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

[Jason Wang]

On 2016å05æ18æ 17:55, Michael S. Tsirkin wrote:
> On Wed, May 18, 2016 at 11:21:29AM +0200, Jesper Dangaard Brouer wrote:
> > On Wed, 18 May 2016 11:21:59 +0300
> > "Michael S. Tsirkin" <mst@xxxxxxxxxx> 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@xxxxxxxxxx> 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.
> This is all assuming you can somehow batch operations.
> We can do this for transmit sometimes (when linux
> is the source of the packets) but not always.
>
> > > > > 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.
> It almost doubles it.
> E.g. queue size of 10000 (rather common) will become 16K, wasting 6K.

It depends on the user, e.g default tx_queue_len is around 1000 for real 
cards. If we really care about the wasting, we can add a threshold and 
fall back to normal linked list during resizing.

> > 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.
> Depends on the usage I guess.
> Entries pointed to are much bigger, and you are
> going to access them - is this really an issue?
> If yes this shouldn't be that hard to fix ...
>
> > --
> > 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