A real-time locking protocol
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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Citations
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4 citations
Cites background or methods from "A real-time locking protocol"
...In the last decade, researchers had proposed various efficient concurrency control protocols to meet transaction deadlines, and, at the same time, to maintain database consistency for RTDBS, e.g., [3, 8 , 9, 12, 16]....
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...For example, the well-known priority ceiling protocol (PCP) [ 8 , 12] (designed for HRT) requires a static system that is usually not true for a system with SRT....
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...The model consists of a transaction manager (TM), a scheduler (S), and a resource manager (RM), as shown in Figure 1 [3, 4, 8 , 16]....
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...The priority ceiling protocol (PCP) [ 8 ] is proposed to bound the worstcase blocking time of any transaction....
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...2.1 Real-time Concurrency Control Protocols for Single Type of Transactions Real-time concurrency control protocols are often extended from traditional concurrency control protocols [3, 8 , 9, 12, 16]....
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4 citations
4 citations
4 citations
Cites background from "A real-time locking protocol"
...In [STDW96], Squadrito et al. extended the read/ write priority ceiling protocol [ SRSC91 ] for real-time object-oriented databases....
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...This section defines the Real-Time Nested Priority Ceiling Protocol(RT-NPCP) - a generalization of Sha, Rajkumar, and Lehoczky’s Priority Ceiling Protocol [SRL90, SRSC91 ] - and proves that it prevents unbounded blocking and deadlocks, and maintains the serializality of a set of root transactions....
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...The proofs of (1) and (2) are refinements of the original proofs of the PCP in [ SRSC91 ], and the proof of (3) is similar to a proof which is presented in [Pap79]....
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...In [ SRSC91 ], Sha et al. proposed a priority-driven two-phase lock protocol called the read/write priority ceiling protocol that avoids deadlocks and guarantees that a job is blocked at most once by a lower priority job....
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...In general, a resource may have a different priority ceiling for each mode in which it may be locked [Bak91, SRSC91 ]....
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References
5,397 citations
3,891 citations
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....
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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....
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...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]....
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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 ]....
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