A real-time locking protocol

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

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.

Full text

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.

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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,

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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.