evaluation of papers

Qiao XIN (qxin@cs.ucsd.edu)
Thu, 20 Apr 2000 03:02:41 -0700 (PDT)

Evaluation of the Paper: Medusa

This paper described Medusa, a multi-user, object oriented
distributed operating
system for a multimicroprocessor system. The goal of Medusa is to
gain an understanding of the building of a distributed system and
to exploit the hardware with three attributes as modularity,
robustness and performance. In this paper, two software issues
partition and communication corresponding with the distribution
and sharing in the hardware are disscussed. Main charisteristics of
the system are the following:

1. The control structure of the operating system is
distributed. The operating system is divided into disjoint
untilities each of which encapslates a single abstraction for
the rest of the system and exists in its own protected
execution environment with firm boundaries. Messages are used
for communication between utilities.

2. Parallism is implicit and expected in the operating system. All
programs are organized as task force which is a collection of
concurrent. cooperating activities. A task force uses PDL and
SDL to access objects.

I think the implementation and design of Medusa is successful in
achieving it goals, especially robustness. Compared with Star OS,
there are tradeoffs in Medusa in that its structure is made to be
closely matched to the hardware. The object mechanism of Medusa
provides a coarser grain of protection for a highly distributed
operating system.

Evaluation of the paper: Star OS

Star OS is a object-oriented, messaged-based multiprocessor
operating system for the support of task forces. The design of
the operating system is to serve the goal of
providing an extremely flexible set of facilities of high-level

The idea of task force
consisting of many cooperating communicating small processes here
is to maximize parallellism, enhance reliability and adapt hardware.

Similar to Hydra, in Star Os system, all information is encoded
and stored in objects and objects are typed. To access an object,
a process must posses a capability for the object to invoke some
function defined in that object. It has module object following
the data abstraction methodology which support for programming.

The system use scheduler and
multiplexor to ensure that concurrent processes execute in
parallel whenever feasible by message communication between
processes. Functions are divided as instructions(nucleus) performed
sequentially with respect to its invoker and those are
asynchronous. The system has both kernel and user address space
roughly corresponding to the kernel/user mode of a conventional

Compared with Medusa, Star OS has more general notion of task
forces which benefits users but makes it hard to implement
coscheduling which is an important issue for a multiprocessor
system, actually Star OS is a more centralized structure.