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If the OS allows this, then it opens the file (creates an entry in the per-process file descriptor table) and returns a handle (file descriptor, index into this table) to the user: the actual access is controlled by the OS, and the handle is a token of that. Conversely, the OS may deny access, and thus neither open the file nor return a handle.
A process with two threads of execution, running on one processor Program vs. Process vs. Thread Scheduling, Preemption, Context Switching. In computer science, a thread of execution is the smallest sequence of programmed instructions that can be managed independently by a scheduler, which is typically a part of the operating system. [1]
The idle threads are scheduled as if they each had a priority lower than can be set for any ordinary thread. Because of the idle process's function, its CPU time measurement (visible through, for example, Windows Task Manager ) may make it appear to users that the idle process is monopolizing the CPU.
This introduces the concept of a thread. A thread may be viewed as a sub-process; that is, a separate, independent sequence of execution within the code of one process. Threads are becoming increasingly important in the design of distributed and client–server systems and in software run on multi-processor systems.
Multiple threads can interfere with each other when sharing hardware resources such as caches or translation lookaside buffers (TLBs). As a result, execution times of a single thread are not improved and can be degraded, even when only one thread is executing, due to lower frequencies or additional pipeline stages that are necessary to accommodate thread-switching hardware.
In multiprogramming systems, a task runs until it must wait for an external event or until the operating system's scheduler forcibly swaps the running task out of the CPU. Real-time systems such as those designed to control industrial robots, require timely processing; a single processor might be shared between calculations of machine movement ...
Another example is a task that has been decomposed into cooperating but partially independent processes which can run simultaneously (i.e., using concurrency, or true parallelism – the latter model is a particular case of concurrent execution and is feasible whenever multiple CPU cores are available for the processes that are ready to run).
A sample thread pool (green boxes) with task queues of waiting tasks (blue) and completed tasks (yellow), in the sense of task as "unit of work". In computing, a task is a unit of execution or a unit of work. The term is ambiguous; precise alternative terms include process, light-weight process, thread (for execution), step, request, or query (for