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What is the difference between a process and a thread



Both processes and threads are independent sequences of execution. The typical difference is that threads (of the same process) run in a shared memory space, while processes run in separate memory spaces.
I'm not sure what "hardware" vs "software" threads might be referring to. Threads are an operating environment feature, rather than a CPU feature (though the CPU typically has operations that make threads efficient).
Erlang uses the term "process" because it does not expose a shared-memory multiprogramming model. Calling them "threads" would imply that they have shared memory.


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.
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