Concurrent Programming

The name given to programming notations and techniques to express potential parallelism and solve the resulting communication and synchronization problems.
Why do we need it ? To allow the expression of potential parallelism so that more than CPU can solve the problem.
To model the parallelism in the real world.
Alternative is difficult: programmer must construct sequential program as a cyclic execution of task to handle various concurrent activities.
This make programming difficult and introduction of structures which are irrelevant to the problem at hand.
Parallel execution on multiple CPU's is difficult.

A concurrent program is a collection of autonomous sequential processes executing logically in parallel.
Each process has a single thread of control.
The actual implementation of these collection of processes can be via:
- Multiprogramming: Multiple processes multiplex their executions on a single processor.
- Multiprocessing: Multiple processes multiplex their executions on a a multiprocessor system where there is access to shared memory.
- Distributed processing: Multiple processes multiplex their executions on several processors where they do not share memory.

Real time systems

A real time system is any system which has to respond to a stimuli within a finite and specified period.
The correctness depends on both the result and the time it was delivered.
Failure to respond is as bad as the wrong response.
Types of real time systems
- Hard real-time — systems where it is absolutely imperative that responses occur within the required deadline. E.g. Flight control systems.
- Soft real-time — systems where deadlines are important but which will still function correctly if deadlines are occasionally missed. E.g. Data acquisition system.
- Firm real-time — systems which are soft real-time but in which there is no benefit from late delivery of service.