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.
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.
Known problem with the io schedulers, and discussed from time to time on
the RAID list. The current io schedulers don't split drive access fairly
between read and write, so when you get a huge batch of write queued
reads suffer. In your case, the vi problem may be an issue of doing a
write to the file and that write being at the end of the io queue.
Note: the optimization is for throughput, not responsiveness, you may
see more pleasing results with the deadline scheduler. You also may want
to look at using NCQ and setting the queue_depth in /sys. I can't
explain it without looking up the details, so there's something for you
to check.
--
Bill Davidsen <davidsen@xxxxxxx>
"We have more to fear from the bungling of the incompetent than from
the machinations of the wicked." - from Slashdot
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