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@xxxxxxxxxx> 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@xxxxxxxxxx> Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html