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Journal ArticleDOI

A real-time locking protocol

01 Jul 1991-IEEE Transactions on Computers (IEEE Computer Society)-Vol. 40, Iss: 7, pp 793-800
TL;DR: It is shown that this protocol leads to freedom from mutual deadlock and can be used by schedulability analysis to guarantee that a set of periodic transactions using this protocol can always meet its deadlines.
Abstract: The authors examine a priority driven two-phase lock protocol called the read/write priority ceiling protocol. It is shown that this protocol leads to freedom from mutual deadlock. In addition, a high-priority transactions can be blocked by lower priority transactions for at most the duration of a single embedded transaction. These properties can be used by schedulability analysis to guarantee that a set of periodic transactions using this protocol can always meet its deadlines. Finally, the performance of this protocol is examined for randomly arriving transactions using simulation studies. >

Summary (2 min read)

1. Introduction

  • In a real-time database context, concurrency control protocols must not only maintain the consistency constraints of the database but also satisfy the timing requirements of the transactions accessing the database.
  • Next, the authors process transactions in priority order.
  • The preemption of T3, and hence the blocking of T1, will continue until T2 and any other pending intermediate-priority level transactions are completed.
  • An objective of this paper is to design an appropriate priority management protocol for a given concurrency control protocol so that deadlocks can be avoided and the duration of blocking can be tightly bounded.

2.1. Basic Concepts

  • Real-time databases are often used by applications such as tracking.
  • The authors assume that an embedded transaction consists of a sequence of read and write operations operating upon the database.
  • Task τ0 will therefore continue and execute its transaction, thereby effectively preempting T2 in its transaction and not encountering any blocking.

2.2. Definitions and Properties

  • Having reviewed the basic concepts, the authors now review their assumptions and state the notation used.
  • The authors also assume that a transaction does not attempt to lock an object that it has already locked and thus deadlock with itself.
  • Under the read- or write-priority ceiling protocol, mutual deadlock of transactions cannot occur and each task can be blocked by at most one embedded transaction until it completes or suspends itself.
  • The authors now develop a set of sufficient conditions under which a set of periodic tasks with hard deadlines at the end of the periods can be scheduled by the rate-monotonic algorithm [15] when the read- or write-priority ceiling protocol is used.

3. Performance Evaluation

  • In the previous section, the authors have assumed that all the tasks are periodic and that all the database objects are in the main memory.
  • For each experiment and for each algorithm tested, the authors collected performance statistics and averaged over 10 runs.
  • The two important factors affecting the performance of locking protocols are their abilities to resolve the locking conflicts and to perform the I/O and transactions in parallel.
  • The main weakness of the read- or write-priority ceiling protocol is its inability to perform I/O and transactions in parallel.
  • In their experiments, each transaction’s deadline is set proportional to its size and system workload (number of transactions), and the transaction with the shorter deadline is assigned a higher priority.

4. Conclusions

  • Real-time database is an important area of research, with applications ranging from surveillance to reliable manufacturing and production control.
  • The authors have investigated the read- or write-priority ceiling protocol, which integrates the two-phase lock protocol with priority-driven real-time scheduling.
  • The authors have also developed schedulability bounds for periodic tasks in a centralized in-core database.
  • Finally, the authors experimentally evaluated the performance of this protocol when the tasks are invoked aperiodically and the database is no longer in-core.

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Content maybe subject to copyright    Report

Technical Report
CMU/SEI-89-TR-018
ESD-89-TR-026
A Real-Time Locking Protocol
Lui Sha
Ragunathan Rajkumar
Sang Son
Chun-Hyon Chang
April 1989

A Real-Time Locking Protocol
AB
Software Engineering Institute
Carnegie Mellon University
Pittsburgh, Pennsylvania 15213
Technical Report
CMU/SEI-89-TR-018
ESD-89-TR-026
April 1989
Lui Sha
Real-Time Scheduling in Ada Project
Ragunathan Rajkumar
Carnegie Mellon University
Sang Son
University of Virginia
Chun-Hyon Chang
Kon Kuk University, Seoul, Korea
Unlimited distribution subject to the copyright.

