netdev - Re: Modification to skb->queue_mapping affecting performance

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Date:   Tue, 13 Sep 2016 22:22:51 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Michael Ma <make0818@...il.com>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: Modification to skb->queue_mapping affecting performance

On Tue, 2016-09-13 at 22:13 -0700, Michael Ma wrote:

> I don't intend to install multiple qdisc - the only reason that I'm
> doing this now is to leverage MQ to workaround the lock contention,
> and based on the profile this all worked. However to simplify the way
> to setup HTB I wanted to use TXQ to partition HTB classes so that a
> HTB class only belongs to one TXQ, which also requires mapping skb to
> TXQ using some rules (here I'm using priority but I assume it's
> straightforward to use other information such as classid). And the
> problem I found here is that when using priority to infer the TXQ so
> that queue_mapping is changed, bandwidth is affected significantly -
> the only thing I can guess is that due to queue switch, there are more
> cache misses assuming processor cores have a static mapping to all the
> queues. Any suggestion on what to do next for the investigation?
> 
> I would also guess that this should be a common problem if anyone
> wants to use MQ+IFB to workaround the qdisc lock contention on the
> receiver side and classful qdisc is used on IFB, but haven't really
> found a similar thread here...

But why are you changing the queue ?

NIC already does the proper RSS thing, meaning all packets of one flow
should land on one RX queue. No need to ' classify yourself and risk
lock contention' 

I use IFB + MQ + netem every day, and it scales to 10 Mpps with no
problem.

Do you really need to rate limit flows ? Not clear what are your goals,
why for example you use HTB to begin with.