43 lines
1.5 KiB
Markdown
43 lines
1.5 KiB
Markdown
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---
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id: zadl
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tags: [operating-systems]
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created: Wednesday, June 26, 2024
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---
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# Threads
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A  is a running instance of a given program. A program
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runs sequentially handling one task at a time, however we may need to run
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certain tasks in parallel.
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Think of a program that is downloading a resource from the internet and wants to
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update the UI at the same time to show the download process.
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If the program is strictly sequential, once the program starts the download, the
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UI is neglected since the CPU time dedicated to the process must be focused
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elsewhere.
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We need the UI update and the download to execute in parallel. This is achieved
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via operating system **threads of execution**.
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> A thread is a schedulable unit of execution within a process. It runs within a
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> process, shares its memory address and can execute any program code loaded
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> within it.
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Each thread has an identifier called a _thread ID_ (TID).
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Both procesess in [user space](User_Space.md) and the [kernel](The_kernel.md)
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utilise threads.
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## Hardware considerations
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Each [processor core](./CPU_architecture.md) can only run one thread at a time
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so the number of cores that a device's CPU has deterimines how many threads can
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run at once. In practice, then, the CPU shuffles between multiple threads in
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[clock segments](./Clock_signals.md). This is known as scheduling and is managed
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by the process that the thread belongs to.
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## Related notes
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