netdev - Re: Initial thoughts on TXDP

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Date:   Thu, 1 Dec 2016 15:13:24 -0500
From:   Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:     Tom Herbert <tom@...bertland.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Initial thoughts on TXDP

On (12/01/16 11:05), Tom Herbert wrote:
> 
> Polling does not necessarily imply that networking monopolizes the CPU
> except when the CPU is otherwise idle. Presumably the application
> drives the polling when it is ready to receive work.

I'm not grokking that- "if the cpu is idle, we want to busy-poll
and make it 0% idle"?  Keeping CPU 0% idle has all sorts
of issues, see slide 20 of
 http://www.slideshare.net/shemminger/dpdk-performance

> > and one other critical difference from the hot-potato-forwarding
> > model (the sort of OVS model that DPDK etc might aruguably be a fit for)
> > does not apply: in order to figure out the ethernet and IP headers
> > in the response correctly at all times (in the face of things like VRRP,
> > gw changes, gw's mac addr changes etc) the application should really
> > be listening on NETLINK sockets for modifications to the networking
> > state - again points to needing a select() socket set where you can
> > have both the I/O fds and the netlink socket,
> >
> I would think that that is management would not be implemented in a
> fast path processing thread for an application.

sure, but my point was that *XDP and other stack-bypass methods needs 
to provide a select()able socket: when your use-case is not about just
networking, you have to snoop on changes to the control plane, and update
your data path. In the OVS case (pure networking) the OVS control plane
updates are intrinsic to OVS. For the rest of the request/response world,
we need a select()able socket set to do this elegantly (not really
possible in DPDK, for example)

> The *SOs are always an interesting question. They make for great
> benchmarks, but in real life the amount of benefit is somewhat
> unclear. Under the wrong conditions, like all cwnds have collapsed or

I think Rick's already bringing up this one.

--Sowmini