From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753752AbeCFOCW (ORCPT ); Tue, 6 Mar 2018 09:02:22 -0500 Received: from mail-sn1nam01on0085.outbound.protection.outlook.com ([104.47.32.85]:36136 "EHLO NAM01-SN1-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753711AbeCFOCU (ORCPT ); Tue, 6 Mar 2018 09:02:20 -0500 Authentication-Results: spf=none (sender IP is ) smtp.mailfrom=Jan.Glauber@cavium.com; Date: Tue, 6 Mar 2018 15:02:01 +0100 From: Jan Glauber To: Arnd Bergmann Cc: Catalin Marinas , Will Deacon , Linux ARM , Linux Kernel Mailing List Subject: Re: [PATCH 2/2] arm64: defconfig: Raise NR_CPUS to 256 Message-ID: <20180306140201.GB7428@hc> References: <20180302143737.10788-1-jglauber@cavium.com> <20180302143737.10788-2-jglauber@cavium.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-Originating-IP: [88.67.129.238] X-ClientProxiedBy: AM5PR0102CA0010.eurprd01.prod.exchangelabs.com (2603:10a6:206::23) To BN3PR07MB2497.namprd07.prod.outlook.com (2a01:111:e400:7bbf::6) X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 3c3aa167-f80b-4ed7-5b34-08d5836adf97 X-Microsoft-Antispam: UriScan:;BCL:0;PCL:0;RULEID:(7020095)(4652020)(5600026)(4604075)(4534165)(4627221)(201703031133081)(201702281549075)(2017052603328)(7153060)(7193020);SRVR:BN3PR07MB2497; 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From my counting, I would have expected > twice that number in both cases: 48 cores, 2 chips and 2x SMT for ThunderX > vs 52 Cores, 2 chips and 4x SMT for ThunderX2. That's what I have on those machines. I counted SMT as normal CPUs as it doesn't make a difference for the config. I've not seen SMT on ThunderX. The ThunderX2 number of 224 is already with 4x SMT (and 2 chips) but there may be other versions planned that I'm not aware of. > > Therefore raise the default number of CPUs from 64 to 256 > > by adding an arm64 specific option to override the generic default. > > Regardless of what the correct numbers for your chips are, I'd like > to hear some other opinions on how high we should raise that default > limit, both in arch/arm64/Kconfig and in the defconfig file. > > As I remember it, there is a noticeable cost for taking the limit beyond > BITS_PER_LONG, both in terms of memory consumption and also > runtime performance (copying and comparing CPU masks). OK, that explains the default. My unverified assumption is that increasing the CPU masks wont be a noticable performance hit. Also, I don't think that anyone who wants performance will use defconfig. All server distributions would bump up the NR_CPUS anyway and really small systems will probably need to tune the config anyway. For me defconfig should produce a usable system, not with every last driver configured but with all the basics like CPUs, networking, etc. fully present. > I'm sure someone will keep coming up with even larger configurations > in the future, so we should try to decide how far we can take the > defaults for the moment without impacting users of the smallest > systems. Alternatively, you could add some measurements that > show how much memory and CPU time is used up on a typical > configuration for a small system (4 cores, no SMT, 512 MB RAM). > If that's low enough, we could just do it anyway. OK, I'll take a look. --Jan