On Thu, Mar 06, 2008 at 11:36:49AM -0500, Bill Davidsen wrote: > 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. > Hi! Do you happen to know if it's possible to check current queue depth "in use"? Meaning how many commands are currently queued.. -- Pasi
