CGS 3763 Operating Systems Concepts Spring 2013 Dan

CGS 3763 Operating Systems Concepts Spring 2013 Dan C. Marinescu Office: HEC 304 Office hours: M-Wd 11: 30 - 12: 30 AM

Lecture 18 – Monday, February 18, 2013 n Last time: Answers to student questions from last week’s lectures ¨ Threads ¨ n Today: ¨ n Next time ¨ n Threads More on threads Reading assignments Chapter 4 of the textbook ¨ Chapters 4 textbook slides ¨ Lecture 18 2

Threads n Thread the smallest sequence of programmed instructions that can be managed independently by an operating system scheduler ¨ a light-weight process ¨ multiple threads can share the same address space On a single processor, multithreading generally occurs by timedivision multiplexing, the processor switches between different threads. This context switching generally happens frequently enough that the user perceives the threads or tasks as running at the same time. On a multiprocessor or a multi-core system the threads actually run at the same time, with each processor or core running a particular thread or task ¨ n n Lecture 18 3

Single- and multi-threaded processes Lecture 18 4

Reasons for multithreading n n Reduce the execution time of a compute-intensive task. Better resource utilization – recall the imbalance between the CPU, memory, and I/O device bandwidth/speed. Improve scalability Example: multithreaded execution on a two core system Lecture 18 5

Challenges n n n Identify activities that can run in parallel. Divide the workload among the threads and balance the load. Minimize the communication among threads. Avoid barrier-synchronization instances when all threads have to wait until all of them reach a certain execution stage. Debugging. Lecture 18 6

Multithreading and client-server systems Lecture 18 7

Common models for threaded programs n n n Manager/worker: a single thread, the manager assigns work to other threads, the workers. Typically, the manager handles all input and parcels out work to the other tasks. At least two forms of the manager/worker model are common: static worker pool and dynamic worker pool. Pipeline: a task is broken into a series of suboperations, each of which is handled in series, but concurrently, by a different thread. An automobile assembly line best describes this model. Peer: similar to the manager/worker model, but after the main thread creates other threads, it participates in the work. Lecture 18 8

Shared memory model for threads n n n All threads have access to the same global, shared memory Threads also have their own private data Programmers are responsible for synchronizing access (protecting) globally shared data. Lecture 18 9

Thread-safeness n n The ability of an application to execute multiple threads simultaneously without "clobbering" shared data or creating "race" conditions. For example, suppose that your application creates several threads, each of which makes a call to the same library routine: ¨ This library routine accesses/modifies a global structure or location in memory. ¨ As each thread calls this routine it is possible that they may try to modify this global structure/memory location at the same time. ¨ If the routine does not employ some sort of synchronization constructs to prevent data corruption, then it is not thread-safe. Lecture 18 10

Multiple threads can share a module Lecture 18 11

User and kernel threads n User threads- thread management done by user-level threads library ¨ POSIX Pthreads Win 32 threads ¨ Java threads ¨ n Kernel threads ¨ ¨ ¨ n n Windows XP/2000 Solaris Linux Tru 64 UNIX Mac OS X Recall that the kernel has to keep track of all processes on the system and uses the PCB to do that. The kernel has to keep track of al threads as well. Lecture 18 12

Relations between user and kernel threads n n n Many-to-One several user-level threads mapped to single kernel thread: ¨ Solaris Green Threads ¨ GNU Portable Threads One-to-One each user-level thread maps to kernel thread: ¨ Windows NT/XP/2000 ¨ Linux ¨ Solaris 9 and later Many-to many user level threads mapped to many kernel threads ¨ Allows the operating system to create a sufficient number of kernel threads ¨ Solaris prior to version 9 ¨ Windows NT/2000 with the Thread. Fiber package Lecture 18 13

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