Arch Linux

hcac

Member

Registered: 2018-01-24

Posts: 7

ionice none: prio 4?!

First I run `ionice` as a non-root user, it says 'none: prio 0'.
But then I `sudo -i` and run the command as root and it results in 'none: prio 4'!
After that, running `ionice` from a non-root account also results in 'none: prio 4'!
I tested this on my Debian machine, it acted normally, never saw prio 4.
According to the manuals, none doesn't even have prio 4.
How is this happening? Is it a bug?

loqs

Member

Registered: 2014-03-06

Posts: 19,104

Re: ionice none: prio 4?!

Best-effort
              This is the effective scheduling class for any process that  has
              not  asked for a specific I/O priority.  This class takes a pri‐
              ority argument from 0-7, with a lower number being higher prior‐
              ity.   Programs  running  at  the  same best-effort priority are
              served in a round-robin fashion.

              Note that before kernel 2.6.26 a process that has not asked  for
              an  I/O  priority  formally uses "none" as scheduling class, but
              the I/O scheduler will treat such processes as if it were in the
              best-effort  class.   The  priority within the best-effort class
              will be dynamically derived from  the  CPU  nice  level  of  the
              process: io_priority = (cpu_nice + 20) / 5.

              For  kernels  after 2.6.26 with the CFQ I/O scheduler, a process
              that has not asked for an I/O priority inherits its CPU schedul‐
              ing  class.  The I/O priority is derived from the CPU nice level
              of the process (same as before kernel 2.6.26).

(0+20)/5=4.  I can not explain the difference from Debian or why the output changes from 0 to 4 after being executed as root.

hcac

Member

Registered: 2018-01-24

Posts: 7

Re: ionice none: prio 4?!

Just realized, the Debian difference was an error on my part. It acted the same as Arch Linux.
I see why 4 is generated, thanks.
It's still bemusing why running ionice on a machine as root would change it's value... .