Priority Ceiling Protocol Research Articles

하드와 소프트 실시간 트랜잭션을 위한 통합된 동시성제어 기법

Most concurrency control protocols have been devised for resolving data conflicts among real-time transactions of a single type. Recent real-time database systems should support various types of real-time transactions due to needs of many different types of applications and steady improvement of hardware. In this paper, we propose integrated concurrency control protocol to resolve data conflicts among hard and soft real-time transactions. Our proposed protocol, based on PCP(Priority Ceiling Protocol) for a hard real-time transactions and MVPR(Multiversion with Precedence Relationship), guarantees that hard real-time transactions meet their deadline, and decreases the deadline miss ratio of soft real-time transactions. We also proved that the proposed protocol guarantees serializable schedules and no deadlocks. The performance of the proposed protocol has been compared with other real-time concurrency protocols.

The implementation of the Priority Ceiling Protocol in Ada-2005

Ada 2005 is an even safer and more agile language than its predecessors, with all of the efficiency that is a hallmark of the Ada language design. It still has a strong focus on real-time response, multi-tasking, and sound engineering. It is the only international standard for object-oriented real-time distributed programming. High-integrity systems rarely make use of high-level language features such as Ada Protected Objects. Protected Types provide a passive mechanism for communication and synchronization between tasks. In this paper, we present some of these features, which are implemented using the Priority Ceiling Protocol (PCP). PCP is an extension of the Priority Inheritance Protocol (PIP), with the added features of preventing deadlocks and priority inversions. Priority inversion occurs when a high priority task is blocked for an unbounded time by lower priority tasks. Unfortunately though, PCP is not known to be supported by any operating systems. This article describes an implementation of PCP in Ada-2005. A detailed discussion of this protocol, and other related issues, are presented.

A hardware accelerator for controlling access to multiple-unit resources in safety/time-critical systems

In multitasking, priority-driven systems, resource access-control protocols such as Priority Ceiling Protocol (PCP) reduce the undesirable effects of resource contention. In general, software implementation of these protocols entails costly computations that can degrade the system performance to unacceptable levels. In this paper, we present the design for a hardware accelerator to execute the PCP functionality for controlling access to multiple-unit resources and illustrate that the proposed implementation accelerates the execution time by a factor of up to 30.

Testing for the Conformance of Real-time Protocols Implemented by Operating Systems

The priority inversion problem arises when prioritized processes concurrently attempt to enter critical sections. This phenomenon results in extremely pessimistic estimations of worst case response times for real-time processes. Various protocols against priority inversion have been proposed in the literature and are available at system call level of operating systems and run-time executives. They belong to two major families of protocols: the priority inheritance protocols (PIP) and to the priority ceiling protocols (PCP). These protocols have in common that they allow to derive more optimistic worst case response times.In contrast to the importance of this predictability aspect in the context of time-critical applications a lot of PIP- and PCP-implementations are not correct and permit the violation of time bounds. This article presents an effective and flexible tool set applied here for the validation of the implementations of protocols of the PCP-family. Besides the manual setup and instrumentation major parts of the black-box validation process are executed automatically.

Two-version based concurrency control and recovery in real-time client/server databases

While there has been a significant amount of research in real-time concurrency control, little work has been done in logging and recovery for real-time databases. This paper proposes a two-version approach which considers both real-time concurrency control and recovery. We propose a network-server-based architecture and algorithms which can not only reduce the blocking time of higher-priority transactions and improve the response time of client-side read-only transactions, but also provide a diskless runtime logging mechanism and an efficient and predictable recovery procedure. The performance of the algorithms was verified by a series of simulation experiments by comparing the algorithms with the well-known Priority Ceiling Protocol (PCP), the Read/Write PCP, the New PCP, and the 2-version two-phase locking protocol, for which we have very encouraging results. The schedulability of higher-priority transactions and the response time of client-side read-only transactions were all greatly improved.

The Impacts of Write-Through Procedures and Checkpointing on Real-Time Concurrency Control

In this paper, we study the impacts of checkpointing and write-through procedures, which are critical in maintaining database recoverability and transaction durability, on the performance of a well-known real-time concurrency control protocol, the Read/Write Priority Ceiling Protocol (RWPCP). Although RWPCP can guarantee the schedulability of real-time transactions, it could be unrecoverable, and the priority inversion problems could be unbounded, when transaction commitment is considered. In this paper, we first propose to extend RWPCP with deferred-commitment and extended-locking-period methods to resolve the problems. Then, we study the impacts of different checkpointing granularities and checkpointing methods on the proposed recoverable RWPCP. A detailed simulation study was conducted to evaluate the performance of the recoverable RWPCP with different checkpointing methods under various workloads.

