Carlos O'Ryan

The Design and Performance of a Pluggable Protocols Framework for Object Request Broker Middleware

To be an effective platform for performance-sensitive real-time and embedded applications, off-the-shelf CORBA middleware must preserve communication-layer quality of service (QoS) properties to applications end-to-end. However, the standard CORBA’s GIOP/IIOP interoperability protocols are not well suited for applications that cannot tolerate the message footprint size, latency, and jitter associated with general-purpose messaging and transport protocols. It is essential, therefore, to develop standard pluggable protocols frameworks that allow custom messaging and transport protocols to be configured flexibly and used transparently by applications. This paper provides three contributions to research on pluggable protocols frameworks for performance-sensitive communication middleware. First, we outline the key design challenges faced by pluggable protocols developers. Second, we describe how TAO, our high-performance, real-time CORBAcompliant ORB, addresses these challenges in its pluggable protocols framework. Third, we present the results of benchmarks that pinpoint the impact of TAO’s OO design on its endto-end efficiency, predictability, and scalability. Our results demonstrate how applying optimizations to communication middleware can yield highly flexible/reusable designs and highly efficient/predictable implementations. In particular, the overall round-trip latency of a TAO two-way method invocation using the standard inter-ORB protocol and using a commercial, off-the-self Pentium II Xeon 400 MHz workstation running in loopback mode is 125 secs. The ORB middleware accounts for approximately 48% or 60 secs of the total round-trip latency. These results illustrate that (1) communication middleware performance is largely This work was supported in part by Boeing, DARPA contract 9701516, GDIS, NSF grant NCR-9628218, Nortel, Siemens, and Sprint. an implementation detail and (2) the next-generation of optimized, standards-based CORBA middleware can replace ad hoc and proprietary solutions. Subject areas: Frameworks; Design Patterns; Distributed and Real-Time Systems

Towards highly configurable real-time object request brokers

This paper discusses the software architecture of a real-time CORBA object request broker (ORB), called ZEN, written in real-time Java, which is designed to eliminate common sources of overhead and non-determinism in ORB implementations. We illustrate how ZEN can be configured to select the minimal set of components used by an application. Our experience with ZEN indicates that combining real-time Java with real-time CORBA is a major step forward towards simplifying the development and maintenance of distributed middleware and applications with stringent quality of service requirements.

Designing and Optimizing a Scalable CORBA Notification Service

Many distributed applications require a scalable event-driven communication model that decouples suppliers from consumers and simultaneously supports advanced quality of service (QoS) properties and event filtering mechanisms. The CORBA Notification Service provides a publish/subscribe mechanism that is designed to support scalable event-driven communication by routing events efficiently between many suppliers and consumers, enforcing various QoS properties (such as reliability, priority, ordering, and timeliness), and filtering events at multiple points in a distributed system.This paper provides several contributions to research on scalable notification services. First, we present the CORBA Notification Service architecture and illustrate how it addresses limitations with the earlier CORBA Event Service. Second, we explain how we addressed key design challenges faced when implementing the Notification Service in TAO, which is our high-performance, real-time ORB. We discuss the optimizations used to improve the scalability of TAOs Notification Service. Finally, we present empirical results of the performance of our implementation.

Patterns and performance of distributed real-time and embedded publisher/subscriber architectures

This paper makes four contributions to the design and evaluation of publisher/subscriber architectures for distributed real-time and embedded (DRE) applications. First, it illustrates how a flexible publisher/subscriber architecture can be implemented using standard CORBA middleware. Second, it shows how to extend the standard CORBA publisher/subscriber architecture so it is suitable for DRE applications that require low latency and jitter, periodic rate-based event processing, and event filtering and correlation. Third, it explains how to address key performance-related design challenges faced when implementing a publisher/ subscriber architecture suitable for DRE applications. Finally, the paper presents benchmarks that empirically demonstrate the predictability, latency, and utilization of a widely used real-time CORBA publisher/subscriber architecture. Our results demonstrate that it is possible to strike an effective balance between architectural flexibility and real-time quality of service for important classes of DRE applications.

Applying a scalable CORBA event service to large-scale distributed interactive simulations

Next-generation distributed interactive simulations (DISs) will have stringent quality of service (QoS) requirements for throughput, latency and scalability, as well as requirements for a flexible communication infrastructure to reduce software lifecycle costs. The CORBA event service provides a flexible model for asynchronous communication among distributed and collocated objects. However, the standard CORBA event service specification lacks important features and QoS optimizations required by DIS systems. This paper makes five contributions to the design, implementation and performance measurement of DIS systems. First, it describes how the CORBA event service can be implemented to support key QoS features. Second, it illustrates how to extend the CORBA event service so that it is better suited for DISs. Third, it describes how to develop efficient event dispatching and scheduling mechanisms that can sustain high throughput. Fourth, it describes how to use multicast protocols to reduce network traffic transparently and to improve system scalability. Finally, it illustrates how an event service framework can be strategized to support configurations that facilitate high throughput, predictable bounded latency, or some combination of each.

