Date: Thu, 6 Dec 2007 23:07:54 +0100
From: Andi Kleen <andi@firstfloor.org>
To: Chris Friesen <cfriesen@nortel.com>
Cc: Andi Kleen <andi@firstfloor.org>, Renzo Davoli <renzo@cs.unibo.it>,
       linux-kernel@vger.kernel.org
Subject: Re: New Address Family: Inter Process Networking (IPN)
Message-ID: <20071206220754.GF20595@one.firstfloor.org>
References: <20071205164055.GA2082@cs.unibo.it> <p73bq94k8yd.fsf@bingen.suse.de> <20071206053016.GA1464@cs.unibo.it> <20071206163538.GC20595@one.firstfloor.org> <47585D66.3000404@nortel.com> <20071206212644.GA26725@one.firstfloor.org> <47586E7F.1020903@nortel.com>
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On Thu, Dec 06, 2007 at 03:49:51PM -0600, Chris Friesen wrote:
> Andi Kleen wrote:
> >>Latency was very 
> >>important, so we ended up doing essentially a multicast unix socket 
> >>rather than taking the extra penalty for UDP multicast.
> >
> >
> >What extra penalty? Local UDP shouldn't be much more expensive than Unix.
> 
> On a 1.4GHz P4 I measured a 44% increase in latency between a unix 
> datagram and a UDP datagram.

That's weird.

> 
> For UDP it has to go down the udp stack, then the ip stack, then through 

UDP doesn't really have much stack. IP is also very little assuming
cached route (connect called first) 

I would expect the copies to dominate in both cases.

-Andi
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