This report was prepared for the SEI Joint Program Office HQ ESC/AXS
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FOR THE COMMANDER
(signature on file)
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Copyright 1989 by Carnegie Mellon University.
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CMU/SEI-89-TR-18 1
A Real-Time
Locking Protocol
Abstract: When a database system is used in a real-time application, the concur-
rency control protocol must satisfy not only the consistency of shared data but
also the timing constraints of the application. In this paper, we examine a priority-
driven two-phase lock protocol called the read- or write-priority ceiling protocol.
We show that this protocol is free of deadlock, and in addition a high-priority trans-
action can be blocked by lower priority transactions for at most the duration of a
single embedded transaction. We then evaluate system performance experimen-
tally.
1. Introduction
In a real-time database context, concurrency control protocols must not only maintain the
consistency constraints of the database but also satisfy the timing requirements of the trans-
actions accessing the database.
Both concurrency control [2, 3, 4, 5, 7, 16, 17, 18, 20, 21, 23, 26] and real-time scheduling
algorithms [10, 11, 13, 14, 15, 19, 22, 27] are active areas of research in their own right. It
may seem that the development of a real-time locking protocol is a simple matter of combin-
ing priority scheduling with a locking protocol. For example, we may require each trans-
action to use a well-known concurrency protocol such as the two-phase lock protocol [6] and
assign priorities to transactions according to some well-known scheduling algorithms such
as the earliest deadline algorithm [19]. Next, we process transactions in priority order. Un-
fortunately, such an approach may lead to unbounded priority inversion, in which a high-
priority task would wait for lower priority tasks for an indefinite period of time.
Example 1: Suppose T
1
, T
2
, and T
3
are three transactions arranged in descending order of
priority, with T
1
having the highest priority. Assume that transaction T
1
and T
3
share the
same data object O. Suppose that at time
t
1
transaction T
3
obtains a write-lock on O.
During the execution of T
3
, the high-priority task T
1
arrives and attempts to read-lock the
object O. Transaction T
1
will be blocked, since O is already write-locked. We would expect
that T
1
, being the highest priority transaction, will be blocked no longer than the time for T
3
to complete and unlock O. However, the duration of blocking may, in fact, be unbounded.
This is because transaction T
3
can be preempted by the intermediate-priority transaction T
2
that does not need to access O. The preemption of T
3
, and hence the blocking of T
1
, will
continue until T
2
and any other pending intermediate-priority level transactions are com-
pleted.
The blocking duration in Example 1 can be arbitrarily long. This situation can be partially
remedied if transactions are not allowed to be preempted; however, this solution is only ap-
propriate for very short transactions, because it creates unnecessary blocking. For instance,

Citations
More filters
Proceedings ArticleDOI
03 Dec 1997
TL;DR: This approach reduces the gap between dynamic priority and fixed priority scheduling with the goal of reducing the average response time of soft tasks.
Abstract: The problem of effectively scheduling soft tasks whilst guaranteeing the behaviour of hard tasks has been addressed in many papers and a large number of techniques have been proposed. The dual priority mechanism is an intuitively simple method with low overheads. A hard task is assigned two priorities. Upon invocation, the task starts executing with a low priority and it is promoted to a high priority at a time that will guarantee that its deadline is met. Soft tasks are assigned medium priorities; they can thus preempt any hard task that is executing before its promotion time. To increase the capacity for soft tasks, and therefore the effectiveness of the real-time system, hard tasks may be assigned a (/sub m//sup n/)-hard (read n in m) temporal constraint. This implies that the task must meet n deadlines in any m invocations. This paper addresses the combination of such constraints and dual priority scheduling. This approach reduces the gap between dynamic priority and fixed priority scheduling with the goal of reducing the average response time of soft tasks.

70 citations

Journal ArticleDOI
Azer Bestavros1
01 Mar 1996
TL;DR: More recent advances in RTDB systems research are presented, including the first collection of papers with structured treatments and extensive discussions on RTDB, marking the first significant milestone of the RTDB research area.
Abstract: A Real-Time DataBase (RTDB) system can be viewed as an amalgamation of a conventional DataBase Management System (DBMS) and a real-time system. Like a DBMS, it has to process transactions and guarantee basic correctness criteria. Furthermore, it has to operate in real-time, satisfying timing constraints imposed on transaction commitments and on the temporal validity of data. A RTDB system may exist as a stand-alone system or as an embedded component in a larger multidatabase system. Although various issues related to the concurrency control and transaction scheduling with timing constraints have been studied earlier, the publication of a 1988 special issue of ACM SIGMOD Record on Real-Time DataBases [22] presented the first collection of papers with structured treatments and extensive discussions on RTDB, marking the first significant milestone of the RTDB research area. Today, almost nine years later, we are pleased to present in this book more recent advances in RTDB systems research.

69 citations


Cites background from "A real-time locking protocol"

  • ...Performance evaluation studies of these concurrency control techniques can be found in [3, 29, 57, 31, 1, 28, 30]....

