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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
22 Feb 1993
TL;DR: The author describes an efficient algorithm for determining the feasibility of a transaction with runtime linearly proportional to the number of lock requests in the transaction.
Abstract: The author describes an efficient algorithm for determining the feasibility of a transaction with runtime linearly proportional to the number of lock requests in the transaction. The schedulability checker based on the game-board representation determines transaction schedulability one at a time (incrementally), and thus it also has linear runtime complexity once the checker is initialized. Given the current configuration of the database and the characteristics of the newly arrived transaction, the feasibility test can determine a priori whether the new transaction can be completed by its specified deadline. An example is presented which illustrates how the transaction feasibility checker and the runtime monitor work. >

6 citations

Proceedings ArticleDOI
13 Sep 1995
TL;DR: A new deadlock detection algorithm specially designed for distributed real-time database systems is proposed and results indicated that the performance of the new algorithm is substantially better for softreal-time than that of firm real- time systems.
Abstract: Recently the concurrency control issue of real-time transactions is gaining increasing attention of researchers in the database community. One of the major design issue in concurrency control of real-time transactions is the resolution of local as well as distributed deadlocks while at the same time meeting the timing requirements of the transactions. In this paper, a new deadlock detection algorithm specially designed for distributed real-time database systems is proposed. The performance of the proposed algorithm is evaluated through extensive simulation experiments. Studies have also been carried out to compare the performance of the real-time deadlock detection algorithm with a non real-time algorithm for both firm and soft real-time transactions. Results indicated that the real-time deadlock detection algorithm performs better than the non real-lime deadlock detection algorithm. Results also indicated that the performance of the new algorithm is substantially better for soft real-time than that of firm real-time systems.

6 citations

Journal ArticleDOI
TL;DR: Experimental results show that the proposed hybrid concurrency control algorithm results in more than 35 % reduction in the number of aborts in high-transaction rates as compared to strict two-phase locking algorithm that is used in many commercial database systems.
Abstract: Concurrency control is the activity of synchronizing operations issued by concurrent executing transactions on a shared database. The aim of this control is to provide an execution that has the same effect as a serial (non-interleaved) one. The optimistic concurrency control technique allows the transactions to execute without synchronization, relying on commit-time validation to ensure serializability. Effectiveness of the optimistic techniques depends on the conflict rate of transactions. Since different systems have various patterns of conflict and the patterns may also change over time, so applying the optimistic scheme to the entire system results in degradation of performance. In this paper, a novel algorithm is proposed that dynamically selects the optimistic or pessimistic approach based on the value of conflict rate. The proposed algorithm uses an adaptive resonance theory–based neural network in making decision for granting a lock or detection of the winner transaction. In addition, the parameters of this neural network are optimized by a modified gravitational search algorithm. On the other hand, in the real operational environments we know the writeset (WS) and readset (RS) only for a fraction of transactions set before execution. So, the proposed algorithm is designed based on optional knowledge about WS and RS of transactions. Experimental results show that the proposed hybrid concurrency control algorithm results in more than 35 % reduction in the number of aborts in high-transaction rates as compared to strict two-phase locking algorithm that is used in many commercial database systems. This improvement is 13 % as compared to pure-pessimistic approach and is more than 31 % as compared to pure-optimistic approach.

5 citations


Cites background or methods from "A real-time locking protocol"

  • ...Calculation of TE parameter is similar to priority inheritance idea [37, 38], which is used in solving the priority inversion problem....

    [...]

  • ...The 2VPC is designed based on priority ceiling protocol (PCP) [38] and so it assumes that RS and WS are known before execution....

    [...]

  • ...In addition, knowledge of WS and RS is necessary in 2VPC and PCP algorithms, but there is not such a constraint in the proposed HCC algorithm....

    [...]

Proceedings ArticleDOI
12 Aug 1996
TL;DR: A new disk system which uses a multiple-disk structure similar to a RAID (Redundant Array of Inexpensive Disks) to improve the I/O performance for RTDBSs is proposed and a new disk scheduling algorithm called ED-PT (Earliest Deadline first-Prior Test) for determining the feasibility ofI/O requests is proposed.
Abstract: Time-constrained I/O scheduling is one of the most critical issues in the design and implementation of real-time database systems (RTDBSs) because the I/O time usually dominates the total execution time of a transaction. The performance of single-disk storage systems may not be appropriate for RTDBSs due to the large discrepancy between the disk I/O access/waiting time and the processor/main memory processing time. This paper proposes a new disk system which uses a multiple-disk structure similar to a RAID (Redundant Array of Inexpensive Disks) to improve the I/O performance for RTDBSs. We also propose a new disk scheduling algorithm called ED-PT (Earliest Deadline first-Prior Test) for determining the feasibility of I/O requests. This algorithm gives a priori knowledge about whether an arriving I/O request can meet its deadline, making it possible to reject an infeasible request before it is executed and thus this avoids wasted disk access effort if late completion of requests is not allowed. Our experimental results show that the percentage of requests missing deadlines decreases dramatically in a multiple-disk system when compared to a single disk system.

5 citations

Journal ArticleDOI
01 Dec 1995
TL;DR: This bibliography presents a collection of papers on various aspects of real-time database systems that have been published or identified so far, a field which has emerged as a result of the growing interest in applying the principles and techniques ofreal-time scheduling to transaction management in DBMSs.
Abstract: 1 I n t r o d u c t i o n This is an annotated bibliography on real-time database systems, a field which has emerged as a result of the growing interest in applying the principles and techniques of real-time scheduling to transaction management in DBMSs. Today, many application areas supported by a DBMS are characterized by the requirement of timely access to the underlying database. Among those application areas are information retrieval systems, airline reservation systems, stock market, banking, and computer-integrated manufacturing. In addition to maintaining database consistency, an essential scheduling goal in those applications is to satisfy timing constraints associated with transactions accessing the database. Real-time database systems entered the computer science spotlight with the publication of a special issue in the ACM SIGMOD Record in March 1988. Since then, the results of a considerable number of works addressing various features of real-time database systems have appeared in the literature. This bibliography presents a collection of papers on various aspects of real-time database systems that have been published or identified so far. To our knowledge, this is the first bibliography to appear concerning real-time database systems. The bibliography is available online for anonymous ftp at gord ion .cs .b i lken t . edu . t r in the p u b / b i b directory. We apologize in advance for any errors or omissions. Remarks, corrections, and additions are most welcome and may be sent to the author at the above address.

5 citations

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.