IO scheduler benchmarking

[Andrew Morton <akpm@digeo.com>]

Following this email are the results of a number of tests of various I/O
schedulers:

- Anticipatory Scheduler (AS) (from 2.5.61-mm1 approx)

- CFQ (as in 2.5.61-mm1)

- 2.5.61+hacks (Basically 2.5.61 plus everything before the anticipatory
  scheduler - tweaks which fix the writes-starve-reads problem via a
  scheduling storm)

- 2.4.21-pre4

All these tests are simple things from the command line.

I stayed away from the standard benchmarks because they do not really touch
on areas where the Linux I/O scheduler has traditionally been bad.  (If they
did, perhaps it wouldn't have been so bad..)

Plus all the I/O schedulers perform similarly with the usual benchmarks. 
With the exception of some tiobench phases, where AS does very well.

Executive summary: the anticipatory scheduler is wiping the others off the
map, and 2.4 is a disaster.

I really have not sought to make the AS look good - I mainly concentrated on
things which we have traditonally been bad at.  If anyone wants to suggest
other tests, please let me know.

The known regressions from the anticipatory scheduler are:

1) 15% (ish) slowdown in David Mansfield's database run.  This appeared to
   go away in later versions of the scheduler.

2) 5% dropoff in single-threaded qsbench swapstorms

3) 30% dropoff in write bandwidth when there is a streaming read (this is
   actually good).

The test machine is a fast P4-HT with 256MB of memory.  Testing was against a
single fast IDE disk, using ext2.