netdev - Re: [RFC PATCH] udp: allow busy_poll on some unconnected sockets

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Date:	Thu, 10 Apr 2014 19:38:24 +0100
From:	Edward Cree <ecree@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	<netdev@...r.kernel.org>, Shawn Bohrer <shawn.bohrer@...il.com>,
	"Shawn Bohrer" <sbohrer@...advisors.com>,
	Jonathan Cooper <jcooper@...arflare.com>
Subject: Re: [RFC PATCH] udp: allow busy_poll on some unconnected sockets

On 10/04/14 19:32, Eric Dumazet wrote:
> On Thu, 2014-04-10 at 19:04 +0100, Edward Cree wrote:
>
>> Tested by setting IFF_SINGLE_NAPI in sfc; a UDP ping-pong test showed a
>>  performance benefit from sysctl net.core.busy_{read,poll}=50 in both the
>>  connected and unconnected case, where previously it only saw the benefit
>>  when the socket had been connected.
> Right, but how often do we have single NAPI devices on hosts wanting
> very low latencies ?
>
Well, sfc only has a single NAPI context per device, and I'm fairly sure
most sfc users want very low latencies.
Or have I misunderstood?

(Note that it doesn't matter if there are other NAPI-using devices on
the host, since the socket is bound to a local address and thus is only
going to receive packets from the one device that has that address.  So
maybe IFF_SINGLE_NAPI is a bad name and it should be
IFF_DEVICE_ONLY_HAS_A_SINGLE_NAPI_CONTEXT.  But that's a bit unwieldy ;)
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