Messages in this thread | | | From | Al Boldi <> | Subject | Re: [PATCH] Direct I/O bio size regression | Date | Tue, 25 Apr 2006 13:45:01 +0300 |
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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
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