Re: [PATCH] io stalls

Andrea Arcangeli
2003-06-12T03:19:19Z

On Thu, Jun 12, 2003 at 01:20:44PM +1000, Nick Piggin wrote:
> Its no less fair this way, tasks will still be woken in fifo
> order. They will just be given the chance to submit a batch
> of requests.

If you change the behaviour with queued_task_nr > batch_requests it is
less fair period. Whatever else thing I don't care about right now
because it is a minor cpu improvement anyways.

I'm not talking about performance, I'm talking about latency and
fariness only. This is the whole point of the ->full logic.

> I think the cpu utilization gain of waking a number of tasks
> at once would be outweighed by advantage of waking 1 task
> and not putting it to sleep again for a number of requests.
> You obviously are not claiming concurrency improvements, as
> your method would also increase contention on the io lock
> (or the queue lock in 2.5).

I'm claiming that with queued_task_nr > batch_requests the
batch_requests logic still has a chance to save some cpu, this is the
only reason I didn't nuke it completely as you suggested some email ago.

> Then you have the cache gains of running each task for a
> longer period of time. You also get possible IO scheduling
> improvements.
>
> Consider 8 requests, batch_requests at 4, 10 tasks writing
> to different areas of disk.
>
> Your method still only allows each task to have 1 request in
> the elevator at once. Mine allows each to have a run of 4
> requests in the elevator.

I definitely want 1 request in the elevator at once or we can as well
drop your ->full and return to be unfair. The whole point of ->full is
to get the total fariness, across the tasks in the queue queue, and for
tasks outside the queue calling get_request too. Since not all tasks
will fit in the I/O queue, providing a very fair FIFO in the
wait_for_request is fundamental to provide any sort of latency
guarantee IMHO (the fact an _exclusive wakeup removal that mixes stuff
and probably has the side effect of being more fair, made that much
difference to mainline users kind of confirms that).

Andrea