Re: I/O Scheduling results in poor responsiveness

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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

--
fedora-list mailing list
fedora-list@xxxxxxxxxx
To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list
[Index of Archives]     [Older Fedora Users]     [Fedora Announce]     [Fedora Package Announce]     [EPEL Announce]     [Fedora Magazine]     [Fedora News]     [Fedora Summer Coding]     [Fedora Laptop]     [Fedora Cloud]     [Fedora Advisory Board]     [Fedora Education]     [Fedora Security]     [Fedora Scitech]     [Fedora Robotics]     [Fedora Maintainers]     [Fedora Infrastructure]     [Fedora Websites]     [Anaconda Devel]     [Fedora Devel Java]     [Fedora Legacy]     [Fedora Desktop]     [Fedora Fonts]     [ATA RAID]     [Fedora Marketing]     [Fedora Management Tools]     [Fedora Mentors]     [SSH]     [Fedora Package Review]     [Fedora R Devel]     [Fedora PHP Devel]     [Kickstart]     [Fedora Music]     [Fedora Packaging]     [Centos]     [Fedora SELinux]     [Fedora Legal]     [Fedora Kernel]     [Fedora OCaml]     [Coolkey]     [Virtualization Tools]     [ET Management Tools]     [Yum Users]     [Tux]     [Yosemite News]     [Gnome Users]     [KDE Users]     [Fedora Art]     [Fedora Docs]     [Asterisk PBX]     [Fedora Sparc]     [Fedora Universal Network Connector]     [Libvirt Users]     [Fedora ARM]

  Powered by Linux