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Date:   Thu, 1 Mar 2018 08:15:44 -0700
From:   Keith Busch <keith.busch@intel.com>
To:     "jianchao.wang" <jianchao.w.wang@oracle.com>
Cc:     Sagi Grimberg <sagi@grimberg.me>, Christoph Hellwig <hch@lst.de>,
        axboe@fb.com, linux-kernel@vger.kernel.org,
        linux-nvme@lists.infradead.org
Subject: Re: [PATCH V2] nvme-pci: assign separate irq vectors for adminq and
 ioq0
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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.