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

Al Boldi
2006-04-25T10:54:04Z

Nick Piggin wrote:
> 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?

Same w/ deadline, as, and cfq.

Thanks!

--
Al