13 concurrency

13 concurrency

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  ICS 313 -Fundamentals of Programming Languages 1 13. Concurrency 13.1 Introduction Concurrency can occur at four levels: 1. Machine instruction level 2. High-level language statement level 3. Unit level 4. Program level Because there are no language issues in instruction and program-level concurrency, they are not addressed here The Evolution of Multiprocessor Architectures Late 1950s - One general-purpose processor and one or more special- purpose processors for input and output operations Early 1960s - Multiple complete processors, used for program-level concurrency Mid-1960s - Multiple partial processors, used for instruction-level concurrency Single-Instruction Multiple-Data (SIMD) machines. The same instruction goes to all processors, each with different data - e.g., vector processors Multiple-Instruction Multiple-Data (MIMD) machines - Independent processors that can be synchronized (unit-level concurrency)
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  ICS 313 -Fundamentals of Programming Languages 2 13.1 Introduction (continued) A thread of control in a program is the sequence of program points reached as control flows through the program Categories of Concurrency Physical concurrency - Multiple independent processors (multiple threads of control) Logical concurrency - The appearance of physical concurrency is presented by time-sharing one processor (software can be designed as if there were multiple threads of control) Coroutines provide only quasi-concurrency Reasons to Study Concurrency It involves a different way of designing software that can be very useful - many real-world situations involve concurrency Computers capable of physical concurrency are now widely used 13.2 Intro to Subprogram-Level Concurrency A task is a program unit that can be in concurrent execution with other program units Tasks differ from ordinary subprograms in that: A task may be implicitly started When a program unit starts the execution of a task, it is not necessarily suspended When a task’s execution is completed, control may not return to the caller Tasks usually work together A task is disjoint if it does not communicate with or affect the execution of any other task in the program in any way Task communication is necessary for synchronization Task communication can be through: Shared nonlocal variables Parameters Message passing
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  ICS 313 -Fundamentals of Programming Languages 3 13.2 Intro to Subprogram-Level Concurrency (continued) Kinds of synchronization Cooperation Task A must wait for task B to complete some specific activity before task A can continue its execution e.g., the producer-consumer problem Competition When two or more tasks must use some resource that cannot be simultaneously used e.g., a shared counter Competition is usually provided by mutually exclusive access Providing synchronization requires a mechanism for delaying task execution Task execution control is maintained by a program called the scheduler, which maps task execution onto available processors Tasks can be in one of several different execution states: New - created but not yet started Runnable or ready - ready to run but not currently running (no available processor) Running Blocked - has been running, but cannot now continue (usually waiting for some event to occur) Dead - no longer active in any sense 13.2 Intro to Subprogram-Level Concurrency (continued) Liveness is a characteristic that a program unit may or may not have In sequential code, it means the unit will eventually complete its execution In a concurrent environment, a task can easily lose its liveness If all tasks in a concurrent environment lose their liveness, it is called deadlock Design Issues for Concurrency How is cooperation synchronization provided? How is competition synchronization provided? How and when do tasks begin and end execution? Are tasks statically or dynamically created? Methods of Providing Synchronization: Semaphores Monitors Message Passing
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  ICS 313 -Fundamentals of Programming Languages 4 13.7 Java Threads The concurrent units in Java are run methods The run method is inherited and overriden in subclasses of the predefined Thread class The Thread Class Includes several methods (besides run) start, which calls run , after which control returns immediately to start yield, which stops execution of the thread and puts it in the task ready queue sleep, which stops execution of the thread and blocks it from execution for the amount of time specified in its parameter Early versions of Java used three more Thread methods suspend, which stops execution of the thread until it is restarted with resume resume, which restarts a suspended thread stop, which kills the thread 13.7 Java Threads (continued) Competition Synchronization with Java Threads A method that includes the synchronized modifier disallows any other method from running on the object while it is in execution If only a part of a method must be run without interference, it can be synchronized Cooperation Synchronization with Java Threads The wait and notify methods are defined in Object, which is the root class in Java, so all objects inherit them The wait method must be called in a loop Example - the queue See Queue class (pp. 549-550) and the Producer and Consumer classes (p. 550)