Subject: Re: Bypass at packet-page level (Was: Optimizing instruction-cache, more packets at each stage)

From: Jesper Dangaard Brouer <[email-masked]>
Date: Mon, 25 Jan 2016 23:10:16 +0100

On Mon, 25 Jan 2016 09:50:16 -0800 John Fastabend <[email-masked]> wrote:

> On 16-01-25 09:09 AM, Tom Herbert wrote:
> > On Mon, Jan 25, 2016 at 5:15 AM, Jesper Dangaard Brouer
> > <[email-masked]> wrote:  
> >>
[...]
> >>
> >> There are two ideas, getting mixed up here.  (1) bundling from the
> >> RX-ring, (2) allowing to pick up the "packet-page" directly.
> >>
> >> Bundling (1) is something that seems natural, and which help us
> >> amortize the cost between layers (and utilizes icache better). Lets
> >> keep that in another thread.
> >>
> >> This (2) direct forward of "packet-pages" is a fairly extreme idea,
> >> BUT it have the potential of being an new integration point for
> >> "selective" bypass-solutions and bringing RAW/af_packet (RX) up-to
> >> speed with bypass-solutions.
>
[...]
> 
> Jesper, at least for you (2) case what are we missing with the
> bifurcated/queue splitting work? Are you really after systems
> without SR-IOV support or are you trying to get this on the order
> of queues instead of VFs.

I'm not saying something is missing for bifurcated/queue splitting work.
I'm not trying to work-around SR-IOV.

This an extreme idea, which I got while looking at the lowest RX layer.

Before working any further on this idea/path, I need/want to evaluate
if it makes sense from a performance point of view.  I need to evaluate
if "pulling" out these "packet-pages" is fast enough to compete with
DPDK/netmap.  Else it makes no sense to work on this path.

As a first step to evaluate this lowest RX layer, I'm simply hacking
the drivers (ixgbe and mlx5) to drop/discard packets within-the-driver.
For now, simply replacing napi_gro_receive() with dev_kfree_skb(), and
measuring the "RX-drop" performance.

Next step was to avoid the skb alloc+free calls, but doing so is more
complicated that I first anticipated, as the SKB is tied in fairly
heavily.  Thus, right now I'm instead hooking in my bulk alloc+free
API, as that will remove/mitigate most of the overhead of the
kmem_cache/slab-allocators.

-- 
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