linux-kernel - Re: [PATCH v6 net-next 2/5] net: implement support for low latency socket polling

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Date:	Thu, 30 May 2013 08:58:33 +0300
From:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Or Gerlitz <or.gerlitz@...il.com>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Don Skidmore <donald.c.skidmore@...el.com>,
	e1000-devel@...ts.sourceforge.net,
	Willem de Bruijn <willemb@...gle.com>,
	Andi Kleen <andi@...stfloor.org>, HPA <hpa@...or.com>,
	Eilon Greenstien <eilong@...adcom.com>,
	Alex Rosenbaum <alexr@...lanox.com>,
	Eliezer Tamir <eliezer@...ir.org.il>
Subject: Re: [PATCH v6 net-next 2/5] net: implement support for low latency
 socket polling

On 29/05/2013 22:08, Eric Dumazet wrote:
> On Wed, 2013-05-29 at 21:52 +0300, Or Gerlitz wrote:
>> Eliezer Tamir <eliezer.tamir@...ux.intel.com> wrote:
>>> Or Gerlitz wrote:
>>
>>>> Unlike with TCP sockets, UDP sockets may receive packets from multiple
>>>> sources and hence the receiving context may be steered to be executed
>>>> on different cores through RSS or other Flow-Steering HW mechanisms
>>>> which could mean different napi contexts for the same socket, is that
>>>> a problem here? what's the severity?
>>
>>> Nothing will break if you poll on the wrong queue.
>>> Your data will come through normal NAPI processing of the right queue.
>>
>> Can you elaborate a little further, why you call this "wrong" and "right"?
>> --
>
> This definitely need some documentation, because before llpoll, device
> RX path was serviced by the cpu receiving the harwdare interrupt.
>
> So the "wrong" queue could add false sharing, and wrong NUMA
> allocations.

Yes,
To work properly when you have more than one NUMA node, you have to have 
packet steering set up, either by your NIC or by HW accelerated RFS.

I would like to add a short writeup of the design and suggested 
configuration. Where should it go?
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