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virtual processors and threads

'virtual processors and threads'
Virtual Processors and Threads 11-20037-1 © 2001, 2003 International Business Machines Corporation Virtual Processors and Threads Module 7 7-2Virtual Processors and Threads Objectives 7-2 At the end of this module, you will be able to: ?Define a thread ?Describe the multithreaded architecture ?Describe how the virtual processors are implemented in UNIX ?Use onstat to monitor VPs and threads ?List and explain the virtual processor classes ?Describe how network connections are handled by the server ?Set server configuration parameters related to VPs and threads ?Dynamically add and remove virtual processors Virtual Processors and Threads7-3 Before looking at server architecture, it is helpful to first examine the general concept of a thread. A thread can be thought of as a sequence of instructions being executed in a program. When multiple threads are run within the same entity (in our case an entity is a process), it is called multithreading. A thread is sometimes referred to as a lightweight process in some literature because it makes fewer demands on the operating system. In the following pages, multithreading is explained by comparing the relationship between a thread and a process to the relationship between a process and an operating system. What is a Thread? 7-3 A thread is a sequence of instructions executed in a program. CPU oninit A thread is a sequence of instructions executed in a program. 7-4Virtual Processors and Threads A regular UNIX process that does not implement threads can be thought of as a single-threaded process, although we do not normally call it that. One sequence of instructions is being executed for this process, and the operating system is responsible for scheduling and running the process. Multithreading is a method of executing many iterations of a process for different users without having to form many instances of that process at the operating-system level. A multithreaded process can have multiple threads running within a UNIX process, each running sequentially and giving up control to other threads at a specific point in time. How this is accomplished makes multithreading different from simply receiving and executing requests from a single user’s process. Multithreading is a systems level concept whereby the program actually executes machine-level instructions to manipulate the process so that it executes for many users instead of just one user. The program executes these instructions entirely at the user level, not at the UNIX kernel level. As far as UNIX is concerned, this multithreaded process is a single process just like any other process. Single Threaded Vs. Multithreaded 7-4 One process, multiple threads Three processes, single threads processprocessprocess process Virtual Processors and Threads7-5 Consider how UNIX accomplishes multiprocessing. Every UNIX process has an address space that consists of three segments: text, data, and stack. The text segment contains the machine instructions that form the pr
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