Re: [PATCH 2/2] block: loop: avoiding too many pending per work I/O

[Tejun Heo <tj@kernel.org>]

Hello, Ming.

On Tue, May 05, 2015 at 10:46:10PM +0800, Ming Lei wrote:
> On Tue, May 5, 2015 at 9:59 PM, Tejun Heo <tj@kernel.org> wrote:
> > It's a bit weird to hard code this to 16 as this effectively becomes a
> > hidden bottleneck for concurrency.  For cases where 16 isn't a good
> > value, hunting down what's going on can be painful as it's not visible
> > anywhere.  I still think the right knob to control concurrency is
> > nr_requests for the loop device.  You said that for linear IOs, it's
> > better to have higher nr_requests than concurrency but can you
> > elaborate why?
> 
> I mean, in case of sequential IO, the IO may hit page cache a bit easier,
> so handling the IO may be quite quick, then it is often more efficient to
> handle them in one same context(such as, handle one by one from IO
> queue) than from different contexts(scheduled from different worker
> threads). And that can be made by setting a bigger nr_requests(queue_depth).

Ah, so, it's about the queueing latency.  Blocking the issuer from
get_request side for the same level of concurrency would incur a lot
longer latency before the next IO can be dispatched.  The arbitrary 16
is still bothering but for now it's fine I guess, but we need to
revisit the whole thing including WQ_HIGHPRI thing.  Maybe it made
sense when we had only one thread servicing all IOs but w/ high
concurrency I don't think it's a good idea.

Please feel free to add

 Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun