[RFC/RFT] callout(9): major changes

[Davide Italiano]

Howdy.
As part of the Google Summer of Code project, in the last weeks, I
worked on some changes in the callout(9) subsystem, under the
mentorship of Alexander Motin (mav@).
The projects aims to adapt the callout() backend in order to exploit
better precision allowed by the recently introducted eventtimers(4/9)
subsystem.
A patch may be found here: http://people.freebsd.org/~davide/callout_patch.diff
If you want to try  the code may be found in FreeBSD svn projects/
repository: http://svnweb.freebsd.org/base/projects/calloutng/
At the time of writing, the code cannot be considered ready for hit
the tree, some work is still missing, but some goals have been
reached. In particular:

- The callout(9) backend  was completely switched from a tick-based
approach to a tickless one.
- The code has been integrated with the eventtimers
- An experimental new KPI has been introduced so that timeouts for
callout_* may be specified also in terms of bintime other than ticks,
as previously allowed
- Support for execution of callouts from hw interrupt context has been
introduced
- In order to prove effectiveness of the approach, some consumers (in
particular sleep/pool/select) have been ported to the new KPI, and the
changes have been microbenchmarked
- Some experiments of event aggregation have been done, as well as the
definition of  new KPI in which consumers may specify granularity at
which events may be aggregated.

For my benchmarks I used the same program luigi@ has recently used,
even though results are a bit different
(http://lists.freebsd.org/pipermail/freebsd-arch/2012-February/012413.html).
Just a bit of results:
Sequential usleep():
This graph shows on the x-axes the timeout se and on y-axes the real
sleep time (in the range [1;1000]). Timeout is increased sequentially
by one unit every iteration. Green line represents the "ideal" case,
Red line represents results after the changes made.
http://people.freebsd.org/~davide/sequential_new.png
Random sleep():
same as before, but in this case I plotted the delta among the
expected sleep time and the actual sleep time.
http://people.freebsd.org/~davide/delta_random.png
Random select():
http://people.freebsd.org/~davide/delta_random.png

I'd like to have some feedback/comments on the implementative choices
I've done, as well as on the new defined KPI or future directions.
If you've any question, feel free to ask.

Davide