Date: Sat, 25 Oct 2003 22:28:54 -0700 From: David Schultz <das@FreeBSD.ORG> To: "Dag-Erling =?us-ascii:iso-8859-1?Q?Sm=F8rgrav?=" <des@des.no> Cc: Kris Kennaway <kris@obsecurity.org> Subject: Re: Some mmap observations compared to Linux 2.6/OpenBSD Message-ID: <20031026052854.GA20701@VARK.homeunix.com> In-Reply-To: <xzpk76sc425.fsf@dwp.des.no> 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> <xzpk76sc425.fsf@dwp.des.no>
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On Sun, Oct 26, 2003, Dag-Erling Smrgrav wrote: > Q <q_dolan@yahoo.com.au> 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. But regardless of the approach, someone has yet to demonstrate that this is actually a performance problem in the real world. ;-) [1] http://www.usenix.org/event/usenix01/full_papers/bonwick/
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