Takes no arguments. Shows a breakdown of processes by their Linux IO scheduling class and priority level within that class.
Sample usage:
$ ionice -c3 ./iopriostat
none: total 95
idle: total 1
best-effort: total 2 by-level 2 0 0 0 0 2 0 0
realtime: total 0 by-level 0 0 0 0 0 0 0 0
ioprio.h shamelessly copied from Jens Axboe's ionice.c (util-linux-ng)