From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261815AbTHYQyT (ORCPT ); Mon, 25 Aug 2003 12:54:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S261830AbTHYQyT (ORCPT ); Mon, 25 Aug 2003 12:54:19 -0400 Received: from kinesis.swishmail.com ([209.10.110.86]:531 "HELO kinesis.swishmail.com") by vger.kernel.org with SMTP id S261815AbTHYQyR (ORCPT ); Mon, 25 Aug 2003 12:54:17 -0400 Message-ID: <3F4A4362.4020601@techsource.com> Date: Mon, 25 Aug 2003 13:12:02 -0400 From: Timothy Miller User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0 X-Accept-Language: en-us, en MIME-Version: 1.0 To: Ricardo Nabinger Sanchez CC: linux-kernel@vger.kernel.org, Frank.Cornelis@elis.ugent.be Subject: Re: [PATCH] sched: CPU_THRESHOLD References: <20030823195740.7133874d.rnsanchez@terra.com.br> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Ricardo Nabinger Sanchez wrote: > hello Frank, > > >>- *imbalance = (max_load - nr_running) / 2; >>+ *imbalance = (max_load - nr_running) >> 1; > > > I think it is a good coding practice to keep things human-readable. > In this code snippet, the division by 2 is quickly understood by most > readers (specially those who didn't write it). The right shift may > obfuscate the real meaning of this operation, which is a single > division by 2, not a bit-oriented expression. > > Assuming that sched.c will be compiled with optimizations enabled, the > compiler will change the human-readable division by a fast machine > right shift operation, whenever possible (gcc surely will). > > Thus, we keep the kernel code more readable, and sometimes let the > compiler apply newer (and hopefully faster) optimizations than some > tricks we have known as fastest available. > > Regards, and please let me know what do you think about it. > Note that right-shift rounds towards negative infinity, while divide rounds towards zero. If the operands are unsigned, then rsh and divide will yield the same instructions (on x86). OTOH, if they are signed, then you get different results. For this rsh, obviously, you get a shift, but for divide, you get something like this: ; divide edx by 2 mov ecx,edx lsr ecx,31 asr edx,1 add edx,ecx