John E. Neilson

The stochastic rendezvous network model for performance of synchronous client-server-like distributed software

Distributed or parallel software with synchronous communication via rendezvous is found in client-server systems and in proposed open distributed systems, in implementation environments such as Ada, V, remote procedure call systems, in transputer systems, and in specification techniques such as CSP, CCS and LOTOS. The delays induced by rendezvous can cause serious performance problems, which are not easy to estimate using conventional models which focus on hardware contention, or on a restricted view of the parallelism which ignores implementation constraints. Stochastic rendezvous networks are queueing networks of a new type which have been proposed as a modelling framework for these systems. They incorporate the two key phenomena of included service and a second phase of service. This paper extends the model to also incorporate different services or entries associated with each task. Approximations to arrival-instant probabilities are employed with a mean-value analysis framework, to give approximate performance estimates. The method has been applied to moderately large industrial software systems. >

A toolset for performance engineering and software design of client-server systems

Abstract TimeBench/SRVN is a prototype toolset for computer-aided design and performance analysis of software, with an emphasis on distributed client-server systems. The performance behaviour of such systems may defy intuition because it involves factors in the software design (such as the partitioning of the functionality and the frequency with which requests will be made to each server) and in the configuration of the distributed system (including replication of services, the distribution of data, and the speed of network access). The novelty of the tool consists in providing support both for developing design specifications and also for performance analysis. The integrated approach avoids the semantic gap between a designers domain and the performance modeling domain, and assists the designer to explore factors that impact the performance of a design. The performance models are based on the Stochastic Rendezvous Network (SRVN) formalism for client-server systems with synchronous service requests. The distinctive features of SRVNs are nested services (since servers can also act as clients to other servers) and the existence of two or more phases of service (the first executed while the client is blocked, and the others executed in parallel with the client). TimeBench/SRVN is intended as a demonstration of the concept of an integrated designer/performance interface, and as a research environment for fast analytic solvers for the models. Besides a simulation solver, it offers three approximate analytic solvers based on recent research, a Markovian solver, a technique for finding bounds on the throughput without too many assumptions, and a tool for rapidly exploring the space of possible parameter values.

Performance bounds for concurrent software with rendezvous

Abstract Synchronous message-passing communication, or rendezvous, occurring between software tasks can have a significant effect on system performance. The rendezvous style of communication is coming into wider use in programming languages and operating systems for parallel and distributed environments. Understanding the performance implications of this style of inter-task communication is becoming a matter of practical importance. The dual nature of a task which acts both like a customer as well as a server, makes the performance analysis of rendezvous-based multitasking systems quite different from the analysis of the other queueing systems with known results. This research focuses on rendezvous-based systems in which the execution behavior of the software has a nondeterministic component of a very general nature which may for example be the manifestation of a data dependent behavior. Based on a model called the Stochastic Rendezvous Network the computation of bounds on task throughputs for multitasking systems characterized by rendezvous style communication is presented. Although the behavior of tasks is called stochastic, it is very general and the results are valid for general distributions of computation times and the number of messages generated by tasks. The inter-relationship among task throughputs, however, makes it difficult to extract the bounds in closed analytic form. The notion of a feasible throughput region which encloses the set of feasible tasks throughputs and captures the inter-relationship among the behavior of tasks is introduced. Variations of this basic bounding approach that are useful in the context of different types of multitasking systems are considered in the article. For example, a novel technique based on interval arithmetic is proposed for the computation of numerical values for the bounds. The applicability of the bounds and their tightness are analyzed through case studies. Issues such as the inter-relationship between the software architecture and system performance, and the effect of processor contention on the performance bounds are discussed.

Performance of Concurrent Rendezvous Systems with Complex Pipeline Structures

The term “complex pipeline” describes a set of tasks which process incoming data in a sequence, like a pipeline, but have various kinds of parallel execution steps coupled into the main stream of execution. Examples are, splitting off of parallel streams, and shared server tasks. Examples are found in processing to interpret radar data, and other real-time systems. Rendezvous systems like Ada have static tasks, static processor allocations and synchronous inter-task communications, which can cause potential performance problems. The growing importance of rendezvous-based environments, including Ada, requires that we be able to predict these problems. Models such as Petri nets, are often too expensive to solve; fast approximation techniques are needed. The approach of “stochastic rendezvous networks” is adapted here to deal with complex pipelines. This paper describes an algorithm and evaluates its accuracy; the algorithm is the major feature of the paper, including a “Conditional Mean Value Analysis” step. It includes processor queueing, which was not modelled in the earlier work. The method is several orders of magnitude faster than Petri net analysis even on small examples. The accuracy obtained is generally better than 10%.

Throughput of stochastic rendezvous networks with caller-specific service and processor contention

Stochastic rendezvous networks (SRVN) are models for the throughput of distributed programs executed concurrently and synchronously on distributed computing nodes. Server tasks respond to requests from user tasks and execute in a two-phase pattern (in-rendezvous phase, then postrendezvous phase), and may themselves act as user tasks to further servers, to any depth. The authors extend the model to permit the execution time and the occurrence of nested rendezvous calls to depend on the identity of the calling user task. An iterative approximation method is developed, and compared to exact throughput calculations found with timed Petri nets. The approximation has errors of a few percent in most cases, and executes very much faster than the Petri-net calculation.<<ETX>>

Robust box bounds: throughput guarantees for closed multiclass queueing networks with minimal stochastic assumptions

To use queuing theory to analyze real systems such as computer communications networks, one makes assumptions that are, strictly speaking, untrue. The authors provide an exact analysis for cases with greatly relaxed assumptions. Service times can have general increasing failure rate distributions, different by class even at FIFO nodes. Routing can be arbitrary, including dependencies along the route, provided the number of visits to each node is a random variable. Only the mean service time and mean visit rates at nodes need be specified. A lower throughput bound is found which gives a minimum guaranteed throughput for each class; together with the familiar multiclass asymptotic upper bounds they give a convex feasible region in a multidimensional throughput space. A detailed analysis is given for systems with FIFO and infinite-server nodes, and the extension to processor-sharing nodes is described. The results can be reinterpreted as a set of bounds on the separate throughputs. This is equivalent to a circumscribed rectangular region called the robust box bounds.<<ETX>>

An analytic performance model of a multiprogrammed batch-timeshared computer

The paper presents an analytic performance model of a multiprogrammed batch and timeshared context swapped system. The model predicts resource utilizations, batch throughput and timesharing response times as functions of the system hardware characteristics, including core size; load characteristics, differing between batch and interactive users; and the numbers of batch and interactive users. Thus, it allows one to study the effects of different hardware configurations and/or load patterns on system performance.

Interval Arithmetic for Computing Performance Guarantees in Client-Server Software

Performance analysis of client-server software systems through bounds on task throughputs is presented. The upper and lower bounds (performance guarantee) are both independent of any assumptions about the stochastic behavior of the client tasks, and make only weak assumptions on the server. The analytic expressions for the bounds however, are not in closed form. A new technique based on interval arithmetic is developed to compute numerical values for the bounds.