>
> 1. # vmstat 1 (or some other bio mon)
> 2. < change vt >
> 3. # cat /dev/hda > /dev/null &
> 4. # cat /dev/hda > /dev/null
> Let this second cat run for a sec, then ^C.
> Depending on your hw specifics the bio should either go up or down by a 
> factor of 2 (on my system 25mb/s-48mb/s).  You may have to repeat step 4 a 
> few times to aggravate the situation.
>
> Note that this is not specific to cat, but can also be observed during normal 
> random disk access, although not in a controlled manner.

*random* disk access?

What io scheduler are you using? Can you try with as?

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