Multiple Processes can execute concurrently in a Computer System
Processes may be Created and Deleted dynamically
How do you manually create and delete a Process?
Our Focus is: How to dynamically create and delete Processes
Operating Systems CS 33211
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Operations on Processes Process Creation
We use system call to create a Process
Unix based system call:
fork ()
The Creating Process
Parent Process
Needs Resources (memory, CPU Time, I/O Devices) to perform Tasks
The New Process
Child Process (of the Parent Process)
The Child Process can create one or more processes
Needs Resources (memory, CPU Time, I/O Devices) to perform Tasks
Operating Systems CS 33211
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Operations on Processes Process Creation
Resource Sharing Options
Parent and Child Share no Resources
Child Shares a subset of Parent’s Resources
Parent & Child Share All Resources
Process Tasks
Parent and Child may execute tasks concurrently
Parent suspends execution (waits) until Child terminates
Address Space
Child has Program Loaded into it
Child is duplicate of Parent
Parent and Child have Separate Address Space
Operating Systems CS 33211
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Operations on Processes Tree Process on a Typical Solaris System ( Go to Slide 21)
Process Tasks Summary
pageout: Memory Management fsflush – File System Management init – parent for user processes
Operating Systems CS 33211
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Operations on Processes C Program forking Separate Process ( )
Use the fork () system call to create a new process (Child Process) When the Child Process is Created, the Parent and Child will execute the trailing instruction after fork ()
Use the return value from fork() to make a distinction between Parent and Child Process If fork ( ) returns:
Two different processes running a copy of the same program
Negative value Process Creation Failed (No Child Process) 0 To the Child Process Positive Value The Process ID of the Child Process to the Parent
Always perform a test after a fork ( ) system call
Operating Systems CS 33211
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Operations on Processes C Program forking Separate Process ( ) [ Go to slide 20] Parent Process makes fork () system call
int main () { Pid_t pid;
pid = fork (); if (pid < 0) {
….. }
…….. }
Operating Systems CS 33211
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Operations on Processes C Program forking Separate Process ( ) [ Go to slide 20] Suppose fork () system call is successful Parent Process int main () { Pid_t pid; pid = fork (); if (pid < 0) { ….. } …….. }
Child Process int main () { Pid_t pid; pid = fork (); if (pid < 0) { ….. } …….. }
Both Process resume execution after the system call if (pid < 0) • Both Processes have identical variables and values but different address spaces •Variable changes in a Process will not affect the other Process Operating Systems CS 33211
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Operations on Processes C Program forking Separate Process ( ) [ Go to slide 20] Suppose fork () system call is successful Parent Process int main () { Pid_t pid; pid = fork (); pid 123 if (pid < 0) { ….. } else if (pid == 0){ execlp (“/bin/ls”, “ls, NULL); } else{ wait (NULL); printf (“Child Complete”); exit (0); } …….. }
Parent calls wait () Operating Systems CS 33211
Child Process int main () { Pid_t pid; pid = fork (); pid 0 if (pid < 0) { ….. } else if (pid == 0){ execlp (“/bin/ls”, “ls, NULL); } else{ wait (NULL); printf (“Child Complete”); exit (0); } …….. }
Child calls execlp () 9
Process Termination System Call exit( ) [ Go to slide 22]
Process Terminates itself by issuing System Call
exit(status)
Calling Process Resources are deallocated by OS
File Descriptors etc..
Operating Systems CS 33211
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Process Termination Scenarios
Suppose Child Process executes system call exit (stats), while Parent is executing system call wait(int *stat_loc):
OS will notify Parent about Terminated Child Process
Suppose Child Process executes system call exit (stats); but Parent is not executing wait( )
Child process becomes a zombie process
wait( ) returns the pid of the Terminated Child process
No user or kernel space/resources Only has entry in the process table
Suppose Parent terminates a child process using system call abort (pid) [ Child exceeds its resources]
All children processes of the child are also terminated (Cascading Termination)
Operating Systems CS 33211
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Inter-process Communication Cooperating Processes
Processes may run Independently Process A can not affect or be affected by Process B’s execution It is advantageous for several Processes to Cooperate (i.e work together): Information Sharing – Several users may want concurrent access to the same information Computational Speed-up - Divide complex tasks to smaller tractable subtasks and assign sub-tasks to PEs for concurrent executions Modularity – Divide system functions into separate processes (why? – efficiency, code maintenance, teamwork..) Convenience – A User may choose to run several tasks at the same time
Operating Systems CS 33211
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Inter-Process Communication (IPC) Cooperating Processes
Basic Approach for IPC
Shared Memory – Processes exchange information by reading and writing data to a shared region in memory
Useful for large data Systems calls mainly to create and attach to memory followed by routine memory access calls Maximum speed-up
Message Passing – Processes exchange information by means of messages (packets)
Advantageous for small data Disadvantage – Overhead, several system calls, kernel interventions
Operating Systems CS 33211
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Inter-Process Communication Shared Memory
“Producer” Process and “Consumer” Process use a buffer in shared memory
“Producer” Process fills buffer “Consumer” Process empties the buffer Buffer is a circular array
access based on FIFO
Bounded Buffer: fixed size 4th
Producer waits if buffer is full ((in + 1) % BUFFER_SIZE)) == out
in (Next free position)
3rd
2nd 1st
Consumer waits if buffer is empty in == out
out (first full position)
So what is the unbounded buffer? Operating Systems CS 33211
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Inter-Process Communication Message Passing
OS provides a set of system utilities (primitives) to facilitate packet exchange amongst Processes
How links are established between two Processes Link capacity Message size Bi-directional and uni-directional links How to recover lost messages What happens if sender or receiver process dies
Operating Systems CS 33211
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Inter-Process Communication Message Passing: Direct Communication Process P to Q:
send(Q, message) P sends message to Q, then OS blocks processing in P until Q receives the message [Blocking send] receive (P, message)
Q is blocked by the OS until a message is received from P [Blocking receive]
Operating Systems CS 33211
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Inter-Process Communication Message Passing: Indirect Communication
Messages are sent to and received from mailboxes (ports): Sender/Receiver Processes must share the same mailbox – Sender/Receiver Link Communication Link may be established with more than two Processes Communication Link may be bi-directional or uni-directional
OS provides primitives to: Create a new mailbox Send and Receive Messages through the Mailbox Delete a Mailbox
Operating Systems CS 33211
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Inter-Process Communication Indirect Communication: Three or More Processes Assume P1, P2 and P3 share Mailbox A Let P1 send a message to mailbox A; assume P2 and P3 execute the receive( ) Who gets the message from A? It depends on method used to create the links:
Allow a link to be associated with at most two processes Allow only one process at a time to execute a receive operation Allow the Os to select arbitrarily the receiver process
Operating Systems CS 33211
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Client/Server Model
Server
Process (or set of processes) that provides one or more services
Client
Mail server
Process that requests the services from a server Netscape, IE Explorer, Outlook
Client/Server Protocol:
Client sends request message to the server Server performs requested service and sends reply message to Client
Operating Systems CS 33211
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Client/Server Model End to End Communication [Go to Slide 39]
Socket: Endpoint for Communication
IP Address : Port Number
Server Process listens to client request on designated port Well known ports
Ports assigned number < 1024 fro Specific service
Client Process initiates request Host assigns client a port number > 1024
