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

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Date:   Fri, 2 Mar 2018 11:11:22 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Keith Busch <keith.busch@...el.com>
Cc:     Sagi Grimberg <sagi@...mberg.me>, Christoph Hellwig <hch@....de>,
        axboe@...com, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org
Subject: Re: [PATCH V2] nvme-pci: assign separate irq vectors for adminq and
 ioq0

Hi Keith

Thanks for your kindly directive and precious time for this.

On 03/01/2018 11:15 PM, Keith Busch wrote:
> 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.

Yes, indeed, sorry for my bad description.

>> 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.
> 

Yes, indeed

Sincerely
Jianchao