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CPU Scheduler
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== Scheduling in Linux == Multitasking kernels (like Linux) allow more than one process to exist at any given time, and furthermore each process is allowed to run as if it were the only process on the system. Processes do not need to be aware of any other processes unless they are explicitly designed to be. This makes programs easier to develop, maintain, and port. Though each CPU in a system can execute only one thread within a process at a time, many threads from many processes appear to be executing at the same time. This is because threads are scheduled to run for very short periods of time and then other threads are given a chance to run. A kernel's scheduler enforces a thread scheduling policy, including when, for how long, and in some cases where (on Symmetric Multiprocessing (SMP) systems) threads can execute. Normally the scheduler runs in its own thread, which is woken up by a timer interrupt. Otherwise it is invoked via a system call or another kernel thread that wishes to yield the CPU. A thread will be allowed to execute for a certain amount of time, then a context switch to the scheduler thread will occur, followed by another context switch to a thread of the scheduler's choice. This cycle continues, and in this way a certain policy for CPU usage is carried out.
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