This survey covers hard real-time scheduling algorithms and schedulability analysis techniques for homogeneous multiprocessor systems. It reviews the key results in this field from its origins in the late 1960s to the latest research published in late 2009. The survey outlines fundamental results about multiprocessor real-time scheduling that hold independent of the scheduling algorithms employed. It provides a taxonomy of the different scheduling methods, and considers the various performance metrics that can be used for comparison purposes. A detailed review is provided covering partitioned, global, and hybrid scheduling algorithms, approaches to resource sharing, and the latest results from empirical investigations. The survey identifies open issues, key research challenges, and likely productive research directions.

/pdf/a-survey-of-hard-real-time-scheduling-for-multiprocessor-313o35g6ho.pdf

A survey of hard real-time scheduling for multiprocessor systems

The preemptive scheduling of systems of periodic tasks on a platform comprised of several identical processors is considered A scheduling algorithm is proposed for static-priority scheduling of such systems; this algorithm is a simple extension of the uniprocessor rate-monotonic scheduling algorithm It is proven that this algorithm successfully schedules any periodic task system with a worst-case utilization no more than a third the capacity of the multiprocessor platform It is also shown that no static-priority multiprocessor scheduling algorithm (partitioned or global) can guarantee schedulability for a periodic task set with a utilization higher than one half the capacity of the multiprocessor platform

http://www.cse.chalmers.se/edu/year/2010/course/EDA421/Documents/Articles/Andersson01.pdf

Static-priority scheduling on multiprocessors

Real-time multiprocessor systems are now commonplace. Designs range from single-chip architectures, with a modest number of processors, to large-scale signal-processing systems, such as synthetic-aperture radar systems. For uniprocessor systems, the problem of ensuring that deadline constraints are met has been widely studied: effective scheduling algorithms that take into account the many complexities that arise in real systems (e.g., synchronization costs, system overheads, etc.) are well understood. In contrast, researchers are just beginning to understand the trade-offs that exist in multiprocessor systems. In this chapter, we analyze the trade-offs involved in scheduling independent, periodic real-time tasks on a multiprocessor.

/pdf/a-categorization-of-real-time-multiprocessor-scheduling-1pkxhujs1a.pdf

A categorization of real-time multiprocessor scheduling problems and algorithms

A processor capable of secure execution. The processor contains an execution unit and secure partition logic that secures a partition in memory. The processor also contains cryptographic logic coupled to the execution unit that encrypts and decrypts secure data and code.

Method and apparatus for secure execution using a secure memory partition

This chapter deals with the problem of scheduling a set of tasks to meet deadlines on a computer with multiple processors. Static-priority scheduling is considered, that is, a task is assigned a priority number that never changes and at every moment the highest-priority tasks that request to be executed are selected for execution. The performance metric used is the capacity that tasks can request without missing a deadline. It is shown that every static-priority algorithm can miss deadlines although close to 50% of the capacity is requested. The new algorithms in this chapter have the following performance. In periodic scheduling, the capacity that can be requested without missing a deadline is: 33% for migrative scheduling and 50% for non-migrative scheduling. In aperiodic scheduling, many performance metrics have been used in previous research. With the aperiodic model used in this chapter, the new algorithms in this chapter have the following performance. The capacity that can be requested without missing a deadline is: 50% for migrative scheduling and 31% for non-migrative scheduling.

Static-Priority Scheduling on Multiprocessors

The problem of jointly scheduling both hard deadline periodic tasks and soft aperiodic tasks has been the subject of considerable research in real-time systems. One of the most widely accepted solutions for this problem are slack stealing algorithms. However, these algorithms are rather impractical, since they all imply a considerable scheduler overhead. This paper faces the overhead problem by introducing a complete hardware architecture that implements slack stealing in hardware using an optimal algorithm that has been completely redesigned to perform efficiently in hardware. The proposed solution is a circuit that behaves as a kind of sophisticated interrupt controller that takes the task workload and the interrupts as inputs, and provides an output to inform the CPU which is the highest priority task. From the point of view of hardware design, the algorithm involves two main problems: first, to select the highest priority task at every moment and, second, to locate a set of slack gaps in a real-time computation. Locating slack gaps in a real-time computation is a problem that requires to "look forward in time" into the forecast schedule of a given workload. This paper presents a novel architecture that shows how to solve this problem in an efficient way using an event-driven simulation. A timing analysis of the proposed design is also presented.

