netdev - Re: [PATCH] rps: send IPIs ASAP

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Date:	Tue, 20 Apr 2010 17:17:26 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] rps: send IPIs ASAP

On Tue, Apr 20, 2010 at 1:45 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le lundi 19 avril 2010 à 22:15 -0700, Tom Herbert a écrit :
>> On Mon, Apr 19, 2010 at 9:08 PM, Changli Gao <xiaosuo@...il.com> wrote:
>> > rps: send IPIs ASAP
>> >
>> > In order to reduce latency, we'd better send IPIs ASAP to schedule the
>> > corresponding NAPIs.
>> >
>> A design point of RPS is that we generate at most one IPI per CPU per
>> device interrupt, which at least offers some predictable coalescing.
>> With your changes, we would get at most one IPI per packet-- that
>> could represent a lot more of them.  Did you test this to see what the
>> impact is in this regard?
>>
>
> I agree with you Tom. Coalescing IPI is probably better.
>
> If the receiver CPU got a single packet in its RX handling, latency will
> be the same anyway.

I did the "ping -f" test again, and found that the differences of RTT
I got before were noises. It seems your "shortcut net_rps_action()"
patch eliminates the differences.

>
> If the receiver CPU got many packets, chance is high we are in a stress
> situation, and coalescing is a win in this case.
>
> I am currently testing a patch to call net_rps_action() at the beginning
> of process_backlog() (if we have a non null ipi_rps_list pointer)
>
> Will post a patch with bench results
>

It sounds like a better idea.


-- 
Regards，
Changli Gao(xiaosuo@...il.com)
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