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=-0.8 required=3.0 tests=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 686FBC38A2A for ; Sun, 10 May 2020 18:58:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 446562082E for ; Sun, 10 May 2020 18:58:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729113AbgEJS62 convert rfc822-to-8bit (ORCPT ); Sun, 10 May 2020 14:58:28 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:37822 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728756AbgEJS62 (ORCPT ); Sun, 10 May 2020 14:58:28 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mtapsc-2-ai4aFd-aMnmImmhU2s39WA-1; Sun, 10 May 2020 19:58:24 +0100 X-MC-Unique: ai4aFd-aMnmImmhU2s39WA-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Sun, 10 May 2020 19:58:24 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Sun, 10 May 2020 19:58:24 +0100 From: David Laight To: 'Joe Perches' , Arnd Bergmann , "Oleksandr Natalenko" CC: "Jason A. Donenfeld" , LKML , X86 ML , Andrew Morton Subject: RE: [PATCH] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE_O3 for gcc >= 10 Thread-Topic: [PATCH] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE_O3 for gcc >= 10 Thread-Index: AQHWJUo8bK79Vz6Er0qWTasVui4yHqihRWbAgABE0gCAABxpkA== Date: Sun, 10 May 2020 18:58:24 +0000 Message-ID: References: <20200507224530.2993316-1-Jason@zx2c4.com> <20200508090202.7s3kcqpvpxx32syu@butterfly.localdomain> <20200508113336.2vdfdnc6tqyx4pu6@butterfly.localdomain> <9590a4674863448e8b13fee5086fcf73@AcuMS.aculab.com> In-Reply-To: Accept-Language: en-GB, en-US Content-Language: 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 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Joe Perches > Sent: 10 May 2020 18:45 > > On Sun, 2020-05-10 at 12:47 +0000, David Laight wrote: > > From: Joe Perches > > > Sent: 08 May 2020 16:06 > > > On Fri, 2020-05-08 at 13:49 +0200, Arnd Bergmann wrote: > > > > Personally, I'm more interested in improving compile speed of the kernel > > > > > > Any opinion on precompiled header support? > > > > When ever I've been anywhere near it it is always a disaster. > > A disaster? Why? The only time I've had systems that used them they always got out of step with the headers - probable due to #define changes. If auto-generated by the compiler then parallel makes also give problems. > For a large commercial c only project, it worked well > by reducing a combined multi-include file, similar to > kernel.h here, to a single file. Certainly reducing the number of directories searched can make a big difference. I've also compiled .so by merging all the sources into a single file. > That was before SSDs though and the file open times > might have been rather larger then. The real killer is lots of directory names in the -I especially over NFS. I've also looked at system call stats during a kernel compile. open() dominated and my 'gut feeling' was that most were failing opens. I also suspect that modern compilers remember that an include file contained an include guard - and don't even both looking for it a second time. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)