netdev - Re: [PATCH net-next-2.6] net: speedup udp receive path

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Message-Id: <1272721749.14499.50.camel@bigi>
Date:	Sat, 01 May 2010 09:49:09 -0400
From:	jamal <hadi@...erus.ca>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Changli Gao <xiaosuo@...il.com>,
	David Miller <davem@...emloft.net>, therbert@...gle.com,
	shemminger@...tta.com, netdev@...r.kernel.org,
	Eilon Greenstein <eilong@...adcom.com>,
	Brian Bloniarz <bmb@...enacr.com>
Subject: Re: [PATCH net-next-2.6] net: speedup udp receive path

On Sat, 2010-05-01 at 15:22 +0200, Eric Dumazet wrote:

> You must understand that the whole 'bench' is mostly governed by
> scheduler artifacts. The regression you mention is probably a side
> effect.

likely.

> By slowing down one part, its possible to zap all calls to scheduler and
> go maybe 300% faster (Because consumer threads can avoid 3/4 of the time
> to schedule)
> 
> Reciprocally, optimizing one part of the network stack might make
> threads hitting an empty queue, and need to call more often the
> scheduler.

It is fair to say that what i am seeing is _not_ fatal because it is rps
that is regressing; non-rps is fine. I would consider non-rps to be the
common use scenario and if that was doing badly then it is a problem.
The good news is it is getting better - likely because of some changes
made on behalf of rps ;->
With rps, one could follow some instructions on how to make it better.
I am hoping that some of the system "magic" is documented as Tom
mentioned he will.

> This is why some higly specialized programs never block/schedule and
> perform busy loops instead.

Agreed. My brain cells should learn to accept this fact ;->

cheers,
jamal

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