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 53E11C433F5 for ; Tue, 14 Sep 2021 08:23:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 37C336103B for ; Tue, 14 Sep 2021 08:23:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230040AbhINIZB convert rfc822-to-8bit (ORCPT ); Tue, 14 Sep 2021 04:25:01 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.85.151]:55755 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229458AbhINIZA (ORCPT ); Tue, 14 Sep 2021 04:25:00 -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-2-spjJskWLOtKyxaxI53WEXQ-1; Tue, 14 Sep 2021 09:23:41 +0100 X-MC-Unique: spjJskWLOtKyxaxI53WEXQ-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; Tue, 14 Sep 2021 09:23:41 +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; Tue, 14 Sep 2021 09:23:40 +0100 From: David Laight To: 'Willy Tarreau' CC: Douglas Gilbert , LKML Subject: RE: how many memset(,0,) calls in kernel ? Thread-Topic: how many memset(,0,) calls in kernel ? Thread-Index: AQHXp5KEgy5ggGKXu0iAcdzvqjgoc6uiIXMA///yLYCAAR6/oA== Date: Tue, 14 Sep 2021 08:23:40 +0000 Message-ID: <15cd0a8e72b3460db939060db25dd59a@AcuMS.aculab.com> References: <1c4a94df-fc2f-1bb2-8bce-2d71f9f1f5df@interlog.com> <20210912045608.GB16216@1wt.eu> <88976a40175c491fb5e3349f6686ad67@AcuMS.aculab.com> <20210913160945.GA2456@1wt.eu> In-Reply-To: <20210913160945.GA2456@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 From: Willy Tarreau > Sent: 13 September 2021 17:10 > > On Mon, Sep 13, 2021 at 04:03:09PM +0000, David Laight wrote: > > > 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. > > The exact point is, here it's up to the compiler to decide thanks to > its builtin what it considers best for the target CPU. It already > knows the fixed size and the code is emitted accordingly. It may > very well be a call to the memset() function when the size is large > and a power of two because it knows alternate variants are available > for example. > > The compiler might even decide to shrink that area if other bytes > are written just after the memset(), leaving only holes touched by > memset(). You might think the compiler will make sane choices for the target CPU. But it often makes a complete pig's breakfast of it. I'm pretty sure 6 'rep stos' is slower than 6 write an absolutely everything - with the possible exception of an 8088. By far the worst ones are when the compiler decides to pessimise a loop by using the simd (eg avx512) instructions to do 4 (or 8) loop iterations in one pass. It might be fine if the loop count is in the 100s - but not when it is 3. One compiler I've used nicely converted any byte copy loop into a 'rep movsb' instruction. That was contemporary with P4 netburst - where it was terribly slow. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)