From owner-freebsd-hackers@FreeBSD.ORG Thu Oct 18 16:42:27 2012 Return-Path: Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 154702AF; Thu, 18 Oct 2012 16:42:27 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from mail.zoral.com.ua (mx0.zoral.com.ua [91.193.166.200]) by mx1.freebsd.org (Postfix) with ESMTP id 877128FC14; Thu, 18 Oct 2012 16:42:26 +0000 (UTC) Received: from skuns.kiev.zoral.com.ua (localhost [127.0.0.1]) by mail.zoral.com.ua (8.14.2/8.14.2) with ESMTP id q9IGgUJM079973; Thu, 18 Oct 2012 19:42:31 +0300 (EEST) (envelope-from kostikbel@gmail.com) Received: from deviant.kiev.zoral.com.ua (kostik@localhost [127.0.0.1]) by deviant.kiev.zoral.com.ua (8.14.5/8.14.5) with ESMTP id q9IGgIYL086939; Thu, 18 Oct 2012 19:42:18 +0300 (EEST) (envelope-from kostikbel@gmail.com) Received: (from kostik@localhost) by deviant.kiev.zoral.com.ua (8.14.5/8.14.5/Submit) id q9IGgI11086938; Thu, 18 Oct 2012 19:42:18 +0300 (EEST) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: deviant.kiev.zoral.com.ua: kostik set sender to kostikbel@gmail.com using -f Date: Thu, 18 Oct 2012 19:42:18 +0300 From: Konstantin Belousov To: John Baldwin Subject: Re: syncing large mmaped files Message-ID: <20121018164218.GR35915@deviant.kiev.zoral.com.ua> References: <20121018083537.GQ35915@deviant.kiev.zoral.com.ua> <201210180939.34861.jhb@freebsd.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="DI3e56nQDAJ1LWZd" Content-Disposition: inline In-Reply-To: <201210180939.34861.jhb@freebsd.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-Virus-Scanned: clamav-milter 0.95.2 at skuns.kiev.zoral.com.ua X-Virus-Status: Clean X-Spam-Status: No, score=-4.0 required=5.0 tests=ALL_TRUSTED,AWL,BAYES_00 autolearn=ham version=3.2.5 X-Spam-Checker-Version: SpamAssassin 3.2.5 (2008-06-10) on skuns.kiev.zoral.com.ua Cc: freebsd-hackers@freebsd.org, Tristan Verniquet X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 18 Oct 2012 16:42:27 -0000 --DI3e56nQDAJ1LWZd Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Oct 18, 2012 at 09:39:34AM -0400, John Baldwin wrote: > On Thursday, October 18, 2012 4:35:37 am Konstantin Belousov wrote: > > On Thu, Oct 18, 2012 at 10:08:22AM +1000, Tristan Verniquet wrote: > > >=20 > > > I want to work with large (1-10G) files in memory but eventually sync > > > them back out to disk. The problem is that the sync process appears to > > > lock the file in kernel for the duration of the sync, which can run > > > into minutes. This prevents other processes from reading from the file > > > (unless they already have it mapped) for this whole time. Is there > > > any way to prevent this? I think I read in a post somewhere about > > > openbsd implementing partial-writes when it hits a file with lots of > > > dirty pages in order to prevent this. Is there anything available for > > > FreeBSD or is there another way around it? > > > > > No, currently the vnode lock is held exclusive for the whole duration > > of the msync(2) syscall or its analog from the syncer. > >=20 > > Making a change to periodically drop the vnode lock in > > vm_object_page_clean() might be possible, but requires the benchmarking > > to make sure that we do not pessimize the common case. Also, this opens > > a possibility for the vnode reclamation meantime. >=20 > You can simulate this in userland by breaking up your msync() into multip= le > msync() calls where each call just syncs a portion of the file. Be aware that this is much-much slower than msyncing the whole file, even if file is very large. The reason is that pager initiates asynchronous _immediate_ clustered write for such situations. Async writes (AKA bdwrite()) are only specified for full range msyncing. >=20 > > Anyway, note that you cannot 'work with large files in memory', even if > > you have enough RAM and no pressure to hold all the file pages resident. > > The syncer will do a writeback periodically regardless of the applicati= on > > calling msync(2) or not, with the interval of approximately 30 seconds. >=20 > You can mmap with MAP_NOSYNC to prevent the syncer from writing the file = out > every 30 seconds. This also prevents msync(2) from syncing the region. The flag is fine for throw-away data, but not for the scenario that was described, I think. --DI3e56nQDAJ1LWZd Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAlCAMWoACgkQC3+MBN1Mb4iV7ACfeO+DqO2Onc8uMS29tjTbykJF Ek0An1i+6oS2OaxLly9sI5pAGmKlXw8F =UG69 -----END PGP SIGNATURE----- --DI3e56nQDAJ1LWZd--