[ANNOUNCE] bfq-v3r1 for .38, .39, 3.0 and 3.1

[Paolo Valente]

Hi,
once again I am not announcing just a port to 3.1, but a new version of bfq.
More precisely, it is a revision of v3, available for 2.6.38, 2.6.39,
23.0 and 3.1. This revision contains, basically, an important bug fix
(see below the list of new features).
Curiously, this bug interacted with the block layer in some obscure way
that improved latencies :)
So, after removing the bug, latencies got worse. We had to investigate
the problem, and fortunately it seems we have solved it by adding the
additional mechanism described below. This mechanism did not yield just
the same latencies as before: actually, it allows bfq to achieve even
lower latencies. Unfortunately, the latency of interactive applications
is still, occasionally, unusually high (it happens, for example, about
one time out of ten command-startup attempts). But, leaving out these
outliers, the minimum, average and maximum latency is now much improved
(from two to ten times lower than before).

This is the full list of the fixes and new features:
- Fixed an important request-dispatch bug causing occasional IO hangs
- Added a new mechanism to reduce the latency of low-bw async queues.
This should reduce the latency of also the sync queues synchronized with
the above async queues.
- Fixed a minor bug in iocontext locking (port of commits 9b50902 and
3181faa
from CFQ).

As usual, Arianna has prepared patches for every possible transition.

I hope bfq will continue to serve you well,
Paolo