Using principle patterns to optimize real-time ORBs

Increasing quality-of-service requirements pose challenges in both design and optimization for real time CORBA middleware. The authors describe TAO (The Ace ORB), an object request broker that provides end-to-end QoS guarantees. In discussing TAO, we focus on object-adapter and ORB-core optimizations in three key dimensions in high performance, real time ORB end system design: server-side concurrency, memory management, and CORBA request demultiplexing.

Operating system performance in support of real-time middleware

Commercial-off-the-shelf (COTS) hardware and software is being evaluated and/or used in an increasing range of mission-critical distributed real-time and embedded (DRE) systems. Due to substantial R&D investment over the past decade, COTS middleware has matured to the point where it is no longer the dominant factor in the overhead, non-determinism, and priority inversion incurred by DRE systems. As a result, the focus has shifted to the COTS operating systems and networks, which are once again responsible for the majority of end-to-end latency and jitter. We compare and evaluate the suitability of popular COTS operating systems for real-time COTS middleware, such as Real-time CORBA. We examine real-time operating systems (VxWorks and QNX), general-purpose operating systems with real-time thread scheduling classes (Windows NT Windows 2K, and Linux), and a hybrid real-time/general-purpose operating system (Linux/RT). While holding the hardware and ORB constant, we vary these operating systems systematically to measure platform-specific variations in context switch overhead, throughput of the ORB in terms of two-way operations per-second and memory-footprint of the ORB libraries. We also measure how the latency and jitter of high-priority operations are affected as the number of low-priority operations increase.

Evaluating policies and mechanisms for supporting embedded, real-time applications with CORBA 3.0

To be an effective platform for performance-sensitive real time systems, commercial-off-the-shelf (COTS) distributed object computing (DOC) middleware must support application quality of service (QoS) requirements end-to-end. However, conventional DOC middleware does not provide this support, which makes it unsuited for applications with stringent latency, determinism, and priority preservation requirements. It is essential, therefore, to develop standards based, COTS DOC middleware that permits the specification, allocation, and enforcement of application QoS requirements end-to-end. The Real-time CORBA and Messaging specifications in the forthcoming CORBA 3.0 standard are important steps towards defining standards based COTS DOC middleware that can deliver end-to-end QoS support at multiple levels in distributed and embedded real time systems. However these specifications still lack sufficient detail to portably configure and control processor, communication, and memory resources for applications with stringent QoS requirements. The paper provides four contributions to research on real time DOC middleware. First, we show how the CORBA 3.0 Real-time and Messaging specifications provide a starting point for addressing the needs of an important class of applications with stringent real time requirements. Second, we show how the CORBA 3.0 specifications are not sufficient to solve all the issues within this application domain. Third, we describe how we have implemented portions of these specifications, as well as several enhancements, using TAO, which is our open-source real time CORBA ORB. Finally, we empirically evaluate the performance of TAO to illustrate how its features address the QoS requirements of certain types of real time applications.

Evaluating policies and mechanisms to support distributed real‐time applications with CORBA

To be an effective platform for performance‐sensitive real‐time systems, commodity‐off‐the‐shelf (COTS) distributed object computing (DOC) middleware must support application quality of service (QoS) requirements end‐to‐end. However, conventional COTS DOC middleware does not provide this support, which makes it unsuited for applications with stringent latency, determinism, and priority preservation requirements. It is essential, therefore, to develop standards‐based, COTS DOC middleware that permits the specification, allocation, and enforcement of application QoS requirements end‐to‐end.

A pattern language for efficient, predictable, scalable, and flexible dispatching mechanisms for distributed object computing middleware

In many application domains, the distributed object computing (DOC) middleware is responsible for dispatching upcalls to one or more application objects when events or requests arrive from clients. Implementing efficient, predictable, and scalable middleware dispatching mechanisms is hard and implementing them for multi-threaded systems is even harder. In particular dispatching mechanisms must be prepared to dispatch upcalls to multiple objects, to handle recursive requests originated from application-provided upcalls, and must often collaborate with the application to control object life-cycle. In our DOC middleware research we have implemented many dispatching mechanisms that repeatedly apply common solutions to solve the challenges outlined above. Moreover, we have discovered that the forces constraining dispatching mechanisms often differ slightly thereby requiring alternative solutions. This paper presents two contributions to the design and implementation of efficient, predictable, scalable and flexible DOC middleware and applications. First, it shows how patterns can be applied to the object-oriented systems to capture key design and performance characteristics of proven dispatching mechanisms. Second it presents a pattern language that describes successful solutions that are appropriate for key dispatching challenges that arise in various real-time DOC middleware and applications.