netdev - Re: rps perfomance WAS(Re: rps: question

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Date:	Thu, 15 Apr 2010 07:55:28 -0400
From:	jamal <hadi@...erus.ca>
To:	David Miller <davem@...emloft.net>
Cc:	eric.dumazet@...il.com, therbert@...gle.com,
	netdev@...r.kernel.org, robert@...julf.net, xiaosuo@...il.com,
	andi@...stfloor.org
Subject: Re: rps perfomance WAS(Re: rps: question

On Thu, 2010-04-15 at 01:48 -0700, David Miller wrote:

> A single-queue NIC is actually not a requirement, 
> RPS helps also in cases where you have 'N' application threads 
> and N is less than the number of CPUs your multi-queue NIC is 
> distributing traffic to.

sure..

> Moving the bulk of the input packet processing to the cpus where
> the applications actually sit had a non-trivial benefit.  

This is true regardless of rps though. 

> RFS takes this aspect to yet another level.

rfs looks quiet interesting;-> I think with some twist it could be
used with multiqueue nics as well

> I think for the case where application locality is important,
> RPS/RFS can help regardless of cache details.

Generally true, as long as there's not much shared data across the cpus
or the cost of a cache miss is reasonably tolerable. The socket layer
just happens to be not sharing much with ingress packet path and
for a single processor Nehalem, the caching system works so well that
the cost of cache misses is not as an important a variable. Everything
is on the same die including the MM controller etc.
I am speculating (didnt get any answer to the question i asked) that
people running rps use such hardware;->

I speculate again that it may be too costly to run rps on something like
a tigerton or intel clovertown where you have cores sharing/contending
for an FSB. If I can get answers to the question: "What h/ware are
people running?" i could be proven wrong.
[Note: I am not against RPS - i think it has its place; so i hope my
desire to find out when to use rps doesnt show as hostility towards
rps.]

cheers,
jamal

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