Hi,

I added basic io priority support to the time sliced cfq base. Right now
this is just proof of concept, the interface for setting/querying io
prio will change. There are 8 basic io priorities now, 0 being highest
prio and 7 the lowest. The scheduling type is best effort, in the future
there will be a realtime class as well (and hence the need to change
sys_ioprio_set etc). If a process hasn't set its io priority explicitly,
io priority is determined from the process nice level. CPU nice level of
0 yields io priority 4, cpu nice -20 gives you 0, and finally cpu nice
19 will give you an io priority of 7. Values in-between are
appropriately scaled. If a process sets its io priority explicitly, that
value is used from then on.

A test run with 7 readers are various priorities:

thread1 (read): err=0, prio=0 maxl=634msec, run=30012msec, bw=5884KiB/sec
thread2 (read): err=0, prio=1 maxl=650msec, run=30041msec, bw=5102KiB/sec
thread3 (read): err=0, prio=1 maxl=646msec, run=30057msec, bw=5062KiB/sec
thread4 (read): err=0, prio=3 maxl=687msec, run=30079msec, bw=3551KiB/sec
thread5 (read): err=0, prio=6 maxl=750msec, run=30208msec, bw=1253KiB/sec
thread6 (read): err=0, prio=3 maxl=690msec, run=30100msec, bw=3562KiB/sec
thread7 (read): err=0, prio=4 maxl=758msec, run=30181msec, bw=2631KiB/sec
Run status:
   READ: io=775MiB, aggrb=26927, minl=634, maxl=758, minb=1253, maxb=5884, mint=30012msec, maxt=30208msec

Note that aggregate bandwidth stays the same as without io priorities.
Only io scheduling cares about the io priority currently, request
allocation policy, queue congestion etc doesn't yet.

I have attached a sample ionice.c file, so that you can do:

# ionice -n3 some_process

which will run that process at io priority 3.

Other changes:

- Disable TCQ in the hardware/driver by default. Can be changed (as
  always) with the max_depth setting. If you do that, don't expect
  fairness or priorities to work as well.

- Import thinktime stats from AS. We use this to determine when to
  preempt a queue during its idle window.

- Kill find_best_crq setting. It was on by default before, and it would
  be a bug if it didn't work well.

- Add ability for a given process to preempt another process slice.

- Allow idle window to slide, if there are no other potential queues we
  could service requests from.

- Various little cleanups and optimizations.

2.6.10-rc2-mm4 patch:

http://www.kernel.org/pub/linux/kernel/people/axboe/patches/v2.6/2.6.10-rc2-mm4/cfq-time-slices-10-2.6.10-rc2-mm4.gz

-- 
Jens Axboe