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

[Al Boldi <a1426z@gawab.com>]

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.

Setting max_sectors_kb to ~192 seems to inhibit this problem.

Thanks!

--
Al