RCPCP: A Ceiling-based Protocol for Multiple-disk Environments

Processes running in a multiple-disk environment may share non-preemptible resources on the processor and, at the same time, request services from disk subsystems. In this paper, we propose a methodology which is efficient and easy to implement for scheduling processes in multiple-disk environments. In other words, our methodology addresses the scheduling of real-time processes which may stop to wait for disk I/O without releasing any locked semaphores. Our proposed methodology is a variation of the well-known priority ceiling protocol to schedule processes running in multiple-disk environments. The capability of the proposed methodology is verified by a series of simulation experiments under different workloads of CPU-bound and I/O-bound processes in multiple-disk environments, for which we have some encouraging experimental results.

Timing analysis of compound scheduling policies: application to posix1003.1b

The analysis of fixed priority preemptive scheduling has been extended in various ways to improve its usefulness for the design of real-time systems. In this paper, we define the layered preemptive priority scheduling policy which generalizes fixed preemptive priorities by combination with other policies in a layered structure. In particular, the combination with the Round Robin scheduling policy is studied. Its compliance with Posix 1003.1b requirements is shown and its timing analysis is provided. For this purpose and as a basis for the analysis of other policies, the concept of majorizing work arrival function, is introduced to synthesize essential ideas used in existing analysis of the fixed preemptive priority policy.If critical resources are protected by semaphores, the Priority Ceiling Protocol (PCP) can be used under fixed preemptive priorities to control resulting priority inversions. An extension of the PCP is proposed for Round Robin, to allow a global control of priority inversions under the layered priority policy and to prevent deadlocks. The initial timing analysis is extended to account for the effects of the protocol. The results are illustrated by a small test case.

Response Time Analysis of Asynchronous Real-Time Systems

In asynchronous real-time systems the time when all events occur can not be predicted beforehand. Systems with sporadic tasks, or that operate a protocol for sharing resources like the priority ceiling protocol, for example, are asynchronous real-time systems. In this paper, we present a sufficient and efficient response time based analysis technique for computing Ri(k), the worst case response time at each invocation k of the periodic tasks of real-time asynchronous systems. In addition, efficient idle time computation for asynchronous systems is presented. This analysis technique can be applied to the analysis of several process models including weakly hard real-time systems, and slack management techniques like aperiodic servers and slack stealing algorithms. It is also shown that the pattern of response times of tasks in a hyperperiod is pseudoperiodic and that the maximum response time instants tend to occur evenly separated within the hyperperiod.

Perfecting preemption threshold scheduling for object-oriented real-time system design

In spite of the proliferation of object-oriented design methodologies in contemporary software development, their application to real-time embedded systems has been limited because of the practitioner's conservative attitude toward handling timing constraints. In fact, this conservative attitude is well-grounded because traditional priority-based scheduling techniques cannot be straightforwardly integrated into them. The automated implementation from the object-oriented real-time designs usually incurs a large number of tasks which, under traditional priority-based scheduling techniques, does not scale well due to excessive preemption overheads. Recently, preemption threshold scheduling was introduced to reduce run-time multi-tasking overhead while improving schedulability by exploiting non-preemptibility as much as possible. Unfortunately, the preemption threshold scheduling cannot be directly adopted into the object-oriented design methods due to the lack of real-time synchronization.In this paper, we present the essential basis of real-time synchronization for preemption threshold scheduling. Specifically, we integrate the priority inheritance protocol, the priority ceiling protocol, and the immediate inheritance protocol into preemption threshold scheduling. We also provide their schedulability analyses. Consequently, the integrated scheme, which minimizes worst-case context switches, is appropriate for the automated implementation of real-time object-oriented design models.

The system-on-a-chip lock cache

In this dissertation, we implement efficient lock-based synchronization by a novel, high performance, simple and scalable hardware technique and associated software for a target shared-memory multiprocessor System-on-a-Chip (SoC). The custom hardware part of our solution is provided in the form of an intellectual property (IP) hardware unit which we call the SoC Lock Cache (SoCLC). SoCLC provides effective lock hand-off by reducing on-chip memory traffic and improving performance in terms of lock latency, lock delay and bandwidth consumption. The proposed solution is independent from the memory hierarchy, cache protocol and the processor architectures used in the SoC, which enables easily applicable implementations of the SoCLC (e.g., as a reconfigurable or partially/fully custom logic), and which distinguishes SoCLC from previous approaches. Furthermore, the SoCLC mechanism has been extended to support priority inheritance with an immediate priority ceiling protocol (IPCP) implemented in hardware, which enhances the hard real-time performance of the system. Our experimental results in a four-processor SoC indicate that SoCLC can achieve up to 37% overall speedup over spin-lock and up to 48% overall speedup over MCS for a microbenchmark with false sharing. The priority inheritance implemented as part of the SoCLC hardware, on the other hand, achieves 1.43X speedup in overall execution time of a robot application when compared to the priority inheritance implementation under the Atalanta real-time operating system. Furthermore, it has been shown that with the IPCP mechanism integrated into the SoCLC, all of the tasks of the robot application could meet their deadlines (e.g., a high priority task with 250us worst case response time could complete its execution in 93us with SoCLC, however the same task missed its deadline by completing its execution in 283us without SoCLC). Therefore, with IPCP support, our solution can provide better real-time guarantees for real-time systems. To automate SoCLC design, we have also developed an SoCLC-generator tool, PARLAK, that generates user specified configurations of a custom SoCLC. We used PARLAK to generate SoCLCs from a version for two processors with 32 lock variables occupying 2,520 gates up to a version for fourteen processors with 256 lock variables occupying 78,240 gates.

A dynamic synchronization protocol and scheduling method based on Timestamp Ordering for real‐time transactions

AbstractIn this article, we propose a dynamic synchronization protocol and a scheduling method based on Timestamp Ordering for real‐time transactions. First, we describe a synchronization protocol that avoids transaction abort by reserving shared resources accessed by transactions. This protocol does not incur unnecessary wait for resources, so it shows excellent performance under time constraint, compared with the priority ceiling protocol used in static scheduling environment. Next, we describe a dynamic scheduling method that takes into account both Timestamp Ordering and priority of transactions. This method does not incur deadlock unlike two‐phase locking, for example, and it can decrease violation of time constraint due to resource waiting. We compare the two proposed methods and other methods by simulation, and discuss the effectiveness of the proposed methods. © 2001 Scripta Technica, Syst Comp Jpn, 32(13): 61–70, 2001

Scheduling and Priority Mapping for Static Real-Time Middleware

This paper presents a middleware real-time scheduling technique for static, distributed, real-time applications. The technique uses global deadline monotonic priority assignment to clients and the Distributed Priority Ceiling protocol to provide concurrency control and priorities for server execution. The paper presents a new algorithm for mapping the potentially large number of unique global priorities required by this scheduling technique to the restricted set of priorities provided by commercial real-time operating systems. This algorithm is called Lowest Overlap First Priority Mapping; we prove that it is optimal among direct priority mapping algorithms. This paper also presents the implementation of these real-time middleware scheduling techniques in a Scheduling Service that meets the interface proposed for such a service in the Real-Time CORBA 1.0 standard. Our prototype Scheduling Service is integrated with the commercial PERTS tool that provides schedulability analysis and automated generation of global and local priorities for clients and servers.

Scheduling transactions with stringent real-time constraints

Many real-time database systems (RTDBS) are used to support mission-critical applications such as avionics control systems and defense systems, in which transactions have stringent real-time constraints. In these hard real-time systems, a transaction missing its deadline could lead to catastrophic consequences. The difficulties of providing a guarantee of meeting transaction deadlines in hard RTDBS lie in the problems of priority inversion and deadlocks. The transaction blocking delay due to priority inversion can be unbounded, which is unacceptable in most hard RTDBS. Several priority ceiling protocols have been proposed to solve these two problems. However, they are too conservative in scheduling transactions for the single-blocking and deadlock-free properties, leading to many unnecessary transaction blockings. The main reason for their conservatism is their implicit assumption that if a transaction's operation conflicts with that of another executing transaction, it is presumably unable to preempt the conflicting transaction. In this paper, we propose a new priority ceiling protocol that allows a higher priority transaction to preempt a lower priority transaction on data conflicts by using the notion of dynamic adjustment of serialization order. We devise a worst-case schedulability analysis for the new protocol which provides a better schedulability condition compared to other protocols. The simulation results show that the new priority ceiling protocol outperforms other protocols.

