Re: [PATCH V2] nvme-pci: assign separate irq vectors for adminq and ioq0

[Keith Busch]

On Thu, Mar 01, 2018 at 06:05:53PM +0800, jianchao.wang wrote:
> When the adminq is free, ioq0 irq completion path has to invoke nvme_irq twice, one for itself,
> one for adminq completion irq action.

Let's be a little more careful on the terminology when referring to spec
defined features: there is no such thing as "ioq0". The IO queues start
at 1. The admin queue is the '0' index queue.

> We are trying to save every cpu cycle across the nvme host path, why we waste nvme_irq cycles here.
> If we have enough vectors, we could allocate another irq vector for adminq to avoid this.

Please understand the _overwhelming_ majority of time spent for IRQ
handling is the context switches. There's a reason you're not able to
measure a perf difference between IOQ1 and IOQ2: the number of CPU cycles
to chain a second action is negligible.