Lecture 7: Process & Thread Introduction

Transcription

1 COMP 150-CCP Concurrent Programming Lecture 7: Process & Thread Introduction Dr. Richard S. Hall Concurrent programming February 7, 2008

2 Operating System Concepts Definition of a process An executing program Similar to an FSP process (i.e., a set of actions that cause state transformations) A conceptual bookkeeping unit for the OS, is used to keep track of Program counter Keeps track of the next instruction to execute All CPU register contents Call stack Open files Memory (including actual program text/code) Any other resources owned by the process program counter CPU register values memory (data and text) open files... call stack

3 Operating System Concepts Reinterpreting the previous slide, the notion of a process actually embodies two distinct concepts Resource ownership Flow of execution

4 Operating System Concepts Assume that we divide a process into two pieces A resource container An execution flow After making this division, we name the resource container a process and the execution flow a thread A thread cannot exist without a process, thus a process is also a container for threads A process may contain multiple threads A process with a single thread is equivalent to our original definition of a process

5 Operating System Concepts The new process contains resources shared by all its threads, whereas the thread contains its own private resources program counter CPU register values memory (data and text) open files... call stack Original process memory (data and text) open files... New process program counter value CPU register values call stack Thread

6 Operating System Concepts Process/thread relationship Process contains threads Threads share process resources Process memory (data and text) open files... program counter value program counter value CPU program register counter values value CPU program register counter values value CPU program register counter values value call CPU stack register values call CPU stack register values call stack call stack call stack Threads

7 Operating System Concepts Why do we want threads? Performance multi-processor machines Responsiveness user interfaces Efficiency blocking calls Naturalness related, but separate activity streams

8 Operating System Concepts Technical benefits of threads Takes less time to create a new thread than a process Less time to terminate a thread than a process Less time to switch between two threads within the same process Since threads within the same process share memory and files, they can communicate with each other without invoking the kernel

9 Threading Example Implementing a web server in a single process Three possible approaches Single thread No parallelism, service only one client request at a time Finite state machine Parallelism, requires the use of nonblocking system calls, interrupts As we know, we can simulate multiple threads of controls using a finite state machine, but we no longer have a sequential process, so it is much more complicated to implement Multiple threads Parallelism with simple blocking system calls

10 Relationship to FSP Modeling processes as finite state machines using FSP/LTS. Implementing processes as threads in Java. To avoid confusion, the term process is used when referring to models, and thread when referring to an implementation in Java.

11 Java Thread Class java.lang.thread Represents an execution flow Can create new threads dynamically using standard object-oriented approach (i.e., new Thread();) A thread can execute arbitrary code So, how do we tell it which code to execute?

12 Threads in Java A Thread class manages a single sequential thread of control. Threads may be created and deleted dynamically. Thread run() MyThread run() class MyThread extends Thread { public void run() { //... } } The Thread class executes instructions from its method run(). The actual code executed depends on the implementation provided for run() in a derived class.

13 Threads in Java Since Java does not permit multiple inheritance, we often implement the run() method in a class not derived from Thread but from the interface Runnable. Runnable target Thread run() MyRun run() public interface Runnable { public abstract void run(); } class MyRun implements Runnable { public void run() { // } }

14 Thread Subclass vs Runnable A Runnable is potentially more re-usable since other pieces of the Java class library also use the Runnable interface A Runnable does not "waste" inheritance Runnable may promote better design Separation of concerns a thread is a flow of execution that executes some set of actions This creates a Thread/Runnable distinction where the Thread is the flow of execution and the Runnable is the set of actions