Instructore: Tasneem Darwish1 University of Palestine Faculty of Applied Engineering and Urban Planning Software Engineering Department Concurrent and Real Time programming Introduction

Instructore: Tasneem Darwish2 Outlines  Concurrency  Real time systems

Instructore: Tasneem Darwish3 Concurrency  Concurrent programming is the name given to the notations that are used to express parallelism in an application program.  Concurrent programming also expresses the techniques for dealing with communication and synchronization between parallel entities.

Instructore: Tasneem Darwish4 Concurrency  why to use concurrency? To fully utilize the processor (the processor is so fast with respect to the I/O devices) To allow more than one processor to solve a problem ( A sequential program can only be executed by one processor. A concurrent program is able to exploit parallelism and obtain faster execution). To model parallelism in the real world (Real-time and embedded programs have to control and interface with real- world entities (robots, conveyor belts, etc.) that are inherently parallel).

Instructore: Tasneem Darwish5 Concurrency  Concurrency problems:  Concurrent activities need to coordinate their actions.  the coordination can involve intricate patterns of communication and synchronization.  Not managing communication and synchronization very well can result in new error arising.

Instructore: Tasneem Darwish6 Concurrency  Examples of problems in concurrent programs:  deadlock may occur where each concurrent activity is waiting for another to perform an operation  interference may occur when two or more concurrent activities attempt to update the same object; this can result in the object's data becoming corrupt;  starvation may happen where one or more concurrent activities are continually denied resources as a result of the actions of the others.

Instructore: Tasneem Darwish7 Concurrency  The desired concurrent program  The desired behaviour of a concurrent program is safety and aliveness.  The safety property expresses the requirement for "nothing bad to happen“ (the concurrent activities do not interfere with each other and cause data corruption).  The aliveness property expresses the requirement that "something good will happen". (concurrent activities are able to make progress and do not suffer from deadlocks or starvation).

Instructore: Tasneem Darwish8 Concurrency  How deadlocks happen?  There are four necessary conditions that must exist if deadlock is to occur. 1.Mutual exclusion (only one concurrent activity can use a resource at once) 2.Hold and Waite (there must exist concurrent activities that are holding resources while waiting for others resources to be acquired) 3.No pre-emption (resources acquired by a concurrent activity cannot be forcibly taken away from it by another activity.) 4.Circular wait (a circular chain of concurrent activities must exist)

Instructore: Tasneem Darwish9 Real Time systems  real-time systems are systems that have to respond to externally generated input within a finite and specified time interval.  They are inherently concurrent because they are often embedded in a larger engineering system and have to model the parallelism that exists in the real-world objects that they are monitoring and controlling.  The RTSJ extends the Java concurrency model to allow it to interact with external events.

Instructore: Tasneem Darwish10 Real Time systems  Some real-time systems characteristics: 1.Large and complex. 2.Extremely reliable and safe. 3.Interaction with hardware interfaces.

Instructore: Tasneem Darwish11 Real Time systems  Hard and soft real time systems: Hard real-time systems are those where it is absolutely important that responses occur within the specified deadline. Soft real-time systems are those where response times are important, but the system will still function correctly if deadlines are missed. In interactive systems there are no explicit deadlines.