Transcript process
Operating Systems COMP 4850/CISG 5550 Processes Introduction to Threads Dr. James Money Processes • A process is the abstraction of a running program. This includes the code, associated memory, among other items. • Most operating systems allow more than one process to be runnable at a given time. Process Model • A process is an executing program • We mentally think of the process having a virtual CPU. • However, in reality, the CPU switches between active processes in a rapid fashion. • This switching is called multiprogramming. Process Model Process Creation • We need a way to create and terminate a process • There are a number of ways a process is created. Process Creation • Processes are created when: – System initialization – Execution of a process creation system call by a running process – A user request to create a new process – Initiation of a batch job Types of processes • There are two types of processes: – Foreground – interact with user – Background – run a particular function. • These are called daemons. • Typically these handle mail, web pages, remote file requests, etc. Creating Processes • UNIX: – fork() – Followed by exec() • Windows – CreateProcess • Separate address space for each process Process Termination • Eventually a processes ends or terminates due to one of the following: – Normal, voluntary exit – Error exit, voluntary – Fatal error, involuntary – Killed by another process, involuntary Process Hierarchies • When processes create new processes, you get a hierarchy in UNIX, which also called a process group. • One process creates a new one, which is called the child process. • The process that create the new process is called the parent process. • Windows has no hierarchy. Process States • Many times processes interact with other process • Example: – cat chapter1 chapter2 chapter3|grep tree • Grep may run before cat starts it’s output • This state is called blocked. Process States • There are three states – Running – using the CPU – Ready – runnable, but stopped – Blocked – waiting for an external event to occur Process States • Examples of transitions between states: Process States • This model results in a scheduler, which handles which process is executing a given instant. • We think of the scheduler being the base of the operating system • The scheduler handles interrupts and scheduling. Implementing Processes • To implement, we use a process table, with one entry per process. • Each entry is a called a process control block. Implementing Processes • Each PCB has – Process state – Program counter – Stack pointer – Memory allocations – Open files – Scheduling information – Other info needed for running PCBs Handling Interrupts Threads • Normally, each process has one thread of execution, or shortened, a thread. • Many times it is desirable to have multiple threads • Sometimes called lightweight processes. • We use the term multithreading to refer to the fact multiple threads are running in a single process. Thread Model Thread Model • Different threads != different processes • No memory protection in threads, so they can wipe other threads memory values • Threads have unique: – Program counters – Registers – Stack – State information Thread Model Thread Model • Threads have three states like processes, plus a final one: – Running – Blocked – Ready – And terminated • Threads have independent stacks Thread Model Thread Functions • thread_create() – create a new thread from a function pointer • thread_exit() – exit a thread • thread_wait() – wait for a thread to finish • thread_yield() – voluntarily give up CPU to other threads