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28
RealTime Computing with LockFree Shared Objects
 ACM Transactions on Computer Systems
, 1995
"... This paper considers the use of lockfree shared objects within hard realtime systems. As the name suggests, lockfree shared objects are distinguished by the fact that they are not locked. As such, they do not give rise to priority inversions, a key advantage over conventional, lockbased objects ..."
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Cited by 54 (8 self)
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This paper considers the use of lockfree shared objects within hard realtime systems. As the name suggests, lockfree shared objects are distinguished by the fact that they are not locked. As such, they do not give rise to priority inversions, a key advantage over conventional, lockbased objectsharing approaches. Despite this advantage, it is not immediately apparent that lockfree shared objects can be employed if tasks must adhere to strict timing constraints. In particular, lockfree object implementations permit concurrent operations to interfere with each other, and repeated interferences can cause a given operation to take an arbitrarily long time to complete. The main contribution of this paper is to show that such interferences can be bounded by judicious scheduling. This work pertains to periodic, hard realtime tasks that sharelockfree objects on a uniprocessor. In the first part of the paper, scheduling conditions are derived for such tasks, for both static and dynamic pri...
A Framework for Implementing Objects and Scheduling Tasks in LockFree RealTime Systems
 In Proceedings of the 17th IEEE RealTime Systems Symposium
, 1996
"... We present an integrated framework for developing realtime systems in which lockfree algorithms are employed to implement shared objects. There are two key objectives of our work. The first is to enable functionality for object sharing in lockfree realtime systems that is comparable to that in lo ..."
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Cited by 27 (16 self)
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We present an integrated framework for developing realtime systems in which lockfree algorithms are employed to implement shared objects. There are two key objectives of our work. The first is to enable functionality for object sharing in lockfree realtime systems that is comparable to that in lockbased systems. Our main contribution toward this objective is an efficient approach for implementing multiobject lockfree operations and transactions. A second key objective of our work is to improve upon previously proposed scheduling conditions for tasks that share lockfree objects. When developing such conditions, the key issue is to bound the cost of operation "interferences". We present a general approach for doing this, based on linear programming. 1. Introduction Mostworkon implementing shared objects in preemptive realtime uniprocessor systems has focused on using critical sections to ensure object consistency. The main problem that arises when using critical sections is that ...
Efficient Object Sharing in QuantumBased RealTime Systems  Summary by Andreas Haeberlen
 In Proceedings of the 19th IEEE RealTime Systems Symposium
, 1998
"... The authors present a new scheme for implementing shared objects on a realtime system [1]. They assume that the system allocates processor time in discrete quanta, and that the quantum is large compared to the length of an object call. Under these conditions, most object calls are likely to execute ..."
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Cited by 21 (5 self)
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The authors present a new scheme for implementing shared objects on a realtime system [1]. They assume that the system allocates processor time in discrete quanta, and that the quantum is large compared to the length of an object call. Under these conditions, most object calls are likely to execute without preemption, thus allowing the usage of simpler and more efficient access mechanisms.
Efficient and Flexible Fair Scheduling of Realtime Tasks on Multiprocessors
 University
, 2003
"... ..."
An improved schedulability test for uniprocessor periodic task systems
 In Proceedings of EuroMicro Conference on RealTime Systems
, 2003
"... We present a sufficient lineartime schedulability test for preemptable, asynchronous, periodic task systems with arbitrary relative deadlines, scheduled on a uniprocessor by an optimal scheduling algorithm. We show analytically and empirically that this test is more accurate than the commonlyused d ..."
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Cited by 18 (0 self)
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We present a sufficient lineartime schedulability test for preemptable, asynchronous, periodic task systems with arbitrary relative deadlines, scheduled on a uniprocessor by an optimal scheduling algorithm. We show analytically and empirically that this test is more accurate than the commonlyused density condition. We also present and discuss the results of experiments that compare the accuracy and execution time of our test with that of a pseudopolynomialtime schedulability test presented previously for a restricted class of task systems in which utilization is strictly less than one. 1
On NonPreemptive Scheduling of Recurring Tasks Using Inserted Idle Times
 Information and Computation
, 1995
"... We consider the problem of nonpreemptively scheduling periodic and sporadic task systems on one processor using inserted idle times. For periodic task systems, we prove that the decision problem of determining whether a periodic task system is schedulable for all start times with respect to the cla ..."
