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Shutemov" , Michal Hocko , "Matthew Wilcox" , Minchan Kim , "Vlastimil Babka" , Chris Kennelly , Linux MM , Linux API Subject: Re: [RFC] Hugepage collapse in process context Thread-Topic: [RFC] Hugepage collapse in process context Thread-Index: AQHXBOTGyS5qDuITxk+obyTeBp1b4KpdkXWA Date: Thu, 18 Feb 2021 08:11:13 +0000 Message-ID: <9B5BFA9A-E945-4665-B335-A0B8E36D4463@fb.com> References: In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-mailer: Apple Mail (2.3654.60.0.2.21) authentication-results: google.com; dkim=none (message not signed) header.d=none;google.com; dmarc=none action=none header.from=fb.com; x-originating-ip: [24.6.81.164] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: e7f18446-82ca-4456-b363-08d8d3e4c26e x-ms-traffictypediagnostic: SJ0PR15MB4710: x-microsoft-antispam-prvs: x-fb-source: Internal x-ms-oob-tlc-oobclassifiers: OLM:10000; x-ms-exchange-senderadcheck: 1 x-microsoft-antispam: BCL:0; x-microsoft-antispam-message-info: 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 x-forefront-antispam-report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:BYAPR15MB2999.namprd15.prod.outlook.com;PTR:;CAT:NONE;SFS:(136003)(39860400002)(366004)(376002)(396003)(346002)(33656002)(8676002)(6512007)(8936002)(83380400001)(66946007)(6506007)(478600001)(5660300002)(4326008)(66476007)(66556008)(2906002)(316002)(7416002)(186003)(53546011)(54906003)(2616005)(6916009)(64756008)(26005)(6486002)(66446008)(86362001)(76116006)(36756003)(71200400001)(45980500001);DIR:OUT;SFP:1102; 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charset="us-ascii" Content-ID: <5335E776C975DA4FB02DFDCA65E3546D@namprd15.prod.outlook.com> Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: BYAPR15MB2999.namprd15.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: e7f18446-82ca-4456-b363-08d8d3e4c26e X-MS-Exchange-CrossTenant-originalarrivaltime: 18 Feb 2021 08:11:14.0004 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: lpS6PCN2wIMwqtNGSdRFB0fWkQPefqoliaknuqMtrOoDeSENmxg9/H3OJ7JxVVwH4HIxReq65RN+Z7hiz91RgQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: SJ0PR15MB4710 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.369,18.0.761 definitions=2021-02-18_03:2021-02-18,2021-02-18 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 bulkscore=0 adultscore=0 lowpriorityscore=0 mlxlogscore=999 clxscore=1011 malwarescore=0 spamscore=0 priorityscore=1501 suspectscore=0 phishscore=0 mlxscore=0 impostorscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2102180067 X-FB-Internal: deliver X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: > On Feb 16, 2021, at 8:24 PM, David Rientjes wrote: >=20 > Hi everybody, >=20 > Khugepaged is slow by default, it scans at most 4096 pages every 10s. =20 > That's normally fine as a system-wide setting, but some applications woul= d=20 > benefit from a more aggressive approach (as long as they are willing to=20 > pay for it). >=20 > Instead of adding priorities for eligible ranges of memory to khugepaged,= =20 > temporarily speeding khugepaged up for the whole system, or sharding its= =20 > work for memory belonging to a certain process, one approach would be to= =20 > allow userspace to induce hugepage collapse. >=20 > The benefit to this approach would be that this is done in process contex= t=20 > so its cpu is charged to the process that is inducing the collapse. =20 > Khugepaged is not involved. >=20 > Idea was to allow userspace to induce hugepage collapse through the new=20 > process_madvise() call. This allows us to collapse hugepages on behalf o= f=20 > current or another process for a vectored set of ranges. >=20 > This could be done through a new process_madvise() mode *or* it could be = a=20 > flag to MADV_HUGEPAGE since process_madvise() allows for a flag parameter= =20 > to be passed. For example, MADV_F_SYNC. >=20 > When done, this madvise call would allocate a hugepage on the right node= =20 > and attempt to do the collapse in process context just as khugepaged woul= d=20 > otherwise do. This is very interesting idea. One question, IIUC, the user process will=20 block until all small pages in given ranges are collapsed into THPs. What=20 would happen if the memory is so fragmented that we cannot allocate that=20 many huge pages? Do we need some fail over mechanisms?=20 >=20 > This would immediately be useful for a malloc implementation, for example= ,=20 > that has released its memory back to the system using MADV_DONTNEED and=20 > will subsequently refault the memory. Rather than wait for khugepaged to= =20 > come along 30m later, for example, and collapse this memory into a=20 > hugepage (which could take a much longer time on a very large system), an= =20 > alternative would be to use this process_madvise() mode to induce the=20 > action up front. In other words, say "I'm returning this memory to the=20 > application and it's going to be hot, so back it by a hugepage now rather= =20 > than waiting until later." >=20 > It would also be useful for read-only file-backed mappings for text=20 > segments. Khugepaged should be happy, it's just less work done by generi= c=20 > kthreads that gets charged as an overall tax to everybody. Mixing sync-THP with async-THP (khugepaged) could be useful when there are= =20 different priorities of THPs. In one of the use cases, we use THP for both= =20 text and data. The ratio may look like 5x THPs for text, and 2000x THPs for= =20 data. If the system has fewer than 2005 THPs, we wouldn't wait, but we woul= d=20 prioritize THPs for text. With this new mechanism, we can use sync-THP for= =20 the text, and async-THP for the data.=20 Thanks, Song