Date: Wed, 19 Jan 2000 10:49:52 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Jonas Bulow <jonas.bulow@parallelconsulting.com> Cc: FreeBSD Hackers <freebsd-hackers@FreeBSD.ORG> Subject: Re: UVM vs FreeBSD VM system Message-ID: <200001191849.KAA39586@apollo.backplane.com> References: <3885CDCD.E033801@parallelconsulting.com>
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:Hi,
:
:How does the UVM system compare to the VM system in FreeBSD? Are there
:any benchmark tests or research results in this area?
Well, UVM is a much better design then the *original* Mach VM subsystem
in 4.4. FreeBSD, however, does not use the original Mach VM subsystem
anymore (thanks to John Dyson). UVM and FreeBSD's VM subsystem should
have roughly similar performance.
http://www.ccrc.wustl.edu/pub/chuck/tech/uvm/
http://www.daemonnews.org/200001/freebsd_vm.html
The main focus of UVM was to deal with Mach's deep-stacking of VM Objects.
They replaced the traditional shadow chains with a two-layer system.
FreeBSD wound up keeping the shadow chains but adding an optimization
that tended to restrict their depth. See the "All Shadowed" case in
my paper on daemonnews.
UVM has a lot of cool ideas especially in regards to avoiding program
stalls when the system issues write-I/O. However, it also has some
problems. I think FreeBSD handles VM/Filesystem coherency better at
the moment (as of USENIX NetBSD/UVM did not have a coherent VM/Filesystem
cache). UVM incorporated a number of FreeBSD ideas such as page
clustering. FreeBSD is not without problems either. FreeBSD's VM system
has more stall points then UVM's.
In anycase, the UVM paper listed at the URL contains a good description
of UVM and also compares it (roughly) with other operating systems,
including FreeBSD.
-Matt
Matthew Dillon
<dillon@backplane.com>
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