Communications of the ACM

Communications of the ACM

Multiparadigm Communications in Java For Grid Computing

By Vladimir Getov, Gregor von Laszewski, Michael Philippsen, Ian Foster
Communications of the ACM, October 2001, Vol. 44 No. 10, Pages 118-125
10.1145/383845.383872
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The computational science community has long been at the forefront of advanced computing, due to its need to solve problems requiring resources beyond those provided by the most powerful computers of the day. Examples of such high-end applications range from financial modeling and vehicle design simulation to computational genetics and weather forecasting. Over the years, these considerations have led computational scientists to be aggressive and innovative adopters of vector computers, parallel systems, clusters, and other novel computing technologies.

More recently, the widespread availability of high-speed networks and the growing awareness of the new problem-solving modalities made possible when these networks are used to couple geographically distributed resources have stimulated interest in so-called Grid computing [6]. The term "the Grid" refers to an emerging network-based computing infrastructure providing security, resource access, information, and other services that enable the controlled and coordinated sharing of resources among "virtual organizations" formed dynamically by individuals and institutions with common interests [7]. A number of ambitious projects are today applying Grid computing concepts to challenging problems like the distributed analysis of experimental physics data, community access to earthquake engineering facilities, and the creation of "science portals," or thin clients providing remote access to the information sources and simulation systems supporting a particular scientific discipline.

Underpinning both parallel and Grid computing is the common need for coordination and communication mechanisms allowing multiple resources to be applied in a concerted fashion to these complex problems. Scientific and engineering applications have for the most part addressed this requirement in an ad hoc and low-level fashion, using specialized message-passing libraries within parallel computers and communication mechanisms among networked computers.

While low-level approaches have let users achieve their application performance goals, an unfortunate consequence is that the computational science community has not benefited to any great extent from the advances in software engineering that have occurred in industry over the past 10 years. In particular, the Java programming environment, which seems ideal for multiparadigm communications, is hardly exploited at all. Java's platform-independent "bytecode" can be executed securely on many platforms, making the language an attractive basis for portable Grid computing. In addition, Java's performance on sequential codes, a prerequisite for developing such "Grande" applications, has increased substantially over the past few years [4] (see the sidebar "Java Grande"). Inspired originally by coffeecup jargon, the buzzword Grande is now also commonplace for distinguishing this emerging type of high-end applications when written in Java. Java also provides a sophisticated graphical user interface framework, as well as a paradigm for invoking methods on remote objects. These features are of particular interest for steering scientific instruments from a distance (see the sidebar "10 Reasons to Use Java in Grid Computing").

The rapid development of Java technology now makes it possible to support, in a single OO framework, the various communication and coordination structures in scientific applications. Here, we outline how this integrated approach can be achieved, reviewing in the process the state-of-the-art in communication paradigms within Java. We also present recent evaluation results indicating this integrated approach can be achieved without compromising performance.

Communication requirements. Communication and coordination within scientific and engineering applications combine stringent performance requirements (especially in Grid contexts) with highly heterogeneous and dynamic computational environments. A number of communication frameworks have been introduced over the years as a result of the development of computer networks and distributed and parallel computing systems (see Figure 1). The pioneering framework—the remote procedure call (RPC)—has been around for at least 20 years. The message-passing paradigm arrived along with distributed memory parallel machines. More recently, other frameworks have been developed based on the RPC concepts: remote method invocation (RMI) and component frameworks. Three communication and coordination programming approaches have emerged, each of which can be expressed effectively in Java:

• Message passing. Within parallel computers and clusters, communication structures and timings are often highly predictable for both senders and receivers and may involve multiparty, or "collective," operations. Efficiency is the main concern, and message-passing libraries, such as the Message Passing Interface (MPI) [8], have become the technology of choice.
• Remote method invocation. When components of a single program are distributed over less tightly coupled elements or when collective operations are rare, communication structures may be less predictable, and such issues as asynchrony, error handling, and ease of argument passing become more prominent. In this context, such technologies as CORBA and Java's RMI have benefits.
• Component frameworks. When constructing programs from separately developed components, the ability to compose and discover the properties of components is critical. Component technologies, including JavaBeans, and their associated development tools become very attractive, as do proposed high-performance component frameworks [2].

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Message Passing

Java includes several built-in mechanisms allowing the exploitation of the parallelism inherent in a given program. Threads and concurrency constructs are well suited for shared-memory computers, but not for large-scale distributed-memory machines. For distributed applications, Java provides sockets and the RMI mechanism. For the parallel computing world, the explicit use of sockets is often too low-level, while RMI is oriented too much toward client/server-type systems and does not specifically support the symmetric model adopted by many parallel applications. Obviously, there is a gap within Java's set of programming models, especially for parallel programming support on clusters of tightly coupled processing resources. A solution inevitably builds on the message-passing communication framework, one of the most popular parallel programming paradigms since the 1980s.

