From mboxrd@z Thu Jan  1 00:00:00 1970
From: okaya@codeaurora.org (Sinan Kaya)
Date: Mon, 17 Jul 2017 18:46:11 -0400
Subject: [PATCH] nvme: Acknowledge completion queue on each iteration
In-Reply-To: <20170717224551.GA1496@localhost.localdomain>
References: <1500330983-27501-1-git-send-email-okaya@codeaurora.org>
 <20170717224551.GA1496@localhost.localdomain>
Message-ID: <6d10032c-35ec-978c-6b8f-1ab9c07adf7f@codeaurora.org>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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.