Concurrent Execution

1. Concurrent Execution CS561 Computer Architecture Hye Yeon Kim

2. Chapter topics • Processes • represent independent threads of control, each of which executes sequential code. • Process creation • declaration • Unabbreviated form of processes • Concurrent invocation statement • Process scheduling & priorities

3. Processes Declarations • Processes in a resources or global are created when the resource or global is created. • Processes are created as their declarations are encountered in executing the resource's or global’s initial code. (the initial code continues as soon as a process is created. It does not wait for the process to terminate.) • For process declarations that contains quantifiers, one instance of the process is created for each combination of values of the bound variables.

4. Non- deterministic executionthe order of the p1, p2 execution is not knownwrite(“in p1”,x) # added to process p1write (“in p2”,x) # added to process p2The output from one write will not one interleaved with the output from the other.

5. Program with multiple processes terminates when all processes have terminated, a deadlock has occurred, or a stop statement is executed. • Any final code in the main resource is then executed, and globals imported by the main resources are destroyed (unless a stop statement is executed with a non-zero exit status)

6. Unabbreviated form of processes • When processes need to be created as a program executes, we can use unabbreviated forms. • proc and send invocation has been abbreviated in process declaration. • send: a asynchronous invocation (i.e. non-blocking) • call: a synchronous invocation (i.e. blocking) • a send does not wait for the invoked proc to return any results; it terminates immediately after passing the arguments to the proc. A new process is created to execute the proc; it executes in parallel with the process that executed send

7. Concurrent invocation without postprocessing code • co p(3) // q() // a:= r(x,y) oc • final invocation assigns the value that r returns to a. • It terminates when all invocations terminates.

8. Concurrent invocation • It consists of one or more concurrent commands separated by // delimiters: • co concurrent_command // concurrent_command // ... oc • Each concurrent command consists of an invocation and optional block of postprocessing code. • Invocation or invocation -> block

9. Concurrent invocation with postprocessing code • All invocations are first started in parallel. Then as each invocation terminates, the corresponding postprocessing block is executed, if there is one. • Postprocessing blocks are executed one at a time; they are not executed concurrently and thus can change variables without requiring mutual exclusion.

10. Process scheduling & priorities • setproirity function