From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Hemminger Subject: Re: RFC: possible NAPI improvements to reduce interrupt rates for low traffic rates Date: Wed, 12 Sep 2007 16:02:39 +0200 Message-ID: <20070912160239.70a580e8@oldman> References: <200709061416.l86EG0Vb017675@quickie.katalix.com> <1189120020.4259.68.camel@localhost> <46E11A61.9030409@katalix.com> <1189171370.4234.38.camel@localhost> <20070912030428.16059af6.billfink@mindspring.com> <1189599142.4326.38.camel@localhost> <46E7EE89.9060006@katalix.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: hadi@cyberus.ca, Bill Fink , netdev@vger.kernel.org, davem@davemloft.net, jeff@garzik.org, mandeep.baines@gmail.com, ossthema@de.ibm.com To: James Chapman Return-path: Received: from smtp2.linux-foundation.org ([207.189.120.14]:56061 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757610AbXILOCy (ORCPT ); Wed, 12 Sep 2007 10:02:54 -0400 In-Reply-To: <46E7EE89.9060006@katalix.com> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Wed, 12 Sep 2007 14:50:01 +0100 James Chapman wrote: > jamal wrote: > > On Wed, 2007-12-09 at 03:04 -0400, Bill Fink wrote: > >> On Fri, 07 Sep 2007, jamal wrote: > > > >>> I am going to be the devil's advocate[1]: > >> So let me be the angel's advocate. :-) > > > > I think this would make you God's advocate ;-> > > (http://en.wikipedia.org/wiki/God%27s_advocate) > > > >> I view his results much more favorably. > > > > The challenge is, under _low traffic_: bad bad CPU use. > > Thats what is at stake, correct? > > By low traffic, I assume you mean a rate at which the NAPI driver > doesn't stay in polled mode. The problem is that that rate is getting > higher all the time, as interface and CPU speeds increase. This results > in too many interrupts and NAPI thrashing in/out of polled mode very > quickly. But if you compare this to non-NAPI driver the same softirq overhead happens. The problem is that for many older devices disabling IRQ's require an expensive non-cached PCI access. Smarter, newer devices all use MSI which is pure edge triggered and with proper register usage, NAPI should be no worse than non-NAPI.