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Cited by 17 (0 self)
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We consider the problem of nonpreemptively scheduling periodic and sporadic task systems on one processor using inserted idle times. For periodic task systems, we prove that the decision problem of determining whether a periodic task system is schedulable for all start times with respect to the class of algorithms using inserted idle times is NPHard in the strong sense, even when the deadlines are equal to the periods. We then show that if there exists a polynomial time scheduling algorithm which correctly schedules a periodic task system T whenever T is feasible for all start times, then P=NP. We also prove that with respect to the same class of algorithms, the problem of determining whether there exist start times for which a periodic task system is feasible is also NPHard in the strong sense even when the deadlines are equal to the periods. The second part of the paper concentrates on sporadic task systems and inserted idle times. It seems reasonable to suppose that to insert idl...
A LockFree Approach to Object Sharing in RealTime Systems
, 1997
"... This work aims to establish the viability of lockfree object sharing in uniprocessor realtime systems. Naive usage of conventional lockbased objectsharing schemes in realtime systems leads to unbounded priority inversion. A priority inversion occurs when a task is blocked by a lowerpriority ta ..."
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Cited by 16 (0 self)
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This work aims to establish the viability of lockfree object sharing in uniprocessor realtime systems. Naive usage of conventional lockbased objectsharing schemes in realtime systems leads to unbounded priority inversion. A priority inversion occurs when a task is blocked by a lowerpriority task that is inside a critical section. Mechanisms that bound priority inversion usually entail kernel overhead that is sometimes excessive. We propose that lockfree objects offer an attractive alternative to lockbased schemes because they eliminate priority inversion and its associated problems. On the surface, lockfree objects may seem to be unsuitable for hard realtime systems because accesses to such objects are not guaranteed to complete in bounded time. Nonetheless, we present scheduling conditions that demonstrate the applicability of lockfree objects in hard realtime systems. Our scheduling conditions are applicable to schemes such as ratemonotonic scheduling and earliestdeadline...
Feasibility intervals for the deadline driven scheduler with arbitrary deadlines
, 1999
"... In this paper, we extend the determination of feasibility intervals to task sets with arbitrary deadlines, both in the synchronous and the asynchronous case. Meanwhile we also improve the arguments and results generally found in the literature. ..."
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Cited by 15 (6 self)
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In this paper, we extend the determination of feasibility intervals to task sets with arbitrary deadlines, both in the synchronous and the asynchronous case. Meanwhile we also improve the arguments and results generally found in the literature.
The knowledge model of Prot6g62000: combining interoperability and flexibility
 In Proceedings of the 2nd International Conference on Knowledge Engineering and Knowledge Management (EKAW'2000), JuanlesPins
, 2000
"... Optimal online scheduling algorithms are known for sporadic task systems scheduled upon a single processor. Additionally, optimal online scheduling algorithms are also known for restricted subclasses of sporadic task systems upon an identical multiprocessor platform. The research reported in this ar ..."
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Cited by 9 (0 self)
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Optimal online scheduling algorithms are known for sporadic task systems scheduled upon a single processor. Additionally, optimal online scheduling algorithms are also known for restricted subclasses of sporadic task systems upon an identical multiprocessor platform. The research reported in this article addresses the question of existence of optimal online multiprocessor scheduling algorithms for general sporadic task systems. Our main result is a proof of the impossibility of optimal online scheduling for sporadic task systems upon a system comprised of two or more processors. The result is shown by finding a sporadic task system that is feasible on a multiprocessor platform that cannot be correctly scheduled by any possible online, deterministic scheduling algorithm. Since the sporadic task model is a subclass of many more general realtime task models, the nonexistence of optimal scheduling algorithms for the sporadic task systems implies nonexistence for any model which generalizes the sporadic task model. The sporadic task model [18, 16] has received tremendous research attention over the years for its usefulness in modeling recurring processes for hardrealtime systems. A sporadic task τi = (ei, di, pi) is characterized by a worstcase execution requirement ei, a (relative) deadline di, and a minimum interarrival separation pi, which
Algorithms for determining the load of a sporadic task system
, 2005
"... In this report, we discuss a metric that characterizes the load of a sporadic task system. We give an exact, exponentialtime algorithm that determines a task system’s load by essentially simulating the execution of task system. In addition, we also give an algorithm that can determine the load of a ..."
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Cited by 8 (5 self)
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In this report, we discuss a metric that characterizes the load of a sporadic task system. We give an exact, exponentialtime algorithm that determines a task system’s load by essentially simulating the execution of task system. In addition, we also give an algorithm that can determine the load of a task system within an arbitrarily small threshold ɛ> 0. While the worstcase time complexity of the approximation is still possibly exponential, we have empirically observed that this algorithm generally provides a very significant reduction in the time that we must simulate the task system to obtain the load. Additionally, we provide proofs of correctness for our algorithms. 1