From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Tue, 29 Oct 2002 01:41:00 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Tue, 29 Oct 2002 01:40:59 -0500 Received: from alpha1.cc.monash.edu.au ([130.194.1.1]:56588 "EHLO ALPHA1.CC.MONASH.EDU.AU") by vger.kernel.org with ESMTP id ; Tue, 29 Oct 2002 01:40:58 -0500 Date: Tue, 29 Oct 2002 17:10:12 +1100 (EST) From: netdev-bounce@oss.sgi.com To: undisclosed-recipients:; Message-id: <20021029061012.ACCDA12CEA6@blammo.its.monash.edu.au> Content-transfer-encoding: 7BIT Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 23 Oct 2002, Nivedita Singhvi wrote: > "Richard B. Johnson" wrote: > > > No. It's done over each word (short int) and the actual summation > > takes place during the address calculation of the next word. This > > gets you a checksum that is practically free. > > Yep, sorry, word, not byte. My bad. The cost is in the fact > that this whole process involves loading each word of the data > stream into a register. Which is why I also used to consider > the checksum cost as negligible. > > > A 400 MHz ix86 CPU will checksum/copy at 685 megabytes per second. > > It will copy at 1,549 megabytes per second. Those are megaBYTES! > > But then why the difference in the checksum/copy and copy? > Are you saying the checksum is not costing you 864 megabytes > a second?? Costing you 864 megabytes per second? Lets say the checksum was free. You are then able to INF bytes/per/sec. So it's costing you INF bytes/per/sec? No, it's costing you nothing. If we were not dealing with INF, then 'Cost' is approximately 1/N, not N. Cost is work_done_without_checksum - work_done_with_checksum. Because of the low-pass filter pole, these numbers are practically the same. But, you can get a measurable difference between any two large numbers. This makes the 'cost' seem high. You need to make it relative to make any sense, so a 'goodness' can be expressed as a ratio of the cost and the work having been done. Cheers, Dick Johnson Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips). Bush : The Fourth Reich of America