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

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Message-ID: <6d10032c-35ec-978c-6b8f-1ab9c07adf7f@codeaurora.org>
Date:   Mon, 17 Jul 2017 18:46:11 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Keith Busch <keith.busch@...el.com>
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

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.

Sinan

-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.