linux-kernel - inconsistent high io wait when reading from disk

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Date:	Sun, 20 Jan 2008 15:21:43 +0100
From:	AL13N <alien999999999@...rs.sourceforge.net>
To:	linux-kernel@...r.kernel.org
Subject: inconsistent high io wait when reading from disk

(please CC me)

I am copying a large file +-1GB to another machine using scp, this was the
easiest way to reproduce.

I've done this a few dozen times, and there's few odd things about this:

sometimes i have read speeds of around 20MB/s, at that time, the CPU is
about 60% idle and max 10% io wait.

but mostly, i have read speeds around 5MB/s, and CPU is 0% idle and 70% io
waiting.

This is a little odd, but one could argue that there are lots of factors,
etc...

True, however, i should note that, allthough i ran the tests immediately
after each other; that if i had low IO wait less than 10%, it stayed that
way, until the file was completely copied. AND if i had IO wait higher
than 50%, it also stayed that way until the file was copied; immediately
after all copies, there is no IO wait. even when no other process is
writing for several seconds, this continues. same things with the speeds,
either i have a consistent 5MB/s read, or 20MB/s read; never in between or
fluctuating.

I've tried to eliminate lots of other possibilities, by doing cat
/path/to/file >/dev/null ; to just do the IO reading; usually this makes a
extremely high io waiting (around 90%), but the read speeds are the same,
either around 5 or around 20.

stats are observed through iostat and mpstat. (in slow or normal case:
iostat -x gives sda total usage 100%)

I would like to trace this io wait to what process and device (I'm fairly
sure it's the process i'm testing this with (eg: scp or cat)). but i don't
know such a utility to find it.

I've tried to google on this, but allthough there is lots of stuff with
high IO wait in it. no solutions seems to be present.

now, i can accept that sometimes, it's just slow. but the inconsistency is
too consistent to be normal.

can anyone give me some constructive info/help on this?

thanks in advance

-- 
AL13N is my name and head-biting is my game.
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