netdev - Re: [RFC PATCH 0/2] rx zero copy interface for af

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Date:   Tue, 31 Jan 2017 11:30:42 -0500
From:   Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     bjorn.topel@...il.com, jasowang@...hat.com, ast@...com,
        alexander.duyck@...il.com, brouer@...hat.com,
        john.r.fastabend@...el.com, netdev@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] rx zero copy interface for af_packet

On (01/27/17 13:33), John Fastabend wrote:
> 
> This is an experimental implementation of rx zero copy for af_packet.
> Its a bit rough and likely has errors but the plan is to clean it up
> over the next few months.
> 
> And seeing I said I would post it in another thread a few days back
> here it is.

One question/comment about this: sure, this saves us an skb copy
on the rx side, but at least for the Tx side, I think there may
be a trade-off between the overhead from the skb setup and the
ease of offloading checksum (and UFO where it is available) to
consider, even for PF_PACKET.

Using PF_PACKET works well for stateless datagram protocols like 
UDP, and for UDP sockets, we find that just switching to Jumbo
(to simulate a poor-man's-UFO) gives us significant improvement
in both throughput and latency for our RDBMS workloads - and 
having the sk_buff facilitates using existing driver-kernel interfaces
for offload easily, so while we may gain some perf improvment by shaving
of the sk_buff overhead, the trade-off needs to be considered.

--Sowmini