netdev - Re: [PATCH net-next] fq_codel: Fair Queue Codel AQM

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Date:	Fri, 11 May 2012 17:23:30 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Dave Taht <dave.taht@...ferbloat.net>,
	Kathleen Nichols <nichols@...lere.com>,
	Van Jacobson <van@...lere.net>,
	Tom Herbert <therbert@...gle.com>,
	Matt Mathis <mattmathis@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Stephen Hemminger <shemminger@...tta.com>
Subject: Re: [PATCH net-next] fq_codel: Fair Queue Codel AQM

On Fri, 2012-05-11 at 23:03 +0800, Changli Gao wrote:
> On Fri, May 11, 2012 at 9:59 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > Fair Queue Codel implementation.
> >
> > Principles :
> >
> > - Packets are classified (internal classifier or external) on flows.
> > - This is a Stochastic model (as we use a hash, several flows might
> >                              be hashed on same slot)
> > - Each flow has a CoDel managed queue.
> > - Flows are linked onto two (Round Robin) lists,
> >  so that new flows have priority on old ones.
> 
> I don't think it is a good idea, as the old ones may be starved. It isn't
> fair. Why not use the conventional DRR?
> 

Hey, its DRR, but with 64 bytes per flow instead of more than 256.
One cache line per flow, that was my goal, sharing the codel_params and
stats for all flows.

A 'struct fq_codel_flow' can be in three states :

- Detached state
- In new flow list
- In old flow list

And its the dequeue() that can put a flow in detached state, only if
coming from old flow list.

Its possible I missed something, because in my first coding I had 3
lists.

Anyway I'll send a V2 because I left .change method to NULL, while the
intent was to permit a change on fq_codel.

> > +
> > +       /* Queue is full! Find the fat flow and drop packet from it.
> > +        * This might sound expensive, but with 1024 flows, we scan
> > +        * 4KB of memory, and we dont need to handle a complex tree
> > +        * in fast path (packet queue/enqueue) with many cache misses.
> > +        */
> 
> How about the tricks used by SFQ?

They are too expensive in term of cache misses and limits.
Code is complex and difficult to maintain.
That was a nice compromise 20 years ago when memory was expensive.
Now, memory is cheap but still slow.

Also adding the 'priority to new flows' is too difficult with SFQ.




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