From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qg0-f43.google.com (mail-qg0-f43.google.com [209.85.192.43]) by kanga.kvack.org (Postfix) with ESMTP id 13CBA6B00C4 for ; Wed, 18 Feb 2015 19:26:50 -0500 (EST) Received: by mail-qg0-f43.google.com with SMTP id i50so3950802qgf.2 for ; Wed, 18 Feb 2015 16:26:49 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id b89si18966173qgf.111.2015.02.18.16.26.48 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 18 Feb 2015 16:26:49 -0800 (PST) Message-ID: <54E5296C.5040806@redhat.com> Date: Wed, 18 Feb 2015 19:08:12 -0500 From: Rik van Riel MIME-Version: 1.0 Subject: Re: [PATCH v2] mm: incorporate zero pages into transparent huge pages References: <1423688635-4306-1-git-send-email-ebru.akagunduz@gmail.com> <20150218153119.0bcd0bf8b4e7d30d99f00a3b@linux-foundation.org> In-Reply-To: <20150218153119.0bcd0bf8b4e7d30d99f00a3b@linux-foundation.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton , Ebru Akagunduz Cc: linux-mm@kvack.org, kirill@shutemov.name, mhocko@suse.cz, mgorman@suse.de, rientjes@google.com, sasha.levin@oracle.com, hughd@google.com, hannes@cmpxchg.org, vbabka@suse.cz, linux-kernel@vger.kernel.org, aarcange@redhat.com -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 02/18/2015 06:31 PM, Andrew Morton wrote: > On Wed, 11 Feb 2015 23:03:55 +0200 Ebru Akagunduz > wrote: > >> This patch improves THP collapse rates, by allowing zero pages. >> >> Currently THP can collapse 4kB pages into a THP when there are up >> to khugepaged_max_ptes_none pte_none ptes in a 2MB range. This >> patch counts pte none and mapped zero pages with the same >> variable. > > So if I'm understanding this correctly, with the default value of > khugepaged_max_ptes_none (HPAGE_PMD_NR-1), if an application > creates a 2MB area which contains 511 mappings of the zero page and > one real page, the kernel will proceed to turn that area into a > real, physical huge page. So it consumes 2MB of memory which would > not have previously been allocated? This is equivalent to an application doing a write fault to a 2MB area that was previously untouched, going into do_huge_pmd_anonymous_page() and receiving a 2MB page. > If so, this might be rather undesirable behaviour in some > situations (and ditto the current behaviour for pte_none ptes)? > > This can be tuned by adjusting khugepaged_max_ptes_none, The example of directly going into do_huge_pmd_anonymous_page() is not influenced by the tunable. It may indeed be undesirable in some situations, but I am not sure how to detect those... - -- All rights reversed -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBAgAGBQJU5SlsAAoJEM553pKExN6D8DYH/0TQPr38R3lYqxTllOVPIUus +UrgXveOeoMiMbN3e5r9tIJkw+2yUJFZ8hkYx+aFsTD5zNz7xwf9Qz8IdJpcZ3sc PkvOnnZNk/ZzixWrBhWFPsKRN2pi5wXMpfNM2jTs9W4EeyfkV3RYbGxZy/OO1LB5 CwDzteCTb81y1FYxC4vNxLnML417ZjIMq7ICdj6lKW2KC5+TdCIPTOrKCy+2fWBo 4qhqho4RFKHLCxpnryUMzZDXca4vmcgGWwUm5xLF6SnJWWFEiPBLixJiRV3xe0iw rbuGhcIXo/q16oO4QOIl+hSVJr8vE+Y8xRbIJFmWXCmuQHQpg5ZspVZ+9Z/3UaI= =Qf1D -----END PGP SIGNATURE----- -- 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