linux-kernel - Re: [PATCH] nvme: Acknowledge completion queue on each iteration

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Date:   Mon, 17 Jul 2017 18:56:15 -0400
From:   Keith Busch <keith.busch@...el.com>
To:     Sinan Kaya <okaya@...eaurora.org>
Cc:     linux-nvme@...ts.infradead.org, timur@...eaurora.org,
        linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nvme: Acknowledge completion queue on each iteration

On Mon, Jul 17, 2017 at 06:46:11PM -0400, Sinan Kaya wrote:
> Hi Keith,
> 
> On 7/17/2017 6:45 PM, Keith Busch wrote:
> > On Mon, Jul 17, 2017 at 06:36:23PM -0400, Sinan Kaya wrote:
> >> Code is moving the completion queue doorbell after processing all completed
> >> events and sending callbacks to the block layer on each iteration.
> >>
> >> This is causing a performance drop when a lot of jobs are queued towards
> >> the HW. Move the completion queue doorbell on each loop instead and allow new
> >> jobs to be queued by the HW.
> > 
> > That doesn't make sense. Aggregating doorbell writes should be much more
> > efficient for high depth workloads.
> > 
> 
> Problem is that code is throttling the HW as HW cannot queue more completions until
> SW get a chance to clear it. 
> 
> As an example:
> 
> for each in N
> (
> 	blk_layer()
> )
> ring door bell
> 
> HW cannot queue new job until N x blk_layer operations are processed and queue
> element ownership is passed to the HW after the loop. HW is just sitting idle
> there if no queue entries are available.

If no completion queue entries are available, then there can't possibly
be any submission queue entries for the HW to work on either.