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

Jesper Dangaard Brouer
2016-05-18T09:21:37Z

On Wed, 18 May 2016 11:21:59 +0300
"Michael S. Tsirkin" <mst@redhat.com> 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 <jasowang@redhat.com> 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