Subject: Re: [PATCH net-next] tuntap: introduce tx skb ring

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 18 May 2016 11:21:29 +0200

On Wed, 18 May 2016 11:21:59 +0300
"Michael S. Tsirkin" <[email-masked]> wrote:

> On Wed, May 18, 2016 at 10:16:31AM +0200, Jesper Dangaard Brouer wrote:
> > 
> > On Tue, 17 May 2016 09:38:37 +0800 Jason Wang <[email-masked]> wrote:
> >   
> > > >> And if tx_queue_length is not power of 2,
> > > >> we probably need modulus to calculate the capacity.    
> > > > Is that really that important for speed?    
> > > 
> > > Not sure, I can test.  
> > 
> > In my experience, yes, adding a modulus does affect performance.  
> 
> How about simple
> 	if (unlikely(++idx > size))
> 		idx = 0;

So, you are exchanging an AND-operation with a mask, for a
branch-operation.  If the branch predictor is good enough in the CPU
and code-"size" use-case, then I could be just as fast.

I've actually played with a lot of different approaches:
 https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/include/linux/alf_queue_helpers.h

I cannot remember the exact results. I do remember micro benchmarking
showed good results with the advanced "unroll" approach, but IPv4
forwarding, where I know I-cache is getting evicted, showed best
results with the more simpler implementations.

> > > 
> > > Right, this sounds a good solution.  
> > 
> > Good idea.  
> 
> I'm not that sure - it's clearly wasting memory.

Rounding up to power of two.  In this case I don't think the memory
wast is too high.  As we are talking about max 16 bytes elements.

I am concerned about memory in another way. We need to keep these
arrays/rings small, due to data cache usage.  A 4096 ring queue is bad
because e.g. 16*4096=65536 bytes, and typical L1 cache is 32K-64K. As
this is a circular buffer, we walk over this memory all the time, thus
evicting the L1 cache.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer