PROCESS MANAGEMENT

Briefing on communications among processes

• Mechanism for processes to communicate and to synchronize their actions.
• Message system – processes communicate with each other without resorting to shared variables.
• IPC facility provides two operations:
• send(message) – message size fixed or variable
• receive(message)
• If P and Q wish to communicate, they need to:
• establish a communication link between them.
• exchange messages via send/receive.

Mutual Exclusion

• Mutual exclusion algorithms are used in concurrent programming to avoid the simultaneous use of a common resource, such as a global variable, by pieces of computer code called critical sections.
• A critical section is a piece of code in which a process or thread accesses a common resource.
• The critical section by itself is not a mechanism or algorithm for mutual exclusion.
• A program, process, or thread can have the critical section in it without any mechanism or algorithm which implements mutual exclusion.

Synchronization

• Message passing may be either blocking or non-blocking
• Blocking is considered synchronous
• Blocking send has the sender block until the message is received
• Blocking receive has the receiver block until a message is available
• Non-blocking is considered asynchronous
• Non-blocking send has the sender send the message and continue
• Non-blocking receive has the receiver receive a valid message or null
• Synchronization is timekeeping which requires the coordination of events to operate a system in unison.
• The familiar conductor of an orchestra serves to keep the orchestra in time.

Deadlock

• Deadlock can arise if four conditions hold simultaneously.
• Mutual exclusion: only one process at a time can use a resource.
• Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes.
• No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task.

Recognize interaction and synchronization among process

• Many systems provide hardware support for critical section code
• Uniprocessors – could disable interrupts
• Currently running code would execute without Preemption.
• Generally too inefficient on multiprocessor systems
• Operating systems using this not broadly scalable
• Modern machines provide special atomic hardware instructions
• Atomic = non-interruptable
• Either test memory word and set value or swap contents of two memory words