On Thu, 2017-01-12 at 10:41 +0200, Sagi Grimberg wrote: > First, when the nvme device fires an interrupt, the driver consumes > the completion(s) from the interrupt (usually there will be some more > completions waiting in the cq by the time the host start processing it). > With irq-poll, we disable further interrupts and schedule soft-irq for > processing, which if at all, improve the completions per interrupt > utilization (because it takes slightly longer before processing the cq). > > Moreover, irq-poll is budgeting the completion queue processing which is > important for a couple of reasons. > > 1. it prevents hard-irq context abuse like we do today. if other cpu > cores are pounding with more submissions on the same queue, we might > get into a hard-lockup (which I've seen happening). > > 2. irq-poll maintains fairness between devices by correctly budgeting > the processing of different completions queues that share the same > affinity. This can become crucial when working with multiple nvme > devices, each has multiple io queues that share the same IRQ > assignment. > > 3. It reduces (or at least should reduce) the overall number of > interrupts in the system because we only enable interrupts again > when the completion queue is completely processed. > > So overall, I think it's very useful for nvme and other modern HBAs, > but unfortunately, other than solving (1), I wasn't able to see > performance improvement but rather a slight regression, but I can't > explain where its coming from...
Hello Sagi, Thank you for the additional clarification. Although I am not sure whether irq-poll is the ideal solution for the problems that has been described above, I agree that it would help to discuss this topic further during LSF/MM. Bart.-- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
