[dpdk-dev] [PATCH] mlx4: use dummy rxqs when a non-pow2 number is requested

[Wiles, Keith]

Hi Olivier,

>Hi Keith,
>
>On 03/21/2016 06:38 PM, Wiles, Keith wrote:
>>> On Mar 21, 2016, at 11:10 AM, Olivier Matz <olivier.matz at 6wind.com> wrote:
>>>
>>> When using RSS, the number of rxqs has to be a power of two.
>>> This is a problem because there is no API is dpdk that makes
>>> the application aware of that.
>>>
>>> A good compromise is to allow the application to request a
>>> number of rxqs that is not a power of 2, but having inactive
>>> queues that will never receive packets. In this configuration,
>>> a warning will be issued to users to let them know that
>>> this is not an optimal configuration.
>> 
>> Not sure I like this solution. I think an error should be returned with a log message instead. What if the next driver needs power of three or must be odd or even number. 
>> 
>> The bigger problem is the application is no longer portable for any given nic configuration.
>> 
>> We need a method for the application to query the system for these types of information. But as we do not have that API we need to just error the request off.
>
>
>The initial problem is that the driver says "I support a maximum
>of X queues" and if the application configures a lower number, it
>gets an error.
>
>There is no API in DPDK to tell that only specific number of queues
>are supported. Adding an API is a solution, but in this case it's
>probably overkill. With this patch, the driver can present the proper
>number of queues to the application, knowing that the spreading of
>the packets won't be ideal (some queues won't receive packets), but
>it will work.
>
>A step further in this direction would be to configure more queues
>than asked in hardware to do a better spreading, almost similar to
>what is done with RETA tables in mlx5. But this is more complicated
>to do, especially if we want it for 16.04.

Well I guess I must agree with the solution, but I am not real happy. Can we mark this a temp fix until we figure out a cleaner solution as I would not want this type of solution forever or be the standard way to handle these problems.

>
>Hope this is clearer with the explanation.
>
>Regards,
>Olivier
>
>


Regards,
Keith