4

After running
ionice -c2 -n7 <PID> on an I/O-intensive process (VirtualBox VM) and checking the result with ionice -p <PID> it shows best-effort: prio 7 which is expected.

But when monitoring the overall disk I/O of all processes via iotop it shows in PRIO column a value of be/4 for that process, which I assume means best-effort and level (priority) 4, which is average priority and not expected.

Can somebody comment why there's a difference between what ionice and what iotop displays? And is there another way to verify the actual I/O priority of a given process other then with ionice -p ?

From what I understood, there's /proc/<pid>/io but it only displays bytes/characters read and written, but not the priority, cf. Know which process does I/O without iotop
Also How do I view the IO priority of a process? only lists ionice -p

For completeness sake, this is on RHEL 6.7 with iotop 0.3.2, and the scheduler is the default CFQ.

Community
  • 1
doktor5000
  • 2,689
  • 15
  • 30

1 Answers1

2

Seems I already found part of the answer. By default iotop shows all processes and threads, and I usually run it via iotop -o which only shows threads and processes doing I/O. The VirtualBox process shown as be/4 is actually a thread, and when I run iotop -p <pid> against the PID which I changed via ionice then it correctly displays be/7 which is expected.

Would still be interesting to know why one thread of that process has a different I/O class/priority, but my ionice does not have -P option to change the I/O class/priority of a process group. Would be nice if someone knows a workaround/solution for that.

doktor5000
  • 2,689
  • 15
  • 30
  • 1
    *"ionice does not have option to change process group. Would be nice if someone knows a workaround/solution for that."* **Use `ionice -p id1 id2 id3 ...`** – Codebling Mar 03 '20 at 23:25
  • It was for Chrome in my case, I had to collect the PIDs using `ps` – Codebling Mar 03 '20 at 23:31
  • I still wasn't able to change the io prority for threads, only for processes – Codebling Mar 04 '20 at 00:15
  • @Codebling FWIW, the essential part in your quote was "my ionice" ... seems this was added back in 2014 according to util-linux changelog, hence -P probably wasn't available in RHEL 6.x - just adding for completeness sake – doktor5000 Mar 05 '20 at 17:19
  • I saw and grokked that. I was suggesting an alternative, the use of lowercase `-p` with a list of PIDs grepped from `ps` instead of uppercase `-P` with a PGID. Even with my `ionice` which supports `-P`, PGID is not readily available information so it seems easier to always use a list of PIDs. Just through I'd mention it for posterity. – Codebling Mar 05 '20 at 18:51
  • Was also wondering if you had found any addditional info on changing the ionice of threads, whose IO priority doesn't seem to change with the process'. – Codebling Mar 05 '20 at 18:53