Processes CSCI 4534 Chapter 4 Introduction Early computer

Processes CSCI 4534 Chapter 4

Introduction • Early computer systems allowed one program to be executed at a time – The program had complete control of the computer – The program had access to all system resources • Current computer systems allow multiple programs: – To be loaded into memory – To be executed “concurrently”

Introduction • What do these terms mean: – program in execution? – unit of work in a time-sharing environment? • Do not use either of these terms unless you can more clearly embellish and define them.

Introduction • Main concerns of an operating system: – To execute user programs – To execute system programs

Process States • • • New Ready Running Waiting Terminated

Process Control Blocks • Each process is represented in the O. S. by a PCB – – Process State Program Counter CPU Register contents Information about: • scheduling priority and queues • memory limit and base registers, page tables • accounting of CPU time used, time limits, acct #s • I/O status of resources allocated and available

Process Scheduling • Scheduling Queues – Ready Queue is a linked list • A header may have pointers to head and tail • PCBs are linked together as in Figure 4. 4 – I/O device queues • Queueing Diagrams – See Figure 4. 5

Process Scheduling • Schedulers – Long-term: to load into memory • Determines degree of multiprogramming • Mix of CPU-bound and I/O-bound processes – Short-term: allocates CPU to a process • How much overhead do schedulers contribute? – Context switching

Process Scheduling • Adding a medium-term scheduler – To reduce the degree of multiprogramming – To keep down contention for CPU – Remove process(es) from memory and swap them back in later

Context Switching • CPU is switched to a different process – save state of current process – load saved state of another process • as in Figure 4. 3 • this time is pure overhead; no useful work gets done • speed ranges from 1 to 1000 microseconds

Context Switching • Hardware support for context switching – Processors may provide multiple sets of registers; change a pointer to current set – Or register data is copied between registers and memory • Threads are used to avoid performance problems of context switching

Process Creation • New child processes get their own resources or they share with the parent • The parent can either: – Execute concurrently with children – Wait until some or all children have terminated using wait( ) system call

Process Creation • Parent processes can create child processes – Using a fork( ) system call: copy of parent – Or by specifying the new process • Parent must know the child’s process id

Process Termination • A process uses the exit( ) system call to ask the O. S. to terminate it • Resources are deallocated: memory, files, buffers • A parent can ask to have a process killed by using the abort( ) system call

Cooperating Processes • Reasons for processes which can affect each other to cooperate: – to share information – to execute faster – to make more modular • There must be ways then, for processes to communicate --> IPC

Cooperating Processes • Producer-Consumer – Unbounded buffer • No limit on buffer size • Producer can always produce – Bounded buffer • Consumer waits if buffer is empty • Producer waits if buffer is full – See code pp. 108 -109

Interprocess Communication • Needs two operations: – send( ) – receive( ) • Needs a communication link – How are links established? – Are links associated with >2 processes? – What is capacity of link? – Unidirectional or bidirectional?

Interprocess Communication • Buffering: what is the capacity of the link? – Zero: no messages waiting – Bounded: there is a queue with finite length – Unbounded: any number of messages can wait; sender is never delayed • Should sent messages be acknowledged? • What about lost messages?

Client-Server Communication • Sockets – A pair of processes communicating over a network uses sockets – A socket consists of an IP address and a port number – Typically a server waits and listens for an incoming request at a specific port – A connection is made when the server accepts a connection request

Client-Server Communication • Some ports are well-known and under 1024 – ftp (port 21) – telnet (port 23) – http (port 80) • Other ports are assigned when a client process requests a connection – The host assigns it a port of a number > 1024 – The connection consists of the pair of sockets: (server. IP: port) and (host. IP: port) on the host – See Figures 4. 9, 4. 10, and 4. 11

Client-Server Communication • Remote Procedure Calls – Used for message-based communication between processes executing on separate systems – A port is a number which is used for addressing messages – Remote systems can obtain information by sending an RPC to that port number – See Figure 4. 12 – RMI is the Java version of an RPC