One dimension is the scope of
the system -- does it manage one, several, or all the programs in the
environment?  Another dimension is Clearly, the latter is impossible given our constraints
since we have no way of enforcing it.  

degree
of control the service has over programs in the environment.  


as a whole - is there a
single service that mediates between all applications  At one
extreme, the service controls all the resources on the system and
provides reservations to programs.  At the other extreme, the service
controls only the single program it is operating on behalf of.

  Our approach is the latter because it greatly
expands the scope of systems on which the service could be used.
Modifying operating systems is ratre

we are
interested in supporting systems which are unlikely to be modified

.  Clearly, the
greater reliance on having control beyond that of a user-

We want to
provide the service on common platforms without requiring
modifications to them.  This 

  Unless the
system actually controls everything For systems that do not control
everything, it is i

 resource
reservation



 In the space, there is a clear distinction between systems that
control all the programs and systems that
essentially implement or rely on resource reservation, and systems
that provide a 

.  Since this
requires operating system and possibly network changes which are
unlikely on our target systems, we concentrate

 An important axis of the space is the degree to which the
service dependsis more than just an application.  A service that relies on or
implements resource reservation, or requires extensive operating
system modifications may be more reliable than one that, but will certainly be
less ubiquitous than one that does not.  The availability of the
service is important to the user, who would like to be able to run the
program anywhere.


 and the approach we take is aggressively application-oriented,
simple,
%
% Differentiate with other approaches here
% 
%
%



 An important axis of the space is the degree to which the
service is more than just an application.  A service that relies on or
implements resource reservation, or requires extensive operating
system modifications may be more reliable than one that, but will certainly be
less ubiquitous than one that does not.  The availability of the
service is important to the user, who would like to be able to run the
program anywhere.

operating system modifications or extensions, resource reservation
mechanisms, or


 The more imbedded or closely associated the service


One axis is the degree to which it on resource
reservations.  Resource reservations require


At the highest level, the service could either
implement some form of resource reservation, or it could predict the
behavior of the system.  We elliminate the former part of the design
space since it

We concentrate on a small portion of it by adopting the goal
that the service be 

, but we limit it somewhat since we look only at approaches that
operate at user level and require no changes


The service could be an application or it could be part of the operating system.  It could be 

re are many ways that such a service
could be implemented.  The approach we take is aggressively
application oriented and simple.
%
% Differentiate with other approaches here
% 
%
%
The system would then attempt to meet
those bounds by dynamically mapping the nodes of the activation tree
to appropriate hosts as the tree unfolded during execution.



% What is the scope of the system?
%What controls does the system have?
%What does the system measure?
%What 


%The design space of such a service is vast. 
%Consider the following three dimensions: scope,
%controls, and measurement.  

%\begin{itemize}
%\item{foo}
%\end{itemize}

%One way to think of it is In essence, it will
%measure or tabulate some set of properties of the system and then
%apply some form of control.

%One dimension of the space is what control
%the service has over the environment as a whole.  At one extreme are
%designs which which control all the resources in the system and
%mediate among all the programs.  Given our constraints, this sort of
%system is impossible since it would require modificationsAt the other extreme are designs
%which control only the program they are acting on behalf of.