From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 32230 invoked by alias); 3 Apr 2013 16:20:49 -0000 Mailing-List: contact libc-ports-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Post: List-Help: , Sender: libc-ports-owner@sourceware.org Received: (qmail 32198 invoked by uid 89); 3 Apr 2013 16:20:44 -0000 Received: from popelka.ms.mff.cuni.cz (HELO popelka.ms.mff.cuni.cz) (195.113.20.131) by sourceware.org (qpsmtpd/0.84/v0.84-167-ge50287c) with ESMTP; Wed, 03 Apr 2013 16:20:44 +0000 Received: from domone.kolej.mff.cuni.cz (popelka.ms.mff.cuni.cz [195.113.20.131]) by popelka.ms.mff.cuni.cz (Postfix) with ESMTPS id 9C69F50796; Wed, 3 Apr 2013 18:20:39 +0200 (CEST) Received: by domone.kolej.mff.cuni.cz (Postfix, from userid 1000) id 258A66046C; Wed, 3 Apr 2013 18:19:50 +0200 (CEST) Date: Wed, 03 Apr 2013 16:20:00 -0000 From: =?utf-8?B?T25kxZllaiBCw61sa2E=?= To: "Shih-Yuan Lee (FourDollars)" Cc: "Joseph S. Myers" , libc-ports@sourceware.org, Jesse Sung , patches@eglibc.org, YC Cheng , rex.tsai@canonical.com Subject: Re: [Patches] [PATCH] ARM: NEON detected memcpy. Message-ID: <20130403161949.GA6759@domone.kolej.mff.cuni.cz> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.20 (2009-06-14) X-SW-Source: 2013-04/txt/msg00012.txt.bz2 On Wed, Apr 03, 2013 at 11:47:36PM +0800, Shih-Yuan Lee (FourDollars) wrote: > Hi Joseph, > ... > > I was previously told by people at ARM that NEON memcpy wasn't a good idea > > in practice because of raised power consumption, context switch costs etc. > > from using NEON in processes that otherwise didn't use it, even if it > > appeared superficially beneficial in benchmarks. > > > About raised power consumption and context switch costs, I may be able > to add some option in configure for the users to decide if they want > to use this feature or not. > How do you think? > Configure option is bit overkill. You need to compare neon/other implementation speed. Then determine size where neon is faster if we include energy cost and context switch. My first estimate is use neon when larger than 4096 bytes. However to determine context switch cost of neon you must account network effect. If you use neon in one function that is called sufficiently often (to always save registers) then adding neon implementation for additional functions does not increase cost.