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

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Date:   Tue, 31 Jan 2017 20:23:51 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.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 17-01-31 08:30 AM, Sowmini Varadhan wrote:
> 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.
> 

Yes although as Willem suggested and I pushed a quick comment
at the end of the patch, virtio descriptors might be a better
options for a v4 descriptor type because they have mechanisms
to handle checksum and others in place already.

> 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.

Of course but many workloads/environments can not use jumbo
frames nor would it be helpful if your average pkt size is
128B or something around there.


> 
> --Sowmini
> 
> 
>