RFC: Allow block drivers to poll for I/O instead of sleeping

[Ingo Molnar]

* Jens Axboe <axboe at kernel.dk> wrote:

> - With the former note, the app either needs to opt in (and hence
>   willingly sacrifice CPU cycles of its scheduling slice) or it needs to 
>   be nicer in when it gives up and goes back to irq driven IO.

The scheduler could look at sleep latency averages of the task in question 
- we measure that already in most cases.

If the 'average sleep latency' is below a certain threshold, the 
scheduler, if it sees that the CPU is about to go idle, could delay doing 
the context switch and do "light idle-polling", for say twice the length 
of the expected sleep latency - assuming the CPU is otherwise idle - 
before it really schedules away the task and the CPU goes idle.

This would still require an IRQ and a wakeup to be taken, but would avoid 
the context switch.

Yet I have an ungood feeling about depending on actual latency values so 
explicitly. There will have to be a cutoff value, and if a workload is 
just below or just above that threshold then behavior will change 
markedly. Such schemes rarely worked out nicely in the past. [Might still 
be worth trying it.]

Couldn't the block device driver itself estimate the expected latency of 
IO completion and simply poll if that's expected to be very short [such as 
there's only a single outstanding IO to a RAM backed device]? IO drivers 
doing some polling and waiting in the microseconds range isnt overly 
controversial. I'd even do that if the CPU is busy otherwise: the task 
should see a proportional slowdown as load increases, with no change in IO 
queueing behavior.

Thanks,

	Ingo