The architecture of a message-passing system can generally follow one of two approaches: implicit or explicit. Solutions taking the implicit approach usually provide the programmer a single shared-memory system image, hiding the message passing at a lower level of the system hierarchy. Thus, a software developer works within an environment often called the distributed shared memory programming model. Translating the implicit solution to Java leads to development of cluster-aware Java virtual machines (JVMs) providing fully transparent and truly parallel multithreaded programming environments [1]. This approach preserves full compatibility with the standard Java bytecode format. However, these advantages result from adopting a complex nonstandard JVM that introduces additional overhead to the Java runtime system. This extra complexity makes it difficult for such JVMs to keep up with the continuous improvements and performance optimizations of the standard technology.

Unlike sockets and RMI, explicit message passing supports symmetric communications directly, including point-to-point and collective operations, such as broadcast, gather, all-to-all, and others, as defined by the MPI standard. Programming with MPI is relatively straightforward because it supports the single program multiple data (SPMD) model of parallel computing, wherein a group of processes cooperate by executing identical program images on local data values.

With the evident success of Java as a programming language, and its inevitable use in connection with parallel, distributed, and Grid computing, the absence of a well-designed explicit message-passing interface for Java would lead to divergent, nonportable practices. Indeed, in 1998, the message-passing working group of the Java Grande Forum was formed to agree on a common MPI-like application programming interface (API) for message passing in Java (MPJ) [5]. An immediate goal was to provide an ad hoc specification for portable message-passing programming in Java that would also serve as a basis for conversion between programs written in C, C++, Fortran, and Java.

MPJ can be implemented in one of two ways: as a wrapper to existing native MPI libraries or as a pure Java implementation. The former provides a quick solution, usually with negligible runtime overhead introduced by the wrapper software. However, using native code breaks the Java security model and does not allow work with applets (advantages of the pure Java approach). Unfortunately, a direct MPJ implementation in Java is usually much slower than wrapper software for existing MPI libraries. One solution to this problem is to employ more sophisticated design approaches. For instance, the use of native conversion into linear byte representation, often called "marshaling," and advanced compilation technologies for Java can make the two design options comparable in terms of performance. Our experiments have used the statically optimizing IBM High-Performance Compiler for Java (HPCJ), which generates native code for the RS6000 architecture, to evaluate the performance of MPJ on an IBM SP2 distributed-memory parallel machine. The results show that when using such a compiler, the MPJ communication component is as fast as the native message-passing library (see the table).

Closely modeled on the MPI-1 standard, the existing MPJ specification should be regarded as a first phase in a broader program aimed at defining a more Java-centric high-performance message-passing environment. We can expect future work to consider more high-level communication abstractions and, perhaps, layering on other standard transports, as well as on Java-compliant middleware. A primary goal should be to offer MPI-like services to Java programs in an upward-compatible fashion. Middleware developed at this level should allow a choice of emphasis—performance or generality—while always supporting portability.

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Fast Remote Method Invocation

Remote invocation is an established programming concept behind both the original RFC [3] and Java's RMI. To implement a remote invocation, the procedure identifier and its arguments are encoded (marshaled) in a wire format understood by both the caller and the code being called, or the callee. The callee uses a proxy object to decode, or unmarshal, the stream of bytes and then perform the actual invocation. The results travel in the other direction, from callee to caller.

Although RMI inherits this basic design, it has distinguishing features beyond the original RPC. In addition, RMI is no longer meant to bridge OO and procedural languages or to bridge languages with different kinds of elementary types and structures. The main advantages of RMI are that it is truly object-oriented, supports all Java data types, and is garbage collected. Since most practitioners agree that, for sequential code, garbage collection saves programmer time, it is likely the same is true for distributed code as well. These features also allow the caller and the callee to be developed separately, as long as they agree on interfaces. As a result, software development and maintenance of distributed systems becomes much easier.

To illustrate these advantages, consider the remote invocation of a method add(Atom name). The OO nature of RMI allows the caller to pass objects of any subclass of Atom to the callee. The object is encoded into a machine-independent byte representation (Java calls it "object serialization") that also includes information on the class implementation. More precisely, if the callee does not know the concrete class implementation of name, it can load the class implementation dynamically. When the caller invokes an instance method on name, say, name.bond, the bond code of the particular subclass of Atom is executed on the side of the callee. Thus, one of the main advantages of OO programming—reuse of existing code with refined subclasses—can also be exploited for distributed code development.

These novel features come at a cost in terms of runtime overhead. With the regular implementation of RMI on top of Ethernet, a remote invocation takes milliseconds; concrete execution times depend on the number and the types of arguments. About a third of the time is needed for the RMI itself, a third for the serialization of the arguments, and another third for the data transfer (TCP/IP-Ethernet). While such latency might be acceptable for coarse-grain applications with limited communication needs, it is too slow for high-performance applications running on low-latency networks, such as a closely connected cluster of workstations.

