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managing multiple PCBs, and with the built-in facility for sharing memory and other resources among threads of the same process. Instead of a PCB, a thread requires a much smaller thread table entry, primarily to provide storage for register and program counter contents when the OS switches among executing threads. In an operating system that supports threads ( kernel threads, which we will discuss shortly), the object that gets scheduled, and which gains control of the CPU, is the thread. The process is still important, but as a collection of resources. Threads are what get executed, and threads are what enter the states of Ready, Running, and Waiting. Since threads resemble processes, some operating systems refer to threads as lightweight processes (e.g., SunOS). And just as OSs came to be described as multiprogrammed when they began to support multiple processes simultaneously, OSs that support threads are described as multithreaded. Threads are useful because many applications benefit from separating the whole job into semi-independent parts. Earlier we mentioned the word-processor/spell-checker example. Another example could be a timeand-attendance application where one thread does a quick check of the employee ID as the employee enters or leaves the plant, and another thread updates the payroll data base in the background. In this threaded design, the quick gate-check thread can keep people moving through the gates, even if the more complex data-base thread takes a little more time. Web servers are almost universally multithreaded. The main server thread waits for a request to arrive, and, when one does, it creates a new worker thread which then handles the request and provides the reply. The main server s only job is to recognize a request and then to create a thread to do the work of responding. This multithreaded approach keeps the web server responsive to new requests. Multithreading is efficient compared to the alternative of using multiple processes, because creating a new thread requires about one hundredth the time that creating a new process requires. Another advantage of threads appears when the computer contains multiple CPUs. In that case, different threads can be executed on different CPUs, with the potential for completely parallel execution and great speed improvement. Threads can be provided as user threads or kernel threads. Threads provided by the OS are kernel threads. If the OS itself does not support threading, it is possible to implement threading using a user thread library that executes as a user process. For example, the POSIX thread package is a user thread package available for UNIX computers. Kernel threads have the advantages associated with the OS itself being aware of the threads. For instance, since the OS does the scheduling of the threads, if one thread must wait for some reason (e.g., for I/O to complete), the OS can schedule another thread from the same process. Likewise, when one thread exhausts its timeslice, the OS can choose another thread from the same process to receive the next timeslice. User threads have some advantages, too. Since the OS has no knowledge itself of the threads, the OS does not get involved with thread creation and scheduling. This lack of OS involvement means less overhead, so creating threads and switching among threads can be more efficient with user threads. The disadvantages of user threads are the reverse of the advantages of kernel threads. When one thread must wait, the OS will block the entire process, for the OS has no knowledge of the different threads within the user process. Likewise, a user thread may claim all of its process CPU time, to the exclusion of the other threads. When the user thread package transfers control to a thread, the user thread package has no way to interrupt the executing thread after some timeslice, because the user thread package does not gain control on a clock tick as the OS does. In the case of Java and some other languages, threads are provided as part of the language. The implementation of the Java Virtual Machine determines how the Java Thread class is mapped to the thread model of the OS. The Java programmer is protected from having to know the details of the underlying threading model of the computer. SYNCHRONIZATION Often processes or threads must share access to the same information. For instance, imagine an application where one thread, the server thread, adds requests to a list of pending tasks, and another thread, the worker thread, reads from the list, removes the task from the list, and executes the task. Appreciate, too, that adding a request to a list will require more than one machine instruction to accomplish; the list object must be referenced, the address of the last element must be calculated, and some reference to the new task must be calculated (perhaps the address of a String buffer) and stored in the appropriate list memory location. In addition,
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