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Setting I/O priorities on Linux

All us system admins know about nice, which lets you set the CPU priorities of a process. Every now and then I need to run an I/O-heavy process. This inevitably makes the system intermittently unresponsive or slow, which can be a real annoyance.

If your system is slow to respond, you can check to see if I/O is the problem (which it usually will be) using a program called iotop, which is similar to the normal top program except it doesn't show CPU/Memory but disk reads/writes. You may need to install it first:

# aptitude install iotop

The output looks like this:

Total DISK READ: 0.00 B/s | Total DISK WRITE: 0.00 B/s
  TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND                                       
12404 be/4 fboender  124.52 K/s  124.52 K/s  0.00 % 99.99 % cp winxp.dev.local.vdi /home/fboender
    1 be/4 root        0.00 B/s    0.00 B/s  0.00 %  0.00 % init
    2 be/4 root        0.00 B/s    0.00 B/s  0.00 %  0.00 % [kthreadd]
    3 be/4 root        0.00 B/s    0.00 B/s  0.00 %  0.00 % [ksoftirqd/0]

As you can see, the copy process with PID 12404 is taking up 99.99% of my I/O, leaving little for the rest of the system.

In recent Linux kernels (2.6.13 with the CFQ io scheduler), there's an option to renice the I/O of a process. The ionice tool allows you to renice the processes from userland. It comes pre-installed on Debian/Ubuntu machines in the util-linux package. To use it, you must specify a priority scheduling class using the -c option.

  • -c0 is an old deprecated value of "None", which is now the same as Best-Effort (-c2)
  • -c1 is Real Time priority, which will give the process the highest I/O priority
  • -c2X is Best-Effort priority puts the process in a round-robin queue where it will get a slice of I/O every so often. How much it gets can be specified using the -n option which takes a value from 0 to 7
  • -c3 is Idle, which means the process will only get I/O when no other process requires it.

For example, I want a certain process (PID 12404) to only use I/O when no other process requires it, because the task is I/O-heavy, but is not high priority:

# ionice -c3 -p 12404

The effects are noticeable immediately. My system responds faster, there is less jitter on the desktop and the commandline.

Nice.

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