Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751662AbaANUxx (ORCPT ); Tue, 14 Jan 2014 15:53:53 -0500 Received: from mail-ie0-f182.google.com ([209.85.223.182]:35499 "EHLO mail-ie0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751367AbaANUxu (ORCPT ); Tue, 14 Jan 2014 15:53:50 -0500 Message-ID: <52D5A3DC.9030107@gmail.com> Date: Tue, 14 Jan 2014 15:53:48 -0500 From: Austin S Hemmelgarn User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 MIME-Version: 1.0 To: Eric Dumazet CC: Hannes Frederic Sowa , netdev@vger.kernel.org, dborkman@redhat.com, linux-kernel@vger.kernel.org, darkjames-ws@darkjames.pl Subject: Re: [PATCH RFC] reciprocal_divide: correction/update of the algorithm References: <20140113214249.GK6586@order.stressinduktion.org> <1389722825.31367.260.camel@edumazet-glaptop2.roam.corp.google.com> <52D58E6F.4050000@gmail.com> <1389729032.31367.262.camel@edumazet-glaptop2.roam.corp.google.com> In-Reply-To: <1389729032.31367.262.camel@edumazet-glaptop2.roam.corp.google.com> X-Enigmail-Version: 1.6 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2014-01-14 14:50, Eric Dumazet wrote: > On Tue, 2014-01-14 at 14:22 -0500, Austin S Hemmelgarn wrote: > >> I disagree with the statement that current CPU's have reasonably fast >> dividers. A lot of embedded processors and many low-end x86 CPU's do >> not in-fact have any hardware divider, and usually provide it using >> microcode based emulation if they provide it at all. The AMD Jaguar >> micro-architecture in particular comes to mind, it uses an iterative >> division algorithm provided by the microcode that only produces 2 bits >> of quotient per cycle, even in the best case (2 8-bit integers and an >> integral 8-bit quotient) this still takes 4 cycles, which is twice as >> slow as any other math operation on the same processor. > > I doubt you run any BPF filter with a divide instruction in it on these > platform. > > Get real, do not over optimize things where it does not matter. > Actually, I have three Jaguar based routers, and use BPF regularly as part of their iptables rules to log certain packet types. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/