[dpdk-dev] 4 Traffic classes per Pipe limitation

[Dumitrescu, Cristian]

Hi Avinash,

> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Yeddula, Avinash
> Sent: Friday, June 5, 2015 6:06 PM
> To: dev at dpdk.org
> Subject: [dpdk-dev] 4 Traffic classes per Pipe limitation
> 
> Hi,
> This is related to the QOS scheduler functionality provided by dpdk.
> 
> I see a limit on the number of traffic classes to be 4.  I'm exploring the
> available options to increase that limit to 8.

Yes, there are 4x traffic classes (scheduled in strict priority), but each traffic class has 4x queues (scheduled using WFQ); for big weight ratios between queues (e.g. 1:4 or 1:8, etc), WFQ becomes very similar to strict priority, a king of strict priority without starvation. So the 16x queues per pipe can be considered 16x sub-traffic-classes.

You might want to watch this video on DPDK QoS: https://youtu.be/_PPklkWGugs 

> 
> This is what I found when I researched on this topic.
> The limitation on number's of TC (and pipes) comes from the number of
> bits available. Since the QoS code overloads the 32 bit RSS field in
> the mbuf there isn't enough bits to a lot. But then again if you add lots
> of pipes or subports the memory footprint gets huge.

It is not that simple. The number of 4x traffic classes in deeply built into the implementation for performance reasons. Increasing the number of bits allocated to traffic class in mbuf->sched would not help.

> 
> Any more info or suggestions on increasing the limit to 8 ?

Yes, look at the 16x pipe queues as 16x (sub)traffic classes.
> 
> Thanks
> -Avinash