Processes and threads

Vocabulário

English	Significado
operating systems	operating systems
process/ˈprəʊses/	An instance of a program in execution together with its execution state and allocated resources.
states/steɪts/	The possible configurations or conditions of a system at a particular point in its operation.
ready	ready
running	running
blocked/blɒkt/	blocked
thread	An execution flow inside a process — shared space, own stack.
process control block/ˈprəʊses kənˈtrəʊl blɒk/	An operating-system record containing the state and management information associated with a process.
context switch/ˈkɒntekst swɪtʃ/	Saving one task's execution state and restoring another's so the processor can change which task it runs.

English

The first illusion

An operating systems layer turns one processor into many machines. A process is a program in motion: its own address space, registers, open files.
A process lives through five states: new, ready (waiting only for the CPU), running, blocked (waiting on a device), terminated. A thread is an execution flow inside a process — shared space, own stack.

{exercise:uni-cs0701-states}

Prática

A process waiting for disk I/O is —

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English

To suspend a process the OS saves the process control block — registers, program counter, page-table pointer, open files. The context switch is exactly this save-and-restore.
With course timings, saving and restoring $2000$ bytes at $2$ GB/s costs $1$ µs — plus cache pollution. Switching threads skips the address space (shared), so it is the cheaper switch.

{exercise:uni-cs0701-switch}

Prática

Switching threads is cheaper than switching processes because threads —

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English

Process A faults on a page: running to blocked (disk); the scheduler dispatches B; the disk completes; A becomes ready; later running again — the faulting instruction restarts, so no work is lost.
A web server spawns a thread per connection: shared cache, about $50$ µs create-and-switch; as processes, about $500$ µs. Sharing is the thread's point — and unit 7.3's danger.

Example

Blocked is not ready: blocked waits on a device; ready waits only for the CPU.

{exercise:uni-cs0701-thread}

Prática

A crashed thread takes with it —

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