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

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Message-ID: <1271742351.3845.106.camel@edumazet-laptop>
Date:	Tue, 20 Apr 2010 07:45:51 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	Changli Gao <xiaosuo@...il.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] rps: send IPIs ASAP

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.

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



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