聊城大学计算机学院计算机操作系统（2）英文课件 Lecture6.ppt

CS162Operating Systems andSystems ProgrammingLecture 6Synchronization September 19, 2005 Prof. John Kubiatowicz /~cs162 Review: ThreadFork(): Create a New Thread ThreadFork() is a user-level procedure that creates a new thread and places it on ready queue Arguments to ThreadFork() Pointer to application routine (fcnPtr) Pointer to array of arguments (fcnArgPtr) Size of stack to allocate Implementation Sanity Check arguments Enter Kernel-mode and Sanity Check arguments again Allocate new Stack and TCB Initialize TCB and place on ready list (Runnable). Review: How does Thread get started? Eventually, run_new_thread() will select this TCB and return into beginning of ThreadRoot() This really starts the new thread Review: What does ThreadRoot() look like? ThreadRoot() is the root for the thread routine: ThreadRoot() { DoStartupHousekeeping(); UserModeSwitch(); /* enter user mode */ Call fcnPtr(fcnArgPtr); ThreadFinish(); } Startup Housekeeping Includes things like recording start time of thread Other Statistics Stack will grow and shrinkwith execution of thread Final return from thread returns into ThreadRoot() which calls ThreadFinish() ThreadFinish() wake up sleeping threads Goals for Today More concurrency examples Need for synchronization Examples of valid synchronization Threaded Web Server Multithreaded version: serverLoop() { connection = AcceptCon(); ThreadFork(ServiceWebPage(),connection); } Advantages of threaded version: Can share file caches kept in memory, results of CGI scripts, other things Threads are much cheaper to create than processes, so this has a lower per-request overhead What if too many requests come in at once? Thread Pools Problem with previous version: Unbounded Threads When web-site becomes too popular – throughput sinks Instead, allocate a bounded “pool” of threads, representing the maximum level of multiprogramming ATM Bank Server ATM server problem: Service a set of requests Do so