From owner-freebsd-hackers@FreeBSD.ORG Sun Oct 26 04:43:35 2003 Return-Path: Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 1747A16A4B3 for ; Sun, 26 Oct 2003 04:43:35 -0800 (PST) Received: from be-well.ilk.org (lowellg.ne.client2.attbi.com [66.30.200.37]) by mx1.FreeBSD.org (Postfix) with ESMTP id 4E8E143F85 for ; Sun, 26 Oct 2003 04:43:34 -0800 (PST) (envelope-from lowell@world.std.com) Received: by be-well.ilk.org (Postfix, from userid 1147) id BDFA53AB7; Sun, 26 Oct 2003 07:43:33 -0500 (EST) Sender: lowell@be-well.ilk.org To: References: <1066789354.21430.39.camel@boxster.onthenet.com.au> <20031022082953.GA69506@rot13.obsecurity.org> <1066816287.25609.34.camel@boxster.onthenet.com.au> <20031022095754.GA70026@rot13.obsecurity.org> <1066820436.25609.93.camel@boxster.onthenet.com.au> <20031026052854.GA20701@VARK.homeunix.com> From: Lowell Gilbert Date: 26 Oct 2003 07:43:33 -0500 In-Reply-To: <20031026052854.GA20701@VARK.homeunix.com> Message-ID: <44he1wi2yy.fsf@be-well.ilk.org> Lines: 44 User-Agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.3 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Subject: Re: Some mmap observations compared to Linux 2.6/OpenBSD X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 26 Oct 2003 12:43:35 -0000 David Schultz writes: > On Sun, Oct 26, 2003, Dag-Erling Smrgrav wrote: > > Q writes: > > > Yes, it would appear this is a legacy thing that existed in the original > > > 1994 import of the BSD 4.4 Lite source. Both FreeBSD and NetBSD still > > > use this technique, but OpenBSD changed to using Red-Black trees back in > > > Feb 2002. > > > [...] > > > I am wondering if there is a compelling reason why the technique used by > > > OpenBSD could not be adapted to FreeBSD's VM system. > > > > Adapting OpenBSD's red-balck patches would require quite a bit of work > > as FreeBSD and OpenBSD have diverged quite a bit in this area. Though > > it is a good idea to change the list into a tree, I think you'd get > > more mileage by addressing the fundamental problem, which is the lack > > of a free list. The current code (in both FreeBSD and OpenBSD) > > searches a list or tree of allocated extents, sorted by location, > > looking for a pair that have sufficient space between them for the > > extent you want to create. We should instead keep track of free > > extents in a structure that makes it easy to locate one of the correct > > size. We probably need a dual structure, though, because we need to > > keep the free extents sorted both by size (to quickly find what we > > need) and by location (to facilitate aggregation of adjacent extents, > > without which we'd suffer horribly from address space fragmentation). > > > > I have no idea how much this means for real-life workloads though. > > Your idea of using a size-hashed freelist as well as a > location-sorted list is appealing in its simplicity. Though it > can cause a bit of fragmentation, it gives you constant time > lookup. Bonwick's vmem allocator ([1], section 4.4.2 and > following), apparently works quite well using this principle. A hash probably isn't the right data structure for either dimension (DES didn't say it was, I notice). Finding the next-largest available entry is a useful operation, here, so a list would be better than a hash. [Or a tree; the point is that exact-match isn't the only kind of search you need.] > But regardless of the approach, someone has yet to demonstrate > that this is actually a performance problem in the real world. ;-) Yes. Wouldn't be easy to test even if you wanted to, either...