The Need for a Virtual Machine
Total Page：16
File Type：pdf, Size：1020Kb
Virtual Machine The Need for a Virtual Machine Faced with large, heterogeneous information systems and the accelerated rate of technological innovation, software engineers are beginning to rediscover the benefits of targeting virtual machines. The name of the game in software development is return on investment. Companies want to invest money in software that will be useful long enough for the resources spent to be justified. Porting a software package from one platform to another is costly and, depending on the number of platforms supported, can prove to be a Sisyphean nightmare. The worst-case scenario occurs when business logic is stranded on an aging system. Due to historical forces, a large base of code may end up on an obsolete platform that no one wants to deal with. The people who wrote the original code have either quit or been promoted and don't remember anything (sometimes intentionally). The source code has been transformed by time into an encrypted historical artifact that will require years of work by researchers to decipher. If the language and tools used to develop the initial code are linked to the original platform, porting may be out of the question. In pathological cases, the code may be so obscure and illegible that maintenance engineers are too scared to touch anything. I'm sure Y2K programmers are familiar with this type of imbroglio. The only recourse may be a complete rewrite and this is an extremely expensive alternative, fraught with all sorts of dangers and hidden costs. This is the kind of situation CIOs lose sleep over. Using a virtual machine offers a degree of insurance against this kind of thing happening. When confronted with a new hardware platform or operating system, the only application that needs to be ported is the virtual machine itself. If a virtual machine is, say, 100,000 lines of code, you might think that actually porting the code is a bad investment. 100,000 lines of code is not a trivial porting job. However, let's say that the virtual machine runs an application suite that consists of 11 million lines of code. Suddenly, porting the virtual machine is not such a bad deal because the alternative would be to port the 11 million lines of application code. Don't think that this isn't realistic. I once visited an automobile insurance company that had around 30 million lines of application code. Dept. of Computer Engineering, ICOER, Pune. 1 Virtual Machine OBJECTIVE: Objective was to build a virtual machine that satisfied three criterion. In order of priority, these are: 1. Portability 2. Simplicity 3. Performance Portability is the most important feature because being able to work with a uniform software interface, across multiple platforms, is the primary benefit of using a virtual machine. If you can't port a virtual machine, then you're stuck with a given platform, in which case you might as well use tools that compile to the native machine encoding. Portability is also the most important priority in the sense that I had to make sacrifices in terms of simplicity and performance in order to achieve it. There is rarely a solution that is ever optimal under all conditions in software engineering. One way of boosting performance would be to make heavy use of assembly code. However, using an assembler does not necessarily guarantee faster program execution. In order to be able to justify using an assembler, the programmer has to have an intimate knowledge of both the assembly code a compiler generates and the kind of optimizations it performs. The goal is to be able to write faster assembly code than the compiler can. This requires a certain amount of vigilance and skill because, in most cases, the optimizer in a C compiler can do a better job than the average programmer. Another problem with using assembly language is that it ties your code to the hardware you're working on. To port the code to new hardware, you'll need to rewrite every single line of assembly code. This can turn out to be much more work than you think. Again, in the effort to maintain a certain degree of portability, I opted out of low-level, high-performance assembly coding and wrote everything in ANSI C. simplicity has priority over performance by virtue of my desire to make the code maintainable. A year from now, I would like to be able to modify my code without having to call in a team of archaeologists. Code that is optimized for performance can become very brittle and resistant to change. Developers writing high-performance C code will often use all sorts of misleading and confusing conventions, like bitwise shifting to divide by 2. What is produced is usually illegible by anyone but the original developer, and given enough time even the original developer may forget what he had done and why he had done it. In an extreme case, a programmer who has created a mess and is too scared to clean it up may opt to quit and go work somewhere else. I've heard people refer to this as "the final solution" or "calling in for air support and pulling out." Dept. of Computer Engineering, ICOER, Pune. 2 Virtual Machine Run-time Systems A run-time system is an environment in which computer programs execute. A run-time system provides everything a program needs in order to run. For example, a run-time system is responsible for allocating memory for an application, loading the application into the allocated memory, and facilitating the execution of the program's instructions. If the program requests services from the underlying operating system through the invocation of system calls, the run- time system is in charge of handling those service requests. For instance, if an application wants to perform file I/O, the run-time system must offer a mechanism to communicate with the disk controller and provide read/write access. There are different kinds of run-time systems. One way to classify run-time systems is to categorize them based on how they execute a program's instructions. For programs whose instructions use the processor's native machine encoding, the run-time system consists of a tightly knit collaboration between the computer's processor and the operating system. The processor provides a mechanism for executing program instructions. The CPU does nothing more than fetch instructions from memory, which are encoded as numeric values, and perform the actions corresponding to those instructions. The operating system implements the policy side of a computer's native run-time system. The CPU may execute instructions, but the operating system decides how, when, and where things happen. Think of the operating system as a fixed set of rules the CPU has to obey during the course of instruction execution. Thus, for programs written in native machine instructions, the computer itself is the run- time system. Program instructions are executed at the machine level, by the physical CPU, and the operating system manages how the execution occurs. This type of run-time system involves a mixture of hardware and software. Programs whose instructions are not directly executed by the physical processor require a run-time system that consists entirely of software. In such a case, the program's instructions are executed by a virtual machine. A virtual machine is a software program that acts like a computer. It fetches and executes instructions just like a normal processor. The difference is that the processing of those instructions happens at the software level instead of the hardware level. A virtual machine also usually contains facilities to manage the path of execution and to offer an interface to services normally provided by the native operating system. A virtual machine is defined by a specification. A virtual machine is not a particular software implementation, but rather a set of rules. These rules form a contract that the engineer, who builds an instantiation of the virtual machine, must honor. A virtual machine can be implemented in any programming language on any hardware platform, as long as it obeys the specification. You could create a version of the HEC virtual machine on an OS/390 using APL if you really wanted to. This is what makes the idea of a virtual machine so powerful. You can run HEC executables, without recompilation, anywhere there is a run-time system that obeys the specification. Dept. of Computer Engineering, ICOER, Pune. 3 Virtual Machine Definitions Virtual machine was originally defined by Popek and Goldberg as "an efficient, isolated duplicate of a real machine". Current use includes virtual machines which have no direct correspondence to any real hardware. Virtual machines are separated into two major categories, based on their use and degree of correspondence to any real machine. A system virtual machine provides a complete system platform which supports the execution of a complete operating system (OS). In contrast, a process virtual machine is designed to run a single program, which means that it supports a single process. An essential characteristic of a virtual machine is that the software running inside is limited to the resources and abstractions provided by the virtual machine -- it cannot break out of its virtual world. Example: A program written in Java receives services from the Java Runtime Environment (JRE) software by issuing commands to, and receiving the expected results from, the Java software. By providing these services to the program, the Java software is acting as a "virtual machine", taking the place of the operating system or hardware for which the program would ordinarily be tailored.