mjhjmtu

I read the man page of ionice(1), and it says:

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 scheduling class. The I/O priority is derived from the CPU nice level of the process (same as before kernel 2.6.26).

And the calculation of the I/O priority before kernel 2.6.26 is:

io_priority = (cpu_nice + 20) / 5

So I did the following experiments:

root@jacky:~# nice
0
root@jacky:~# sleep 5566 &
[1] 32527
root@jacky:~# ionice -p 32527
none: prio 4
root@jacky:~# ps hp 32527 -o nice
 0
root@jacky:~# nice -n -20 sleep 5566 &
[2] 32538
root@jacky:~# ionice -p 32538
none: prio 4
root@jacky:~# ps hp 32538 -o nice
-20

The first process (pid 32527) has the I/O priority of 4, and it's reasonable because (0 + 20) / 5 = 4. The second process (pid 32538) has CPU nice of -20, so it should have I/O priority of (-20 + 20) / 5 = 0. However, it has I/O priority of 4 too.

Can someone explain that for me? Thanks!

p.s. I am using Arch Linux, and my uname -r is 4.13.4-1-ARCH.