    [...]

Proceedings ArticleDOI
01 Jun 1993
TL;DR: This paper proposes a new cost conscious real-time transaction scheduling algorithm which considers dynamic costs associated with a transaction, and shows its superiority over EDF-HP algorithm in simulations.
Abstract: Real-time databases are an important component of embedded real-time systems. In a real-time database context, transactions must not only maintain the consistency constraints of the database but must also satisfy the timing constraints specified for each transaction. Although several approaches have been proposed to integrate real-time scheduling and database concurrency control methods, none of them take into account the dynamic cost of scheduling a transaction. In this paper, we propose a new cost conscious real-time transaction scheduling algorithm which considers dynamic costs associated with a transaction. Our dynamic priority assignment algorithm adapts to changes in the system load without causing excessive numbers of transaction restarts. Our simulations show its superiority over EDF-HP algorithm.

66 citations

Proceedings ArticleDOI
Bestavros1, Braoudakis1
07 Dec 1994
TL;DR: This work proposes a Speculative Concurrency Control technique (SCC) technique that minimizes the impact of block ages and rollbacks, and presents a number of SCC-based algorithms that differ in the level of speculation they introduce, and the amount of System resources they require.
Abstract: Various concurrency control algorithms differ in the time when conflicts are detected, and in the way they are resolved. Pessimistic (PCC) protocols detect conflicts as soon as they occur and resolve them using blocking. Optimistic (OCC) protocols detect conflicts at transaction commit time and resolve them using rollbacks. For real-time databases, blockages and rollbacks are hazards that increase the likelihood of transactions missing their deadlines. We propose a Speculative Concurrency Control (SCC) technique that minimizes the impact of block ages and rollbacks. SCC relies on added system resources to speculate on potential serialization orders, ensuring that if such serialization orders materialize, the hazards of blockages and roll-back are minimized. We present a number of SCC-based algorithms that differ in the level of speculation they introduce, and the amount of System resources (mainly memory) they require. We show the performance gains (in terms of number of satisfied timing constraints) to be expected when a representative SCC algorithm (SCC-2S) is adopted. >

65 citations

01 Jan 2006
TL;DR: A reconfigurable real-time component model (RTCOM) is defined that describes how a real-Time component, supporting aspects and enforcing information hiding, could efficiently be designed and implemented and a proof-of-concept implementation of a configurable embedded real- time database is presented, called COMET.
Abstract: Our main focus in this thesis is on providing guidelines, methods, and tools for design, configuration, and analysis of configurable and reusable real-time software, developed using a combination of aspect-oriented and component-based software development. Specifically, we define a reconfigurable real-time component model (RTCOM) that describes how a real-time component, supporting aspects and enforcing information hiding, could efficiently be designed and implemented. In this context, we outline design guidelines for development of real-time systems using components and aspects, thereby facilitating static configuration of the system, which is preferred for hard real-time systems. For soft real-time systems with high availability requirements we provide a method for dynamic system reconfiguration that is especially suited for resourceconstrained real-time systems and it ensures that components and aspects can be added, removed, or exchanged in a system at run-time. Satisfaction of real-time constraints is essential in the real-time domain and, for real-time systems built of aspects and components, analysis is ensured by: (i) a method for aspectlevel worst-case execution time analysis; (ii) a method for formal verification of temporal properties of reconfigurable real-time components; and (iii) a method for maintaining quality of service, i.e., the specified level of performance, during normal system operation and after dynamic reconfiguration. We have implemented a tool set with which the designer can efficiently configure a real-time system to meet functional requirements and analyze it to ensure that non-functional requirements in terms of temporal constraints and available memory are satisfied. In this thesis we present a proof-of-concept implementation of a configurable embedded real-time database, called COMET. The implementation illustrates how our methods and tools can be applied, and demonstrates that the proposed solutions have a positive impact in facilitating efficient development of families of realtime systems.

64 citations


Cites background from "A real-time locking protocol"

  • ...Read/write priority ceiling protocol (RWPCP) [154] is an extension of the well-known priority ceiling protocol....

    [...]

References
More filters
Book
03 Jan 1989
TL;DR: In this paper, the problem of multiprogram scheduling on a single processor is studied from the viewpoint of the characteristics peculiar to the program functions that need guaranteed service, and it is shown that an optimum fixed priority scheduler possesses an upper bound to processor utilization which may be as low as 70 percent for large task sets.
Abstract: The problem of multiprogram scheduling on a single processor is studied from the viewpoint of the characteristics peculiar to the program functions that need guaranteed service. It is shown that an optimum fixed priority scheduler possesses an upper bound to processor utilization which may be as low as 70 percent for large task sets. It is also shown that full processor utilization can be achieved by dynamically assigning priorities on the basis of their current deadlines. A combination of these two scheduling techniques is also discussed.

