From owner-freebsd-hackers@FreeBSD.ORG Thu Oct 18 13:47:36 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 5AFDD7B5 for ; Thu, 18 Oct 2012 13:47:36 +0000 (UTC) (envelope-from jhb@freebsd.org) Received: from bigwig.baldwin.cx (bigknife-pt.tunnel.tserv9.chi1.ipv6.he.net [IPv6:2001:470:1f10:75::2]) by mx1.freebsd.org (Postfix) with ESMTP id 2CA8E8FC17 for ; Thu, 18 Oct 2012 13:47:36 +0000 (UTC) Received: from jhbbsd.localnet (unknown [209.249.190.124]) by bigwig.baldwin.cx (Postfix) with ESMTPSA id 7F97AB98E; Thu, 18 Oct 2012 09:47:35 -0400 (EDT) From: John Baldwin To: freebsd-hackers@freebsd.org Subject: Re: syncing large mmaped files Date: Thu, 18 Oct 2012 09:39:34 -0400 User-Agent: KMail/1.13.5 (FreeBSD/8.2-CBSD-20110714-p20; KDE/4.5.5; amd64; ; ) References: <20121018083537.GQ35915@deviant.kiev.zoral.com.ua> In-Reply-To: <20121018083537.GQ35915@deviant.kiev.zoral.com.ua> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-15" Content-Transfer-Encoding: 7bit Message-Id: <201210180939.34861.jhb@freebsd.org> X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.2.7 (bigwig.baldwin.cx); Thu, 18 Oct 2012 09:47:35 -0400 (EDT) Cc: Konstantin Belousov , 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 13:47:36 -0000 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: > > > > 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. > > 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. You can simulate this in userland by breaking up your msync() into multiple msync() calls where each call just syncs a portion of the file. > 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 application > calling msync(2) or not, with the interval of approximately 30 seconds. You can mmap with MAP_NOSYNC to prevent the syncer from writing the file out every 30 seconds. -- John Baldwin