[PATCH 0/5] cfq-iosched: improve latency for no-idle queues (v3)

[Corrado Zoccolo <czoccolo@gmail.com>]

[rebased on top of Jeff's latest changes for 2.6.33. Various code style improvements over v1 & v2]

This patch series is intended to improve I/O latency, addressing an often
neglected, important subset of workloads: the ones for which cfq currently
prefers not to do any idling.

Those are the ones that would benefit most from having low latency, in fact
they are any of:
* processes with large think times (e.g. interactive ones like file managers)
* seeky (e.g. programs faulting in their code at startup)
* or marked as no-idle from upper levels.

The patch series addresses this by:
* reducing queues' timeslice when many queues have pending I/O
* separating queues with different priorities and different characteristics in
different service trees, each with an allocated time slice
* enable idling when switching between service trees, even for queues that
would not have idling enabled otherwise.

This provides various benefits:
* service tree insertion code is simplified, since it doesn't need to cope with
priorities any more.
* high priority no_idle queues are no longer penalized when competing with
lower priority, idling queues
* seeky and no_idle queues have their fair share of disk time, without
penalizing NCQ drives' performances, since they can all dispatch together,
filling up the available NCQ slots.

On a non-NCQ capable drive, a workload of 4 random readers competing with
sequential writer, the maximum latency experienced by readers decreased from >
500ms to about 160ms.