From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752591AbcH3FPL (ORCPT ); Tue, 30 Aug 2016 01:15:11 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:34762 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752203AbcH3FPJ (ORCPT ); Tue, 30 Aug 2016 01:15:09 -0400 X-IBM-Helo: d28dlp03.in.ibm.com X-IBM-MailFrom: khandual@linux.vnet.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org Date: Tue, 30 Aug 2016 10:44:21 +0530 From: Anshuman Khandual User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 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 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 X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16083005-4789-0000-0000-00000335F749 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 16083005-4790-0000-0000-000011E06AA2 Message-Id: <57C5162D.80405@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-08-30_02:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=2 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1604210000 definitions=main-1608300048 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: 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.