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

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Message-ID: <CO2PR11MB00887B710566F1CBEFDF837D97070@CO2PR11MB0088.namprd11.prod.outlook.com>
Date:	Mon, 23 Nov 2015 04:47:29 +0000
From:	Yuval Mintz <Yuval.Mintz@...gic.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
CC:	netdev <netdev@...r.kernel.org>
Subject: Re: What's the benefit of large Rx rings?

>> 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.

So we're basically spending hundred of MBs [at least for high-speed
ethernet devices] on memory that helps us mostly on the first
coalesced interrupt [since later it all goes through napi re-scheduling]?
Sounds a bit... wasteful.

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