Messages in this thread | | | From | Al Boldi <> | Subject | Re: [PATCH] Direct I/O bio size regression | Date | Mon, 24 Apr 2006 23:59:13 +0300 |
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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
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