Nemesis is (also) an experimental operating system, built from the ground
up to support Quality of Service (QoS) guarantees in multimedia applications.
Originally developed at the Computer Lab at Cambridge University, with
later contributions from University of Glasgow (Scotland), University
of Twente (Netherlands) and the Swedish Institute of Computer Science.
Goals for the Operating System include not only providing the necessary
support for real-time and multimedia applications, but also the provision
of a stable and flexible (and largely Open Source) platform for practical
research in multimedia and QoS, particularly Network QoS (largely ATM based) and Disk QoS projects
What is QoS?
Quality of Service, as a general concept in computing, is more or
less what you'd expect from the name. A Quality of Service guarantee states
that resources necessary to complete or maintain a process are guaranteed
to be available within a specified timescale, and can be applied to almost
any resource available in a computer system:
For example, to play a RealVideo
movie might require 56kbps of network
bandwidth, some buffering in physical memory, and a small amount of CPU time
for each frame
. If one of these isn't available at the correct time, frames
will be dropped and the movie will lose quality.
In a traditional operating system, scheduling of resource allocation
to a process is based on a simple prioritized time-sharing system, typically
with the CPU scheduling demand-driving other resource allocation (such
as physical memory). The availability of resources to a program therefore
depends on the activity of other programs on the system. In the above example,
where the movie needs 56kbps of network bandwidth, the movie's playback
would be affected by some other application (e.g.. NewsUpd) requiring
network bandwidth, particularly if the connection was via a 56kbps.
This unintentional interaction between different applications clamoring
for resources is known as crosstalk.
You're probably already familiar with these effects if you've ever tried
to use your computer while watching a DVD or burning a CDR. The cross-talk
across your CPU usage or disk access will adversely affect the behaviour
of the DVD player or CD burner. In the former the result is annoying;
in the latter, you get coasters.
Nemesis schedules resources (primarily CPU time) by allocating
a guaranteed portion of a resource to a process, and ensuring that it's
always available. The DVD player application above would be allocated
it's required CPU time, and regardless of the activities of other CPU-intensive
applications, the required CPU would always be available to the DVD player,
allowing continuous glitch-free operation. If the operating system had already
committed too much CPU time and couldn't allocate the necessary time
to play the DVD properly, it would warn about this, rather than making
a half-assed job of it and at the same time disrupt the operation of other
It seems obvious, doesn't it?
Fundamentally, Nemesis is a microkernel operating system, consisting
of a small supervisor mode executable (the NTSC, or the Nemesis Trusted
Supervisor Code) managing direct access to hardware and driving processor
scheduling etc. Since there's a fairly small amount of architecture specific
code, ports exist for i386 PC hardware, Alpha platforms and various
ARM platforms including RiscPC and Sidewinder.
All other functionality is provided by various layers of code libraries.
Process model, file systems, network protocols etc. are implemented by libraries.
Even applications are fundamentally libraries.
This greatly helps in accounting for CPU usage by an application. under
UNIX and X-Windows, a video player process uses some CPU time to decode
it's data, but also requires some CPU time in the X server process to render
that data to the framebuffer. The necessary CPU time to play the video can't be accounted
for simply as the CPU time used by the video player process: it also
includes the time used by the X server on behalf of the video player. This
is difficult to extract from the time used by the X server since it will
also include time used on behalf of other applications. Nemesis' library
based approach allows the entire activity to be carried out (excluding
window control functions which are still handled by a single server application...)
by the process itself. The precise amount of CPU time needed can be calculated
and guaranteed to that process.
Nemesis is also a Single Address Space operating system. An object
in memory will live at the same address visible to every application. This
reduces context switching overhead by cutting down on the memory mapping
required to move from one process to another, and on systems (such as ARM
systems) which have virtually mapped caches, it eliminates the need
to flush the caches on each context switch, allowing higher resolution
scheduling in small chunks of time. This isn't to say that memory protection
is absent; applications are protected from each other, but only the protection
permissions are changed on a context switch. Other applications' data is
still mapped into memory, but inaccessible unless explicitly shared.
This single address space raises all sorts of problems: A library may be used by more than
one program at the same time. Data local to that library cannot be stored
at a fixed address since multiple instances of the data must be kept. All
libraries must know to keep their data at an address specific to the calling
application. Global state in Nemesis is truly global. Since application programs are
also libraries (and can be invoked as multiple processes), most C programs
which rely on global state will not run without modification.
This has been the primary barrier to Nemesis' development as a useful
platform, but has been overcome with the use of such tools as pre-processors
which move 'static' state to structs referenced by a local pointer.
Libraries in Nemesis export their interfaces as a closure, in the
CLU sense of the word: an interface encapsulating some state and functions
(much like a class in C++, but without all the messy vtables). These
interfaces are defined and compiled to C from an abstract Ada-like
language, much like Mach's interfaces.
Nemesis has a CLU-like scripting language, known as Clanger, which
allows libraries to be scripted, and the global namespace manipulated.
More importantly, however, it allows Nemesis types to say "Yes, I do
speak Clanger..." :-)
Far more information than you could ever possibly want to know is available
from the Nemesis homepage at the Computer Lab: