From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9C0A0C433FE for ; Mon, 13 Sep 2021 16:03:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 857B560FE6 for ; Mon, 13 Sep 2021 16:03:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1346059AbhIMQE4 convert rfc822-to-8bit (ORCPT ); Mon, 13 Sep 2021 12:04:56 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.85.151]:43962 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1346501AbhIMQE3 (ORCPT ); Mon, 13 Sep 2021 12:04:29 -0400 Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-270-naWZEAVdMh-RMZ74W2aCvQ-1; Mon, 13 Sep 2021 17:03:12 +0100 X-MC-Unique: naWZEAVdMh-RMZ74W2aCvQ-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) by AcuMS.aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) with Microsoft SMTP Server (TLS) id 15.0.1497.23; Mon, 13 Sep 2021 17:03:09 +0100 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.023; Mon, 13 Sep 2021 17:03:09 +0100 From: David Laight To: 'Willy Tarreau' , Douglas Gilbert CC: LKML Subject: RE: how many memset(,0,) calls in kernel ? Thread-Topic: how many memset(,0,) calls in kernel ? Thread-Index: AQHXp5KEgy5ggGKXu0iAcdzvqjgoc6uiIXMA Date: Mon, 13 Sep 2021 16:03:09 +0000 Message-ID: <88976a40175c491fb5e3349f6686ad67@AcuMS.aculab.com> References: <1c4a94df-fc2f-1bb2-8bce-2d71f9f1f5df@interlog.com> <20210912045608.GB16216@1wt.eu> In-Reply-To: <20210912045608.GB16216@1wt.eu> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > 36: b9 06 00 00 00 mov $0x6,%ecx > 3b: 4c 89 e7 mov %r12,%rdi > 3e: f3 ab rep stos %eax,%es:(%rdi) > > The last line does exactly "memset(%rdi, %eax, %ecx)". Just two bytes > for some code that modern processors are even able to optimize. Hmmm I'd bet that 6 stores will be faster on ~everything. 'modern' processors do better than some older ones [1], but 6 writes isn't enough to get into the really fast paths. So you'll still take a few cycles of setup. [1] P4 netburst had a ~40 clock setup for any 'rep' operation. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)