[OS] Process 2

Process Part 2

Operations in Processes

• Process creation
• Process termination
• Inter-process communication
• etc…

Process Creation

• A process (parent) can create another processes (children)
• Siblings are children with the same parent process
• Form a tree of processes
• Generally, process identified and managed via a process identiier (PID)
• Resource sharing options
  • Parent and children share all resources
  • Children share subset of parent’s resources
  • Parent and child share no resources
• Execution options
  • Parent and children execute concurrently
  • Parent waits until children terminate
• fork()
  • Create a new process by cloning the current process (allocate memory)
  • Child duplicates the parent’s address space (but have different PCB)
• exec()
  • System call used ater a fork() to replace the process’ memory space with a new program
  • New process overwrite existed process (no more “existed process” exist)
• CPU scheduler decides which one to execute first between parent and child

Process Termination

• Process executes last statement and then asks the operating system to delete it using the exit() system call
  • Returns status data from child to parent via wait()
  • Parent process wait until child process dead
  • Process’ resources are deallocated by operating system
• Parent may terminate the execution of children processes using the abort() system call
  • When child has exceeded allocated resources
  • When the task assigned to child is no longer required
  • When the parent is exiting and the operating system does not allow a child to continue if its parent terminates (cascading termination)
• Parent process may wait for termination of a child process by using the wait() system call
  • The call returns status information and the pid o the terminated process
  • pid = wait(&status)
  • A process which is terminated but parents its parent has not called wait() yet becomes zombie
  • If parent terminated without calling wait(), process is orphan (child is still alive)
  • Zombie and orphan process cause memory waste problem

Multiprocess Architecture (Chrome Browser)

• Many web browsers ran as single process
  • If one web site casues trouble, entire browser can be crashed
• Google Chrome Browser is multiprocess with 3 different types of processes
  • Browser, Renderer, Plug-in
  • But, waste memory resource problem

Interprocess Communication

• Processe within a system may be independent or cooperating
• Independent processes cannot affect or be affected by the execution of another process
• Cooperating process can affect or be affected by other processes, including the procedure of sharing data
  • Information sharing
  • Computation speedup
  • Modularity
  • Convenience
• Cooperating processes need Inter-process communication(IPC)
  • Shared memory
  • Message passing

Communication Models

Producer-Consumer problem

• Producer process produces information that is consumed by a consumer process
• Make sure that the producer does not overflow the buffer
• Make sure that the consumer does not pull from an empty bufer

Interprocess Communication - Shared Memory

• An area of memory shared among the processes that wish to communicate (use buffer)
• Pros
  • The communication is under the control of the users processes not the operating system
  • OS overhead not happen
• Cons
  • Synchronization problem when lots of processes access the shared memory

Interprocess Communication - Message Passing

• OS controls the mechanism for processes to communicate and to synchronize their actions
• Message system : processes communicate with each other without resorting to shared variables
• Two operations
  • send(message)
  • receive(message)
• If processes P and Q wish to communicate, then
  • Establish a communication link between them
  • Exchange messages via send/receive

Direct Communication

• Processes must name each other explicitly
  • send(P, message) : send a message to process P
  • receive(Q, message) : receive a message from process Q
• Properties of communiation link
  • Links are established automatically
  • A link is associated with exactly one pair of communicating processes
  • Between each pair, there exist exactly one pair
  • The link may be undirectional, but is usually bi-directional
• Symmetric links
  • Both the sender and receiver specify the other partner
• Asymmetric links
  • Only the sender specifies the receiver

Indirect Communication

• Messages are directed and received from mailboxes
• Each mailbox has a unique id(POSIX message queues)
• Processes can communicate only if they share a mailbox
• send(A,message) : send a message to mailbox A
• receive(A,message) : receive a message from mailbox A
• Properties of communication link
  • Link established only if processes share a common mailbox
  • A link may be assoicated with many processes
  • Each pair of processes may share several communication links
  • Link may be unidirectional or bi-directional

Synchronization

• Message passing may be either blocking or non-bloking
• Blocking (synchronous)
  • Blocking send : the sender is blocked until the message is received
  • Blocking receive : the receiver is blocked until a message is available
• Non-Blocking (asynchronous)
  • Non-Blocking send : the sender sends the message and continue
  • Non-Bloking receive : the receiver receives a valid message or null

Communication in Client-Server systems

• Shared memory
• Message passing (Pipes)
• Sockets
• Remote Procedure Calls (RPC)

Pipes

• Ordinary pipes
  • cannot be accessed from outside the process that created it
  • Typically, a parent process creates a pipe and uses it to communiate with a child process that it created
  • allow communication in standard producer-consumer style
  • Producer writes to one end (the write-end of the pipe)
  • Consumer reads from the other end(the read-end of the pipe)
  • Unidirectional
  • Require parent-child relationship between communicating processes
• Named pipes
  • can be accessed without a parent-child relationship
  • more powerful than ordinary pipes
  • Bidirectional
  • No parent-child relationship is necessary
  • Several processes can use the named pipe for communication
  • stay alive even after process terminates

Sockets

• A socket is defined as an endpoint for communication
• Concatenation of IP address and port
• The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8
• Communication consists between a pair of sockets
• All ports below 1024 are well known, used for standard services
• Specia IP address 127.0.0.1 to refer to system on which process is running
• Three types of sockets
  • Connection-oriented (TCP)
  • Connectionless (UDP)
  • MulticastSocket class

Remote Procedure Calls (RPC)

• abstracts procedure calls between processes on networked systems
• Stubs : client-side proxy that abstracts the actual procedure
• The client-side stub locates the server and packaging parameters into a form for transmission on network
• The server-side stub receives this message, unpacks parameters and performs the procedure on the server