Process:
- An executing instance of a program is called a process.
- Some operating systems use the term ‘task‘ to refer to a program that is being executed.
- A process is always stored in the main memory also termed as the primary memory or random access memory.
- Therefore, a process is termed as an active entity. It disappears if the machine is rebooted.
- Several process may be associated with a same program.
- On a multiprocessor system, multiple processes can be executed in parallel.
- On a uni-processor system, though true parallelism is not achieved, a process scheduling algorithm is applied and the processor is scheduled to execute each process one at a time yielding an illusion of concurrency.
- Example: Executing multiple instances of the ‘Calculator’ program. Each of the instances are termed as a process.
Thread:
- A thread is a subset of the process.
- It is termed as a ‘lightweight process’, since it is similar to a real process but executes within the context of a process and shares the same resources allotted to the process by the kernel (See kquest.co.cc/2010/03/operating-system for more info on the term ‘kernel’).
- Usually, a process has only one thread of control ? one set of machine instructions executing at a time.
- A process may also be made up of multiple threads of execution that execute instructions concurrently.
- Multiple threads of control can exploit the true parallelism possible on multiprocessor systems.
- On a uni-processor system, a thread scheduling algorithm is applied and the processor is scheduled to run each thread one at a time.
- All the threads running within a process share the same address space, file descriptor, stack and other process related attributes.
- Since the threads of a process share the same memory, synchronizing the access to the shared data withing the process gains unprecedented importance.
I borrowed the above info from the Knowledge Quest! blog available at:http://kquest.co.cc/2010/03/program-process-task-thread/
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