netdev - Re: What's the benefit of large Rx rings?

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Date:	Sun, 22 Nov 2015 13:53:32 -0800
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Yuval Mintz <Yuval.Mintz@...gic.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: What's the benefit of large Rx rings?

On Sun, Nov 22, 2015 at 12:19 PM, Yuval Mintz <Yuval.Mintz@...gic.com> wrote:
> Hi,
>
> This might be a dumb question, but I recently touched this
> and felt like I'm missing something basic -
>
> NAPI is being scheduled from soft-interrupt contex, and it
> has a ~strict quota for handling Rx packets [even though we're
> allowing practically unlimited handling of Tx completions].
> Given these facts, what's the benefit of having arbitrary large
> Rx buffer rings? Assuming quota is 64, I would have expected
> that having more than twice or thrice as many buffers could not
> help in real traffic scenarios - in any given time-unit
> [the time between 2 NAPI runs which should be relatively
> constant] CPU can't handle more than the quota; If HW is
> generating more packets on a regular basis the buffers are bound
> to get exhausted, no matter how many there are.
>
> While there isn't any obvious downside to allowing drivers to
> increase ring sizes to be larger [other than memory footprint],
> I feel like I'm missing the scenarios where having Ks of
> buffers can actually help.
> And for the unlikely case that I'm not missing anything,
> why aren't we supplying some `default' max and min amounts
> in a common header?

The main benefit of large Rx rings is that you could theoretically
support longer delays between device interrupts.  So for example if
you have a protocol such as UDP that doesn't care about latency then
you could theoretically set a large ring size, a large interrupt delay
and process several hundred or possibly even several thousand packets
per device interrupt instead of just a few.

- Alex
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