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/**
\mainpage FSF reference
This reference manual contains the description of the API provided
by the FSF library.
\section intro Introduction to FSF
The First Scheduling Framework (FSF) is the result of a joint
effort from four european research groups to propose an API for
flexible real-time scheduling in Real Time operating systems. See
<a href="http://130.243.76.81:8080/salsart/first/">the FIRST web
page</a> for more details in the project.
\section lib Objectives
Scheduling theory generally assumes that real-time systems are mostly
composed of activities with hard real-time requirements, many systems
are built today by composing different application or components in
the same system, leading to a mixture of many different kinds of
requirements with small parts of the system having hard real-time
requirements and other larger parts with requirements for more
flexible scheduling, taking into account quality of service. Hard
real-time scheduling techniques are extremely pessimistic for the
latter part of the application, and consequently it is necessary to
use techniques that let the system resources be fully utilized to
achieve the highest possible quality.
The FIRST project aims at developing a framework for a scheduling
architecture that provides the ability to compose several applications
or components into the system, and to flexibly schedule the available
resources while guaranteeing hard real-time requirements. The FIRST
Scheduling Framework (FSF) is independent of the underlying
implementation, and can run on different underlying scheduling
strategies. It is based on establishing service contracts that
represent the complex and flexible requirements of the application,
and which are managed by the underlying system to provide the required
level of service.
FSF provides a generalized architecture framework that combines
different kinds of requirements:
- co-operation and coexistence of standard real-time scheduling
schemes, time-triggered and eventtriggered, dynamic and fixed
priority based, as well as off-line based through a common
architecture that is independent on the underlying scheduling
mechanism
- integration of different timing requirements such as hard and soft,
and more flexible notions
- temporal encapsulation of subsystems in order to support the
composability and reusability of available components including
legacy subsystems.
FSF has been implemented in two POSIX compliant real-time operating
systems, <a href="http://marte.unican.es">MaRTE</a> and <a
href="http://shark.sssup.it/">SHARK</a>, which are based on FP and EDF
scheduling schemes, respectively, thus illustrating the platform
independence of the presented approach.
\section Service Contracts
The service contract is the mechanism that we have chosen for the
application to dynamically specify its own set of complex and flexible
execution requirements. From the application s perspective, the
requirements of an application or application component are written as
a set of service contracts, which are negotiated with the underlying
implementation. To accept a set of contracts, the system has to check
as part of the negotiation if it has enough resources to guarantee all
the minimum requirements specified, while keeping guarantees on all
the previously accepted contracts negotiated by other application
components. If as a result of this negotiation the set of contracts is
accepted, the system will reserve enough capacity to guarantee the
minimum requested resources, and will adapt any spare capacity
available to share it among the different contracts that have
specified their desire or ability for using additional capacity.
As a result of the negotiation process, if a contract is accepted, a
server is created for it. The server is a software object that is the
run-time representation of the contract; it stores all the information
related to the resources currently reserved for that contract, the
resources already consumed, and the resources required to handle the
budget consumption and replenishment events in the particular
operating system being used.
Because there are various application requirements specified in the
contract, they are divided into several groups, also allowing the
underlying implementation to give different levels of support trading
them against implementation complexity. This gives way to a modular
implementation of the framework, with each module addressing specific
application requirements. The minimum resources required by the
application to be reserved by the system are specified in the core
module. The requirements for mutual exclusive synchronization among
parts of the application being scheduled by different servers or among
different applications are specified in the shared objects
module. Flexible resource usage is associated with the spare capacity
and dynamic reclamation modules. The ability to compose applications
or application components with several threads of control, thus
requiring hierarchical scheduling of several threads inside the same
server are supported by the hierarchical scheduling module. Finally,
the requirements of distributed applications are supported by the
distributed and the distributed spare capacity modules. We will now
explain these modules together with their associated application
requirements.
*/
/**
\defgroup coremodule Core module
The core module contains the service contract information related to
the application minimum resource requirements, the operations required
to create contracts and negotiate them, and the underlying
implementation of the servers with a resource reservation mechanism
that allows the system to guarantee the resources granted to each
server.
This module includes the basic functions and services that are
provided by any FSF implementation. This module includes basic type
definitions, and functions to
- create a contract and initialize it
- set/get the basic parameters of a contract
- negotiate a service contract, obtaining a server id
- create and bind threads to servers
- create/destroy a synchronization object
- manage bounded workloads
*/
/**
\defgroup sparemodule Spare capacity sharing module
Many applications have requirements for flexibility regarding the
amount of resources that can be used. The spare capacity module allows
the system to share the spare capacity that may be left over from the
negotiation of the service contracts, in a static way. During the
negotiation, the minimum requested resources are granted to each
server, if possible. Then, if there is any extra capacity left, it is
distributed among those applications that have expressed their ability
to take advantage of it.
This module include functions for sharing the spare capacity in the
system between the servers. It allows to mainly to specify
additional contract parameters to allow the sharing of the spare
capacity.
The features provided by this module are different from the
services provided by the Dynamic reclamation module. This module
influences the negotiation procedure (see the \ref core_negotiate
module).
*/
/**
\defgroup hiermodule Hierarchical scheduling module
One of the application requirements that FSF addresses is the ability
to compose different applications, possibly using different scheduling
policies, into the same system. This can be addressed with support in
the system for hierarchical scheduling. The lower level is the
scheduler that takes care of the service contracts, using an
unspecified scheduling policy (for instance, a CBS on top of EDF, or a
sporadic server on top of fixed priorities). The top level is a
scheduler running inside one particular FSF server, and scheduling the
application threads with whatever scheduling policy they were
designed. In this way, it is possible to have in the same system one
application with, for example, fixed priorities, and another one
running concurrently with an EDF scheduler.
We are currently providing three top-level schedulers: fixed
priorities, EDF, and table-driven.
*/
/**
\defgroup shobjmodule Shared Objects module
The shared objects module of FSF allows the application to specify in
the contract attributes all the information required to do the
schedulability analysis.
The set of shared objects present in the system together with the
lists of critical sections specified for each contract are used for
schedulability analysis purposes only. A run-time mechanism for mutual
exclusion is not provided in FSF for two important reasons. One of
them is upward compatibility of previous code using regular primitives
such as mutexes or protected objects (in Ada); this is a key issue if
we want to persuade application developers to switch their systems to
the FSF environment. The second reason is that enforcing worst case
execution time for critical sections is expensive. The number of
critical sections in real pieces of code may be very high, in the tens
or in the hundreds per task, and monitoring all of them would require
a large amount of system resources.
The FSF application does not depend on any particular synchronization
protocol, but there is a requirement that a budget expiration cannot
occur inside a critical section, because otherwise the blocking delays
could be extremely large. This implies that the application is allowed
to overrun its budget for the duration, at most, of the critical
section, and this extra budget is taken into account in the
schedulability analysis.
*/
/**
\defgroup distjmodule Distributed module
@todo Peppe complete description
*/
/**
\defgroup distsparemodule Distributed spare capacity module
@todo Peppe complete description
*/