5,397 citations

Book
01 Feb 1987
TL;DR: In this article, the design and implementation of concurrency control and recovery mechanisms for transaction management in centralized and distributed database systems is described. But this can lead to interference between queries and updates.
Abstract: This book is an introduction to the design and implementation of concurrency control and recovery mechanisms for transaction management in centralized and distributed database systems. Concurrency control and recovery have become increasingly important as businesses rely more and more heavily on their on-line data processing activities. For high performance, the system must maximize concurrency by multiprogramming transactions. But this can lead to interference between queries and updates, which concurrency control mechanisms must avoid. In addition, a satisfactory recovery system is necessary to ensure that inevitable transaction and database system failures do not corrupt the database.

3,891 citations

Journal ArticleDOI
TL;DR: An investigation is conducted of two protocols belonging to the priority inheritance protocols class; the two are called the basic priority inheritance protocol and the priority ceiling protocol, both of which solve the uncontrolled priority inversion problem.
Abstract: An investigation is conducted of two protocols belonging to the priority inheritance protocols class; the two are called the basic priority inheritance protocol and the priority ceiling protocol. Both protocols solve the uncontrolled priority inversion problem. The priority ceiling protocol solves this uncontrolled priority inversion problem particularly well; it reduces the worst-case task-blocking time to at most the duration of execution of a single critical section of a lower-priority task. This protocol also prevents the formation of deadlocks. Sufficient conditions under which a set of periodic tasks using this protocol may be scheduled is derived. >

2,443 citations


"A real-time locking protocol" refers background in this paper

  • ...With only two-phase locking and priority assignment, we can encounter the problem of unbounded priority inversion as illustrated in Example 1. However, the idea of priority inheritance [ 24 ] solves the unbounded priority inversion problem....

    [...]

Journal ArticleDOI
TL;DR: It is argued that a transaction needs to lock a logical rather than a physical subset of the database, and an implementation of predicate locks which satisfies the consistency condition is suggested.
Abstract: In database systems, users access shared data under the assumption that the data satisfies certain consistency constraints. This paper defines the concepts of transaction, consistency and schedule and shows that consistency requires that a transaction cannot request new locks after releasing a lock. Then it is argued that a transaction needs to lock a logical rather than a physical subset of the database. These subsets may be specified by predicates. An implementation of predicate locks which satisfies the consistency condition is suggested.

2,031 citations


"A real-time locking protocol" refers methods in this paper

  • ...Each embedded transaction will follow the two-phase lock protocol [ 6 ], which requires a transaction to acquire all the locks before it releases any lock....

    [...]

  • ...For example, we may require each transaction to use a well-known concurrency protocol such as the two-phase lock protocol [ 6 ] and assign priorities to transactions according to some well-known scheduling algorithms such as the earliest deadline algorithm [19]....

    [...]

Proceedings ArticleDOI
05 Dec 1989
TL;DR: An exact characterization of the ability of the rate monotonic scheduling algorithm to meet the deadlines of a periodic task set and a stochastic analysis which gives the probability distribution of the breakdown utilization of randomly generated task sets are represented.
Abstract: An exact characterization of the ability of the rate monotonic scheduling algorithm to meet the deadlines of a periodic task set is represented. In addition, a stochastic analysis which gives the probability distribution of the breakdown utilization of randomly generated task sets is presented. It is shown that as the task set size increases, the task computation times become of little importance, and the breakdown utilization converges to a constant determined by the task periods. For uniformly distributed tasks, a breakdown utilization of 88% is a reasonable characterization. A case is shown in which the average-case breakdown utilization reaches the worst-case lower bound of C.L. Liu and J.W. Layland (1973). >

1,582 citations


"A real-time locking protocol" refers background in this paper

  • ...An exact characterization of rate-monotonic schedulability can be found in [ 12 ]....

    [...]

Frequently Asked Questions (1)
Q1. What are the contributions mentioned in the paper "A real-time locking protocol" ?

In this paper, the authors examine a prioritydriven two-phase lock protocol called the reador write-priority ceiling protocol. The authors show that this protocol is free of deadlock, and in addition a high-priority transaction can be blocked by lower priority transactions for at most the duration of a single embedded transaction. The authors then evaluate system performance experimentally.