From mboxrd@z Thu Jan 1 00:00:00 1970 From: Evgeniy Polyakov Subject: Re: Extensible hashing and RCU Date: Tue, 20 Feb 2007 22:36:32 +0300 Message-ID: <20070220193632.GA5590@2ka.mipt.ru> References: <200702191913.08125.dada1@cosmosbay.com> <200702201955.15567.dada1@cosmosbay.com> <20070220190634.GA12193@2ka.mipt.ru> <200702202017.31965.dada1@cosmosbay.com> Mime-Version: 1.0 Content-Type: text/plain; charset=koi8-r Cc: "Michael K. Edwards" , David Miller , akepner@sgi.com, linux@horizon.com, netdev@vger.kernel.org, bcrl@kvack.org To: Eric Dumazet Return-path: Received: from relay.2ka.mipt.ru ([194.85.82.65]:58160 "EHLO 2ka.mipt.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030375AbXBTTsC (ORCPT ); Tue, 20 Feb 2007 14:48:02 -0500 Content-Disposition: inline In-Reply-To: <200702202017.31965.dada1@cosmosbay.com> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Tue, Feb 20, 2007 at 08:17:31PM +0100, Eric Dumazet (dada1@cosmosbay.com) wrote: > I shown your test was bogus. All your claims are just bogus. > I claim your 'true random data' is a joke. rand() in your program is a pure > joke. Care to reread your mail about your true random case with hash chain length of 3 and 4? Anyway, I just shown that jenkins hash is simple to crack and to find its collisions - even if you will put there some constant value it will be the same. It is math, not something special speculation about input values. > Given 48 bits of input, you *can* find a lot of addr/port to hit one > particular cache line if XOR function is used. With jhash, without knowing > the 32bits random secret, you *cant*. You seems to do not want to understand that it is exactly the same as searching for collision law. It is simple, and results will be dangerous. > Again, you dont take into account the chain length. > > If all chains were of length <= 1, then yes, xor would be faster. In real > life, we *know* chain length can be larger, especially in DOS situations. I.e. you propose to add a hash, which has broken case for the same ip addresses and different ports compared to good xor? It was shown that hash(const, const, non_const) ends up with _broken_ distribution comapred to xor hash. -- Evgeniy Polyakov