Khawar M. Zuberi

Scheduling messages on controller area network for real-time CIM applications

Scheduling messages on the controller area network (CAN) corresponds to assigning identifiers (IDs) to messages according to their priorities. In this paper we present the mixed traffic scheduler (MTS) which provides higher schedulability than fixed-priority schemes, like deadline-monotonic (DM), while incurring less overhead than dynamic earliest-deadline (ED) scheduling. Through simulations, we compare the performance of MTS with that of DM and ED* (an imaginary scheduler which works like ED, except it incurs less overhead). Our simulations show that MTS performs much better than DM and at the same level as ED*, except under high loads and tight deadlines, when ED* is superior.

Non-preemptive scheduling of messages on controller area network for real-time control applications

Scheduling messages on the controller area network (CAN) corresponds to assigning identifiers (IDs) to messages according to their priorities. If fixed priority scheduling such as deadline monotonic (DM) is used to calculate these priorities, then in general, it will result in low schedulability. Dynamic scheduling schemes such as earliest deadline (ED) can give greater schedulability, but they are not practical for CAN because if the ID is to reflect message deadlines then a long ID must be used. This increases the length of each message to the point that ED is no better than DM. Our solution to this problem is the mixed traffic scheduler (MTS), which is a cross between ED and DM, and provides high schedulability without needing long IDs. Through simulations, we compare the performance of MTS with that of DM and ED* (an imaginary scheduler which works like ED, except it needs only short IDs). We use a realistic workload in our simulations based on messages typically found in computer integrated manufacturing. Our simulations show that MTS performs much better than DM and at the same level as ED*, except under high loads and tight deadlines, when ED* is superior.

Design and implementation of efficient message scheduling for controller area network

The Controller Area Network (CAN) is being widely used in real-time control applications such as automobiles, aircraft, and automated factories. In this pacer, we present the mixed traffic scheduler (MTS) for CAN, which provides higher schedulability than fixed-priority schemes like deadline-monotonic (DM) while incurring less overhead than dynamic earliest-deadline (ED) scheduling. We also describe how MTS can be implemented on existing CAN network adapters such as Motorolas TouCAN. In previous work, we had shown MTS to be far superior to DM in schedulability performance. In this paper, we present implementation overhead measurements showing that processing needed to support MTS consumes only about 5 to 6 percent of CPU time. Considering its schedulability advantage, this makes MTS ideal for use in control applications.

EMERALDS: a small-memory real-time microkernel

EMERALDS (Extensible Microkernel for Embedded, ReAL-time, Distributed Systems) is a real-time microkernel designed for small-memory embedded applications. These applications must run on slow (15-25MHz) processors with just 32-128 kbytes of memory, either to keep production costs down in mass-produced systems or to keep weight and power consumption low. To be feasible for such applications, the OS must not only be small in size (less than 20 kbytes), but also have low-overhead kernel services. Unlike commercial embedded OSs which rely on carefully-crafted code to achieve efficiency, EMERALDS takes the approach of re-designing the basic OS services of task scheduling, synchronization, communication, and system call mechanism by using characteristics found in small-memory embedded systems, such as small code size and a priori knowledge of task execution and communication patterns. With these new schemes, the overheads of various OS services are reduced 20-40% without compromising any OS functionality.

EMERALDS: a microkernel for embedded real-time systems

EMERALDS (Extensible Microkernel for Embedded Real Time Distributed Systems) is a real time microkernel designed for cost conscious small to medium size embedded systems. It not only offers standard OS services like multi threaded processes, real time scheduling, protected address spaces, message passing, semaphores, and timers, but does so in an efficient manner while keeping the kernel size to just tens of kilobytes. For efficiency, EMERALDS uses the novel approach of mapping the kernel into each user level address space, so even with full memory protection, system calls do not need context switches unless a user level server is involved. EMERALDS also provides the flexibility for users to add communication protocol stacks and device drivers as user level servers without modifying the kernel. We have completed a uniprocessor version of EMERALDS for the Motorola 68040 processor whose size is under 13 KBytes. Context switch takes under 12 /spl mu/s and system calls have overhead just 1.8 /spl mu/s more than that of simple subroutine calls.

An efficient semaphore implementation scheme for small-memory embedded systems

In object-oriented programming, updates to the state variables of objects (by the methods of the object) have to be protected through semaphores to ensure mutual exclusion. Semaphore operations are invoked each time an object is accessed, and this represents significant run-time overhead. This is of special concern in cost-conscious, small-size embedded systems-such as those used in automotive applications-where costs must be kept to an absolute minimum. Object-oriented programming can be feasible in such applications only if the OS provides efficient, low-overhead semaphores. The authors present a new semaphore implementation scheme which saves one context switch per semaphore lock operation in most circumstances and gives performance improvements of 18-25% over traditional semaphore implementation schemes.

A causal message ordering scheme for distributed embedded real-time systems

In any distributed system, messages must be ordered according to their cause-and-effect relation to ensure correct behavior of the system. Causal ordering is also essential for services like atomic multicast and replication. In distributed real-time systems, not only must proper causal ordering be ensured, but message deadlines must be met as well. Previous algorithms which ensure such behavior include the /spl Delta/-protocol family and the MARS approach. However, both these algorithms give large response times by delaying all messages for a fixed period of time. In this paper we show that for small- to medium-sized real-time systems (consisting of a few tens of nodes) as are commonly used for embedded applications, it becomes feasible to extend the h-protocol so that instead of delaying all messages for a fixed period, each message is delayed according to its deadline. Our algorithm requires certain message deadlines to be adjusted by the application designer and we show that for small-scale applications such as those used in embedded systems, this adjustment is feasible and can be automated by the use of proper CAD tools.

EMERALDS-OSEK: A Small Real-Time Operating System for Automotive Control and Monitoring

Increasingly, microcontrollers are being used in automotive systems to handle sophisticated control and monitoring activities. As applications become more sophisticated, their design and development becomes complex, necessitating the use of an operating system to manage the complexity and provide an abstraction for improving portability of code. This paper presents EMERALDS-OSEK, an operating system we have designed and implemented based on OSEK/VDX, an open industry standard. We present some of the features and optimizations that make EMERALDSOSEK appropriate for small, low-cost microcontrollers typically found in automotive applications. We also present measurements of operating system performance. We find EMERALDS-OSEK to be efficient, both in terms of processing overheads and memory usage. However, we also find some parts of the OSEK standard that may be improved, and present our ideas for such improvements.

Real-time decentralized control with CAN

This paper deals with two important concerns in decentralized control: (1) communication over field bus and (2) the transition from a single powerful, centralized controller to many smaller, distributed controllers. The former requires real-time network scheduling for the field bus to ensure that messages get delivered in a timely manner, while the latter requires a low-overhead real-time operating system to cater to the specific needs of distributed control. We present solutions to both of these problems in the form of a network scheduling scheme for the controller area network (CAN) field bus and the EMERALDS real-time operating system which we have designed with the needs of the decentralized control application designer in mind.