From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f72.google.com (mail-pa0-f72.google.com [209.85.220.72]) by kanga.kvack.org (Postfix) with ESMTP id 98C9782F64 for ; Tue, 30 Aug 2016 01:14:58 -0400 (EDT) Received: by mail-pa0-f72.google.com with SMTP id ez1so19580126pab.1 for ; Mon, 29 Aug 2016 22:14:58 -0700 (PDT) Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id 68si43241941pfr.68.2016.08.29.22.14.57 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 29 Aug 2016 22:14:57 -0700 (PDT) Received: from pps.filterd (m0098409.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.11/8.16.0.11) with SMTP id u7U53u5l043930 for ; Tue, 30 Aug 2016 01:14:57 -0400 Received: from e28smtp09.in.ibm.com (e28smtp09.in.ibm.com [125.16.236.9]) by mx0a-001b2d01.pphosted.com with ESMTP id 255363s7nv-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Tue, 30 Aug 2016 01:14:56 -0400 Received: from localhost by e28smtp09.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 30 Aug 2016 10:44:52 +0530 Received: from d28relay01.in.ibm.com (d28relay01.in.ibm.com [9.184.220.58]) by d28dlp01.in.ibm.com (Postfix) with ESMTP id A36D8E0040 for ; Tue, 30 Aug 2016 10:43:52 +0530 (IST) Received: from d28av02.in.ibm.com (d28av02.in.ibm.com [9.184.220.64]) by d28relay01.in.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id u7U5EofQ18677934 for ; Tue, 30 Aug 2016 10:44:50 +0530 Received: from d28av02.in.ibm.com (localhost [127.0.0.1]) by d28av02.in.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id u7U5Eni4021697 for ; Tue, 30 Aug 2016 10:44:50 +0530 Date: Tue, 30 Aug 2016 10:44:21 +0530 From: Anshuman Khandual MIME-Version: 1.0 Subject: Re: [PATCH] thp: reduce usage of huge zero page's atomic counter References: <20160829155021.2a85910c3d6b16a7f75ffccd@linux-foundation.org> In-Reply-To: <20160829155021.2a85910c3d6b16a7f75ffccd@linux-foundation.org> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Message-Id: <57C5162D.80405@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton , Aaron Lu Cc: Linux Memory Management List , "'Kirill A. Shutemov'" , Dave Hansen , Tim Chen , Huang Ying , Vlastimil Babka , Jerome Marchand , Andrea Arcangeli , Mel Gorman , Ebru Akagunduz , linux-kernel@vger.kernel.org On 08/30/2016 04:20 AM, Andrew Morton wrote: > On Mon, 29 Aug 2016 14:31:20 +0800 Aaron Lu wrote: > >> > >> > The global zero page is used to satisfy an anonymous read fault. If >> > THP(Transparent HugePage) is enabled then the global huge zero page is used. >> > The global huge zero page uses an atomic counter for reference counting >> > and is allocated/freed dynamically according to its counter value. >> > >> > CPU time spent on that counter will greatly increase if there are >> > a lot of processes doing anonymous read faults. This patch proposes a >> > way to reduce the access to the global counter so that the CPU load >> > can be reduced accordingly. >> > >> > To do this, a new flag of the mm_struct is introduced: MMF_USED_HUGE_ZERO_PAGE. >> > With this flag, the process only need to touch the global counter in >> > two cases: >> > 1 The first time it uses the global huge zero page; >> > 2 The time when mm_user of its mm_struct reaches zero. >> > >> > Note that right now, the huge zero page is eligible to be freed as soon >> > as its last use goes away. With this patch, the page will not be >> > eligible to be freed until the exit of the last process from which it >> > was ever used. >> > >> > And with the use of mm_user, the kthread is not eligible to use huge >> > zero page either. Since no kthread is using huge zero page today, there >> > is no difference after applying this patch. But if that is not desired, >> > I can change it to when mm_count reaches zero. > I suppose we could simply never free the zero huge page - if some > process has used it in the past, others will probably use it in the > future. One wonders how useful this optimization is... Yeah, what prevents us from doing away with this lock altogether and keep one zero filled huge page (after a process has used it once) for ever to be mapped across all the read faults ? A 16MB / 2MB huge page is too much of memory loss on a THP enabled system ? We can also save on allocation time. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org