Lock Requests Research Articles

Performance analysis of dynamic locking with the no-waiting policy

A transaction processing system with two-phase dynamic locking with the no waiting policy (DLNW) for concurrency control is considered. In this method, transactions making conflicting lock requests are aborted and restarted rather than blocked, thereby eliminating blocking delays (and deadlocks), but making it susceptible to cyclic restarts. Cyclic restarts are dealt with by delaying the restart of a transaction encountering a lock conflict or replacing it with a new transaction. Analytic solution methods for evaluating the performance of the variants of the DLNW method are described. The analytic methods, validated against simulation and shown to be acceptably accurate, are used to study the effect of the following parameters on system performance: transaction size and its distribution, degree of concurrency, the throughput characteristic of the computer system, and the mixture of read-only query and update transactions. A comparison of the DLNW and dynamic locking with waiting (DLW) methods shows that DLW provides higher throughput than DLNW, except when there is no hardware resource contention and conflicted transactions can be replaced by new transactions. The DLNW method outperforms the time-stamp ordering method, as observed from simulation results as well as case by case analyses of possible scenarios. >

Snoopy cache test-and-test-and-set without execessive bus contention

This paper analyzes the burst of bus traffic that occurs when a lock is released that is being waited for in a test-and-test-and-set spinloop. It is typically O ( N ) and can be O ( N 2 ), depending on assumptions about system behaviour, where N is the number of processors. Supporting a single feature in the bus interface, abandoning pending lock requests, reduces this to O (1). Such a feature is already possible on some systems and busses, but has not been applied to synchronization.

The effects of two-phase locking on the performance of a distributed database management system

In this paper, the effects of two-phase locking on the performance of a distributed database management system are evaluated. In particular, the relationship between the average transaction response time for four concurrency control algorithms based on two-phase locking has been established with respect to the static degree of interference for a given multiprogramming level, database size, and transaction size. The increase in average probability of one lock request conflicting with another has been measured with respect to the static degree of interference and the percentage of aborted transactions has been measured for each algorithm. In addition, the impact of a number of parameters (e.g. data replication, lock predeclaration, time-out, local processing time, degree of locality of references, interarrival time, number of nodes and transmission time) on the performance of a distributed database management system in the presence of a concurrency control mechanism has also been measured. The study has been made using simulation and the SIMULA programming language.

Locking performance in centralized databases

An analytic model is used to study the performance of dynamic locking. The analysis uses only the steady-state average values of the variables. The solution to the model is given by a cubic, which has exactly one valid root for the range of parametric values that is of interest. The model's predictions agree well with simulation results for transactions that require up to twenty locks. The model separates data contention from resource contention, thus facilitating an analysis of their separate effects and their interaction. It shows that systems with a particular form of nonuniform access, or with shared locks, are equivalent to systems with uniform access and only exclusive locks. Blocking due to conflicts is found to impose an upper bound on transaction throughput; this fact leads to a rule of thumb on how much data contention should be permitted in a system. Throughput can exceed this bound if a transaction is restarted whenever it encounters a conflict, provided restart costs and resource contention are low. It can also be exceeded by making transactions predeclare their locks. Raising the multiprogramming level to increase throughput also raises the number of restarts per completion. Transactions should minimize their lock requests, because data contention is proportional to the square of the number of requests. The choice of how much data to lock at a time depends on which part of a general granularity curve the system sees.

Performance Evaluation of Centralized Databases with Static Locking

The performance of transaction processing systems is determined by the contention for hardware as well as software resources (database locks), due to the concurrency control mechanism of the database being accessed by transactions. We consider a transaction processing system with a set of dominant transcation classes. Each class needs to acquire a certain subset of the locks in the database before it can be processed, i.e., predeclared lock requests with static locking. Straightforward application of the decomposition method requires the numerical solution of a two-dimensional Markov chain. Equivalently, a hierarchical simulation method, where the computer system is represented by a composite queue with exponential service rates, can be used to analyze the system. We propose an inexpensive analytic solution method, also based on hierarchical decomposition, such that the throughput of the computer system ic characterized by the number of active transactions (regardless of class). Numerical results are provided to show that the new method is adequately accurate compared to the other two rather costly methods. It can be used to determine the effect of granularity of locking on system performance. The solution method is also applicable to multiresource queueing systems with multiple contention points.

A simple analysis of exclusive and shared lock contention in a database system

We consider a probabilistic model of locking in a database system in which an arriving transaction is blocked and lost when its lock requests conflict with the locks held by currently executing transactions. Both exclusive and shared locks are considered. We derive a simple asymptotic expression for the probability of blocking which is exact to order 1/ N where N is the number of lockable items in the database. This expression reduces to one recently derived by Mitra and Weinberger for the special case where all locks are exclusive.

DASD sharing in DOS/VSE

AbstractThe first part of this report discusses DASD sharing in loosely‐coupled data processing systems. For the locking of shared files two design concepts are presented.The second part describes DASD sharing in the IBM operating system DOS/VSE. The control program routines which serialize the lock requests within one processing unit and the cross‐system locking facility are discussed.

Analysis of locking policies in database management systems

Quantitative analysis of locking mechanisms and of their impact on the performance of transactionnal systems have yet received relatively little attention. Although numerous concurrency mechanisms have been proposed and implemented, there is an obvious lack of experimental as well as analytical studies of their behaviour and their influence on system performance. We present in this paper an analytical framework for the performance analysis of locking mechanisms in transactionnal systems based on hierarchical analytical modelling. Three levels of modelling are considered: at level 1, the different stages (lock request, execution, blocking) transactions of through during their life-time are described; the organization and operations of the CPU and I/O resources are analysed at level 2; transaction's behaviour during their lock request phase is analysed at modelling level 3. This hierarchical approach is applied to the analysis of a physical locking scheme involving a static lock acquisition policy. A simple probabilistic model of the transaction behaviour is used to derived the probability that a new transaction is granted the locks it requests given the number of transactions already active as a function of the granularity of the database. On the other hand, the multiprogramming effect due to the sharing of CPU and I/O resources by transactions is analysed using the standard queueing network approaches and the solution package QNAP. In a final step, the results on the blocking probabilities and the multiprogrammin effect are used as input of a global performance model of the transactionnal system. Markovian analysis is used to solve this model and to obtain the throughput of the system as a function of the data base granularity and other parameters. The results obtained provide a clear understanding of the various factors which determine the global performance, of their role and improtance. They also raise many new issues which can only be solved by further extensive experimental and analytical studies and show that two particular topics deserve special attention: the modelling of transaction behaviour and the modelling of locking overheads.