From mboxrd@z Thu Jan 1 00:00:00 1970 From: Juliusz Chroboczek Subject: Re: [PATCH] Stochastic Fair Blue queue discipline Date: Thu, 10 Apr 2008 13:17:53 +0200 Message-ID: <7izls2uffi.fsf@lanthane.pps.jussieu.fr> References: <87skxxb8br.fsf@pirx.pps.jussieu.fr> <873apwrc4t.fsf@basil.nowhere.org> <7i3apwbblk.fsf@lanthane.pps.jussieu.fr> <20080408163252.GS16647@one.firstfloor.org> <7iwsn8s107.fsf@lanthane.pps.jussieu.fr> <20080408175353.GA17147@one.firstfloor.org> <7ik5j7ghgh.fsf@lanthane.pps.jussieu.fr> <20080409174954.GD30885@one.firstfloor.org> <47FD6C0E.7040808@trash.net> <47FD6F85.5080003@trash.net> Mime-Version: 1.0 Content-Type: text/plain Cc: Andi Kleen , netdev@vger.kernel.org To: Patrick McHardy Return-path: Received: from shiva.jussieu.fr ([134.157.0.129]:56426 "EHLO shiva.jussieu.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755149AbYDJLST (ORCPT ); Thu, 10 Apr 2008 07:18:19 -0400 In-Reply-To: <47FD6F85.5080003@trash.net> (Patrick McHardy's message of "Thu\, 10 Apr 2008 03\:38\:13 +0200") Sender: netdev-owner@vger.kernel.org List-ID: > - traffic schedulers shouldn't depend on perfect random, > its more about statistical multiplexing. Okay, I've been thinking about this, and I'm not quite sure that what sfq and sfb want is a pseudo-random sequence in the first place. Sfq and sfb only draw random numbers in order to change hashing functions periodically, so as not to have an innocent flow stuck within a hash bucket with a non-reactive flow. For sfb it doesn't matter much (collisions in a Bloom filter are so rare as to be almost nonexistent), but for sfq, it is fairly important. Now assuming jhash is any good, i.e. that a single bit change in the input changes all output bits with roughly similar probability (and I don't know whether it is), it shouldn't matter much what sequence of u32 we use for the perturbation as long as it doesn't repeat values too often. Something as simple as a static counter might actually be good enough. So I'd argue that the wise thing is to make sure of is that jhash is good (in the sense above), and not bother too much with the PRNG. Juliusz