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