Several projects are under way to improve the performance of RMI, including Manta [9] and JavaParty [10]. In addition to improving the implementation of regular RMI by, say, removing layering overhead, they employ a number of novel optimization ideas:

• Precompiling marshaling routines. The goal is to save the runtime overhead for generating these routines via dynamic type inspection.
• Employing an optimized wire protocol. For type encoding, detailed type descriptions are needed only if the objects are stored into persistent storage. For communication purposes alone, a short type identifier may be sufficient.
• Caching, or replicating, objects. The related techniques help avoid retransmission if the object's instance variables do not change between calls.
• Minimizing memory copy operations. When efficiency is important, there should be as few memory copies as possible in either direction between the object and the communication hardware.
• Minimizing thread switching overhead. Because Java is inherently multithreaded, traditional RPC optimizations are generally insufficient for minimizing runtime. In addition, optimized RMI implementations in Java cannot be as aggressive as native approaches because the JVM concept does not allow direct access to raw data and hides the way threads are handled internally.
• Using an efficient communication subsystem. JavaParty's RMI is implemented on a Myrinet-based library (ParaStation.ira.uka.de) employing user-level communication, hence avoiding costly kernel operations.

The JavaParty project has optimized both RMI and the object serialization in pure Java. Remote invocations can be completed within 80 microseconds on a cluster of DEC Alpha computers connected by Myrinet (see Figure 2). With similar optimization ideas, the Manta group compiles to native code and uses a runtime system written in C, making it less portable compared to JavaParty (see Kielmann et al.'s "Enabling Java for High-Performance Computing" in this issue). Nevertheless, both projects report similar remote invocation latencies of 40 microseconds on clusters of Pentium machines.

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Adaptive Grid Computing

Besides making these communication paradigms available and efficient in Java, further advances are needed to realize the full potential of emerging Grids in which users deal with heterogeneous systems, diverse programming paradigms, and the needs of multiple user communities. Adaptive services are needed for security, resource management, data access, instrumentation, policy, and accounting for applications, users, and resource providers.

Java eases this software engineering problem. Because of its OO nature, ability to develop reusable software components, and integrated packaging mechanism, Java offers support for all phases of the life cycle of a software engineering project, including problem analysis and design and program development, deployment, instantiation, and maintenance.

Java's reusable software-component architecture, called JavaBeans, allows users to write self-contained, reusable software units (see the sidebar "Components and JavaBeans"). Using commercially available visual application builder tools, software components can be composed into applets, applications, servlets, and composite components. Components can be moved, queried, and visually integrated with other components, enabling a new level of convenient computer-aided-software-engineering-based programming within the Grid environment.

Component repositories, or containers, allow a number of engineers to work collectively on similar tasks and share the results with the community of scientific users and engineers. Moreover, the Java framework includes a rich set of predefined Java application protocol interfaces, libraries, and components supporting access to databases and directories, network programming, sophisticated interfaces to XML, and more.

We've been evaluating the feasibility of using these advanced Java features for Grid programming as part of our development of several application-specific Grid portals. A portal defines a commonly known access point to the application reachable via a Web browser. Many portal projects use the Java Commodity Grid, or CoG, Kit [11], allowing access to services provided by the Globus Toolkit (www.globus.org) in a way familiar to Java programmers. Thus, the Java CoG Kit is not a simple one-to-one mapping of the Globus API into Java; instead it uses features of the Java language not available in the original C implementation. For example, it includes both the OO programming model and the Java event model.

Another important Java advantage is a graphical user interface for integrating graphical components into Grid-based applications. Our experience with collaborators from various scientific disciplines, including structural biology and climatology, has shown that development of graphical components hiding the complexity of the Grid lets the scientist concentrate on the science, instead of on the Grid's inherent complexity [12] (see Figure 3).

Besides simplifying program development, Java eases development and installation of the client software accessesing the Grid. While trivial for a Java software engineer to install client libraries of the Java CoG Kit on a computer, installation of client software written in other programming languages or frameworks, including C and C++, is much more involved due to differences in compilers and operating systems. Another advantage of using the bytecode-compiled archives is they can also be installed on any operating system supporting Java, including Windows. Using the Java framework allows development of drag-and-drop components enabling information exchange between the desktop and the running Grid application during a program instantiation. Thus, it is possible to integrate Grid services seamlessly into the Windows and the Unix desktops.

Using a commodity technology like Java as the basis for future Grid-based program development represents yet another advantage. The committed support for Java by major vendors in e-commerce allows scientists to exploit a greater range of computer technology—from supercomputers to state-of-the-art commodity devices like cell phones, PDAs, and Java-enabled sensors—all within a Grid-based problem-solving environment.

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Conclusion

Advanced applications like those in science and engineering can require multiple communication abstractions, ranging from message passing to remote method invocation and component frameworks. We've sought to show how a mixture of existing Java constructs and innovative implementation techniques allow Grid-based software engineers and Java programmers to use these communication abstractions efficiently within a single integrated Java framework. The result is a programming approach that appears particularly advantageous for dynamic and heterogeneous Grid environments.

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The Digital Library is published by the Association for Computing Machinery. Copyright © 2001 ACM, Inc.

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