[PATCH] nvme: Acknowledge completion queue on each iteration

[okaya@codeaurora.org (Sinan Kaya)]

On 7/18/2017 10:36 AM, Keith Busch wrote:
> On Mon, Jul 17, 2017 at 07:07:00PM -0400, okaya at codeaurora.org wrote:
>> Maybe, I need to understand the design better. I was curious why completion
>> and submission queues were protected by a single lock causing lock
>> contention.
> Ideally the queues are tied to CPUs, so you couldn't have one thread
> submitting to a particular queue-pair while another thread is reaping
> completions from it. Such a setup wouldn't get lock contention.

I do see that the NVMe driver is creating a completion interrupt on
each CPU core for the completions. No problems with that. 

However, I don't think you can guarantee that there will always be a single
CPU core targeting one submission queue especially with asynchronous IO.

Lock contention counters from CONFIG_LOCK_STAT are pointing to nvmeq->lock
in my FIO tests.

Did I miss something?

> 
> Some machines have so many CPUs, though, that sharing hardware queues
> is required. We've experimented with separate submission and completion
> locks for such cases, but I've never seen an improved performance as a
> result.
> 

I have also experimented with multiple locks with no significant gains. 
However, I was curious if somebody else had a better implementation than mine.

-- 
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.