On Tue, Mar 04, 2008 at 11:37:31PM -0800, Nathan Grennan wrote:
> Why is the command below all that is needed to bring the system to
> it's knees? Why doesn't the io scheduler, CFQ, which is supposed to be
> all about fairness starve other processes? Example, if I open a new file
> in vim, and hold down "i" while this is running it will pause the
> display of new "i"s for seconds, sometimes until the dd write is
> completely finished. Another example is applications like firefox,
> thunderbird, xchat, and pidgin will stop refreshing for 10+ seconds.
>
> dd if=/dev/zero of=test-file bs=2M count=2048
>
> I understand the main difference between using oflag=direct or not
> relates to if the io scheduler is used, and if the file is cached or
> not. I can see this clearly by watching cached rise without
> oflag=direct, stay the same with it, and go way down when I delete the
> file after running dd without oflag=direct.
>
> The system in question is running Fedora 8. It is an E6600, 4gb
> memory, and 2x300gb Seagate sata drives. The drives are setup with md
> raid 1, and the filesystem is ext3. But I also see this with plenty of
> other systems with more cpu, less cpu, less memory, raid, and no raid.
>
What motherboard/chipset do you have? which sata chipset?
Are you using ncq?
Did you try limiting the memory to 2G or even 1G ?
Are you running 32bit or 64bit OS?
> I have tried various tweaks to sys.vm settings, tried changing the
> scheduler to as or deadline. Nothing seem to get it to behave, other
> than oflag=direct.
>
Did you also try noop?
> Using dd if=/dev/zero is just an easy test case. I see this when
> copying large files, creating large files, and using virtualization
> software that does heavy i/o on large files.
>
>
>
> The command below seems to result in cpu idle 0 and io wait 100%. As
> shown by "vmstat 1"
>
Maybe also try iostat.. maybe it shows you something more/important in this
case.
There are also some caching/flushing related vm parameters which might
affect these things..
-- Pasi
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