Some mmap observations compared to Linux 2.6/OpenBSD

[Dag-ErlingSmørgrav]

Q <q_dolan at 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.

DES
-- 
Dag-Erling Smørgrav - des at des.no