طراحی و پیاده سازی ماشین مجازی گسترده برای کامپیوترهای شبکه ای Design and implementation of a distributed virtual machine for networked computers

Abstract This paper describes the motivation, architecture and performance of a distributed virtual machine (DVM) for networked computers. DVMs rely on a distributed service architecture to meet the manageability, security and uniformity requirements of large, heterogeneous clusters of networked computers. In a DVM, system services, such as verification, security enforcement, compilation and optimization, are factored out of clients and located on powerful network servers. This partitioning of system functionality reduces resource requirements on network clients, improves site security through physical isolation and increases the manageability of a large and heterogeneous network without sacrificing performance. Our DVM implements the Java virtual machine, runs on x86 and DEC Alpha processors and supports existing Javaenabled clients. 1. Introduction Virtual machines (VMs) have the potential to play an important role in tomorrow’s networked computing environments. Current trends indicate that future networks will likely be characterized by mobile code [Thorn 97], large numbers of networked hosts per domain [ISC 99] and large numbers of devices per user that span different hardware architectures and operating systems [Hennessy 99, Weiser 93]. A new class of virtual machines, exemplified by systems such as Java and Inferno [Lindholm & Yellin 96, Dorward et al. 97], has recently emerged to meet the needs of such an environment. These modern virtual machines are compelling because they provide a platform-independent binary format, a strong type-safety guarantee that facilitates the safe execution of untrusted code and an extensive set of programming interfaces that subsume those of a general-purpose operating system. The ability to dynamically load and safely execute untrusted code has already made the Java virtual machine a ubiquitous component in extensible systems ranging from web browsers and servers to database engines and office applications. The platform independence of modern virtual machines makes it feasible to run the same applications on a wide range of computing devices, including embedded systems, handheld organizers, conventional desktop platforms and high-end enterprise servers. In addition, a single execution platform offers the potential for unified management services, thereby enabling a small staff of system administrators to effectively administer thousands or even hundreds of thousands of devices. While modern virtual machines offer a promising future, the present is somewhat grim. For example, the Java virtual machine, despite its commercial success and ubiquity, exhibits major shortcomings. First, even though the Java virtual machine was explicitly designed for handheld devices and embedded systems, it has not been widely adopted in this domain due to its excessive processing and memory requirements [Webb 99]. Second, it is the exception, rather than the rule, to find a secure and reliable Java virtual machine [Dean et al. 97]. And third, rather than simplifying system administration, modern virtual machines, like Java, have created a substantial management problem [McGraw & Felten 96], leading many organizations to simply ban virtual machines altogether [CERT 96].