A hardware scheduler for complex real-time systems

The paper introduces improvements in partitioning schemes for multiprocessor real time systems which allow higher processor utilization and enhanced schedulability by using exact feasibility tests to evaluate the schedulability limit of a processor. The paper analyzes how to combine these tests with existing bin packing algorithms for processor allocation and provides new variants which are exhaustively evaluated using two assumptions: variable and fixed number of processors. The problem of evaluating these algorithms is complex, since metrics are usually based on comparing the performance of a given algorithm to an optimal one, but determining optimals is often NP hard on multiprocessors. This problem has been overcome by defining lower or upper bounds on the performance of an optimal algorithm and then defining metrics with respect these bounds. The evaluation has shown that the algorithms exhibit extremely good behavior and they can be considered very close to the maximum achievable utilization. It is also shown that dynamic priority policies produce significantly better results than fixed priority policies when task sets require high processor utilizations.

Using exact feasibility tests for allocating real-time tasks in multiprocessor systems

Linux has revealed in the last few years as an appealing option for developing embedded systems but there are some extra requirements of embedded systems that Linux does not fulfill, such as real-time capabilities, file system size and specific hardware support. Real-time features can be achieved with Real-Time Linux GPL (hereafter RTLinux), which is a small, deterministic, real-time kernel that handles time-critical tasks and runs Linux as its lowest priority execution thread. However RTLinux has also important drawbacks. One of them is that real-time tasks cannot make use of Linux services and, in particular, TCP/IP networking. This paper describes RTL-lwIP, which is a TCP/IP stack for embedded systems based on the lwIP (lightweight) TCP/IP stack that runs on RTLinux and can be used by real-time tasks. RTL-lwIP allows real-time tasks to communicate directly with remote real-time tasks or even with Linux user processes. The importance of introducing TCP/IP on RTLinux is that it enables the possibility of developing real-time distributed embedded systems based on CORBA, thus allowing interoperability with other platforms and Web-integration. This paper describes the porting of the lwIP TCP/IP stack to RTLinux and gives some guidelines in order to implement RTLinux drivers for Ethernet cards using as example the implementation of a RTLinux driver for the 3Com905C-X NIC (Network Interface Card).

Building distributed embedded systems with RTLinux-GPL

The problem of jointly scheduling both hard deadline periodic tasks and soft aperiodic tasks has been the subject of considerable research in real-time systems. The main goal of such a system is to minimize the response time of soft aperiodic tasks, without jeopardizing the hard deadlines of periodic tasks. Although, several approaches have been developed to schedule critical workloads on multiprocessors and to manage aperiodic tasks on monoprocessors, the proposed problem has not been fully solved for multiprocessors and integrated solutions are still required. This paper addresses the problem of scheduling an aperiodic workload on a multiprocessor system, where every processor has been previously allocated a critical periodic workload. Such pre-allocated workload is the result of executing a partitioning algorithm in order to ensure the timeliness of the critical workload. Two different approaches are used in order to minimize the response time of the aperiodic workload: aperiodic on-line assignment and global aperiodic scheduling. On highly loaded systems, the results show that the global aperiodic scheduling obtains aperiodic average response times between 20% and 50% lower than those obtained by the on-line assignment approach.

Soft aperiodic task scheduling on hard real-time multiprocessor systems

This paper describes an architecture for distributed computing on the RTLinux-GPL(GPL version of RTLinux, hereafter RTLinux) platform. The proposed architecture implements the CORBA (common object request broker architecture) model of computation, more specifically the minimum CORBA specification for embedded systems, and also provide the extensions for real-time computing. Since RTLinux lacks networking capabilities, the architecture implements all the required functionality in a layered fashion: network drivers, TCP/IP stack and an ORB (object request broker). Most of this work consists on a set of partings of some widely known open source Linux projects to RTLinux: Linux Ethernet device drivers, the lwIP (lightweight IP) TCP/IP stack and ORBit, which is a C language implementation of CORBA. This paper also describes some ORBit modifications that make ORBit to comply with the minimum CORBA specification

A CORBA based architecture for distributed embedded systems using the RTLinux-GPL platform

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