Multithreading (Concurrency) in Java “Some People, When Confronted
Multithreading (Concurrency) in Java “Some people, when confronted with a problem, think, "I know, I’ll use threads," and then two they hav erpoblesms.” — Ned Batchelder Multi-tasking In the beginning, computers ran in batch mode: a single program with total access to all resources. Later, a multi-tasking operating system enabled multiple (independent) jobs to run (more or less) simultaneously, even though there was only a single CPU. The jobs take turns running some of their code at one time. This type of interleaving of jobs is called concurrency. Multitasking is performing two or more tasks (or jobs) at roughly the same time. Nearly all operating systems are capable of multitasking by using one of two multitasking techniques: process-based multitasking and thread-based multitasking. Each running job was (and sometimes still is) called a process, which has state (a collection of data and handles to system resources such as files and devices) and a single sequence of instructions. It turns out that many tasks are easier to write, debug, and manage if organized as multiple copies of a process (e.g., a web server). Processes are sometimes called tasks, threads, or jobs. (Although there are some distinctions between these concepts, we will ignore them for now.) Time slice, (context) switching, and scheduling are operating system (“OS”) terms related to multi-tasking. The amount of time a task is allowed to use the CPU exclusively is called a time slice, typically 10ms (a hundredth of a second) but can vary depending on your OS and other factors from 1ms to more than 120ms.
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