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

[Olivier Matz <olivier.matz@6wind.com>]

Hi Keith,

On 03/21/2016 06:38 PM, Wiles, Keith wrote:
>> On Mar 21, 2016, at 11:10 AM, Olivier Matz <olivier.matz@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.

Hope this is clearer with the explanation.

Regards,
Olivier