A conditional abortable priority ceiling protocol for scheduling mixed real-time tasks

Priority Ceiling Protocol (PCP) is a well-known resource access protocol for hard real-time systems. However, it has a problem of ceiling blocking which imposes a great hindrance to task scheduling in mixed real-time systems where tasks may have different criticality. In this paper, a new resource access protocol called the Conditional Abortable Priority Ceiling Protocol (CA-PCP) is proposed. It resolves the problem of ceiling blocking by incorporating a conditional abort scheme into the PCP. The new protocol inherits all the desirable properties of the PCP and the Ceiling Abort Protocol (CAP) which is yet another modification of the PCP. In the proposed protocol, a condition is defined to control the abort of a job so that the schedulability of the system will not be affected. Performance study has been done to compare the CA-PCP with the PCP. The results indicate that CA-PCP can significantly improve the performance of a system if the lengths of the abortable critical sections are well chosen.

Dynamic acceptance of aperiodic tasks with periodic tasks under resource sharing constraints

A real-time system is considered in which a set of periodic tasks are scheduled on an earliest-deadline-first basis and exclusively access critical sections through a dynamic priority ceiling protocol. An optimal online acceptance test is described for hard deadline aperiodic tasks. Aperiodic tasks are accepted only if all deadlines can be met. Here, optimality means that the algorithm is able to answer ‘yes’ to the question of the acceptance of a newly occurring task each time it is possible. The time complexity of the test is O(⌈R/p⌉.n+m) where n is the number of periodic tasks, R is the longest deadline, p is the shortest period and m is the number of aperiodic tasks that have previously been accepted but not completed.

The EDL Server for Scheduling Periodic and Soft AperiodicTasks with Resource Constraints

In this paper, we are concerned with the problem of serving soft aperiodic tasks on a uniprocessor system where periodic tasks are scheduled on a dynamic-priority, preemptive basis and exclusively access to critical sections. Scheduling of tasks is handled by the Dynamic Priority Ceiling Protocol working with an Earliest Deadline scheduler. Our analysis determines the maximum processing time which may be stolen from periodic tasks without jeopardizing both their timing constraints and resource consistency. It provides the basis for an on-line scheduling algorithm, the EDL Server, to deal with the minimization of response times for soft aperiodic tasks.

Deadlock Prevention in a Distributed Real-Time System

Deadlock is a circular wait condition that can occur in any multiprogramming, multiprocessing, or distributed computer system. This article focused on the second, third and fourth models of the Knapp hierarchical model called the AND-model, the OR-model and the AND/OR model. In the last model, a task is blocked until it is granted any of the resources it has requested. The classical solution for the AND-model callled the Priority Ceiling Protocol (PCP) is extended to the AND/OR-model. In the aim to extend this solution to distributed systems, the Rajkumar solution, which consists in the utilization of a global super-priority, is chosen.

How to Schedule Periodic and Sporadic Tasks with Resource Constraints in a Real-Time Computer System

We study the problem of scheduling periodic and sporadic tasks on the nodes of a distributed system. A periodic task consists of an infinite number of requests, each of which has a prescribed deadline. Each node is a monoprocessor machine and we assume that the existence of enough processing power for all periodic tasks is verified at system initialization time. Periodic tasks may interact each other by sharing critical resources. The Dynamic Priority Ceiling Protocol is used to guarantee that each task completes execution before its deadline and each resource is never accessed by more than one task simultaneously. In addition, we allow for the unpredictable occurrence of sporadic tasks. We develop a local scheduling scheme that permits first to test on-line whether or not an occurring hard sporadic task can be guaranteed to meet its timing requirements and second to schedule soft sporadic tasks with a minimal response time.

A Dynamic Scheduling Algorithm for Sporadic Tasks under Resource and Timing Constraints

Currently, we are developing a scheduler for hard real-time computer systems. The system of interest is a uniprocessor machine that supports the operating system and the application software composed of tasks that may be periodic or non-periodic (sporadic). All the tasks are said to be hard because they have strict timing requirements that must not be violated under any circumstances. Tasks may interact with each other by sharing critical resources. The problem is to sequence the tasks such that in all cases it is guaranteed that each task completes execution before its deadline and each resource is never accessed by more than one task simultaneously. We present a scheduling scheme that permits to solve this problem by using the Dynamic Priority Ceiling Protocol.