CS 541 Lecture -*- Outline -*-

* problems with parallel systems

ad: what are the important questions in designing a parallel language?
reference: Filman/Friedman, Coordinated Computing (McGraw-Hill 1984)

** General questions
*** What is the problem?   What are your goals?
*** What have others done?


** Computational power
	What kind of turing machine is a parllel language equivalent to?
		TMs with unbounded random number oracle?
		what about reactive systems: interactive behavior

** preventing race conditions

*** mutual exclusion
	deposit 150 and 50 dollars in an account
		read, read, add, add, write, write (oops)

	synchronization of processes orders execution...

	invariants, transactions, and critical regions

	solutions include semaphores, conditional critical regions, monitors...

	a generalization is the readers and writers problem

*** deadlock

	e.g., dining philosphers problem

*** starvation

	often results from priorities

** problem domains (approaches to parallel programming)

*** operating systems
	control of distributed system
	matching language to the machine

	issues: ease of machine control
		efficiency

	common: process = processor
		communications structure static

*** pragmatic
	tools to aid programming process
	may not be easy to implement

*** semantic

	study systems as mathematical objects
	hope to clarify semantics of parallelism and prove program correctness

	issue: elegance

*** analytic
	analyze efficiency of algorithms

*** heuristic organizations

	AI, symbolic processing

	issues: natural translation of ideas into programs

*** others?

	coordination language: sits on top of normal language

** What primitives?

*** process creation
	explicit or implicit?
		If explicit, how do processes communicate? 

*** process dynamics
	
	issues: ease of machine control
		efficiency

	common: process = processor
		communications structure static

	if explicit processes:
		can processes be created at run-time?
			if so, what can be done with the names?
			gc?

*** synchronization

	shared storage?
	messages?
		asynchronous (send and forget) vs. synch
		interaction with process structure

*** buffering
	bounded or unbounded

*** communication control
	rules for establishing communication
		caller-callee (RPC)
		send-recieve (receiver is active)

		pattern matching

	selection of which call to recieve takes many forms:
		queues, guarded commands,...

*** communication connection
	single vs. multiparty
	ports, channels
	labels (proc names)

*** time-out
	specified time?  Instantaneous

*** fairness
	each process gets a turn?

	opposite is notion of priority:
		can this really be defined?

*** failure
	what kinds?
		node crash, communications, node going haywire

	what to do?
		time-out, exception handlers, redundancy, atomic actions,
		functional accuracy (algorithms that tolerate failures)