netdev - Re: GRO after RPS?

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Date:	Mon, 26 Apr 2010 09:47:44 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: GRO after RPS?

On Mon, Apr 26, 2010 at 09:40:35AM +0800, Changli Gao wrote:
> On Mon, Apr 26, 2010 at 9:17 AM, David Miller <davem@...emloft.net> wrote:
> >
> > The goal is to eliminate all packet header references from the pre-RPS
> > path, and let the post-RPS cpu do it.
> 
> If the NIC doesn't provide rxhash, RPS will have to compute one by one
> by itself. Is the hash computation more expensive than GRO? I think
> the hash computation is cheaper than GRO, so we can do RPS ASAP to
> avoid the direct CPU overload.

The dominating cost will be pulling the memory into cache.  So
once you do that the cost of GRO will be negligible.

As I said, if we had hardware rxhashes, we can change GRO to only
pull in the header when we're likely to have a match.

When we do get a mergeable flow, the difference between doing
GRO before/after RPS is going to be redirecting 1 packet vs.
~100 packets.

So if RPS is so cheap that it's able to process 100 packets at
a cost tantamount to 1 packet, then sure we should postpone GRO.

Otherwise I think GRO before RPS is definitely a win.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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