netdev - Re: RPS vs RFS

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Date:	Thu, 3 Apr 2014 16:58:43 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Sharat Masetty <sharat04@...il.com>
Cc:	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: RPS vs RFS

On Thu, Apr 3, 2014 at 12:14 PM, Sharat Masetty <sharat04@...il.com> wrote:
> I am trying to understand the true benefit of RFS over RPS. In the
> kernel documentation scaling.txt, the author talks about data cache
> hitrate, can someone explain what this actually means? In which
> scenarios would RFS be beneficial? Why would it help to have network
> stack run on the same core on which the application for a stream/flow
> is running?
>
Silo'ing processing is typically good, it provides cache locality,
potentially eliminates a cross CPU wakeup, and hopefully reduces lock
contention. There is a secondary benefit in that we get some isolation
of RX processing and application.
>
> Consider a NIC with a single receive queue, single interrupt line, and
> iperf application is pulling data off this NIC card. In case where
> iperf may still be running on the same core on which the interrupts
> are delivered, then in that case the whole stack is pinned to the same
> core, and would not be benefiting a lot from this scheme
>
Consider what happens you have a multi threaded network intensive
application like a web server. Running all the networking an a single
CPU becomes a bottleneck (why we created RPS/RFS in the first place).

>
>
>
> References:
> https://www.kernel.org/doc/Documentation/networking/scaling.txt
>
> https://lwn.net/Articles/382428/
>
> Regards,
> Sharat
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