Concurrent Execution

CS561 Computer Architecture

Hye Yeon Kim

Chapter topicsChapter 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

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.

Non- deterministic execution

the order of the p1, p2 execution is not known

write(“in p1”,x) # added to process p1write (“in p2”,x) # added to process p2

The output from one write will not one interleaved with the output from the other.

• 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)

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

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.

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

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.

Process scheduling & priorities

• setproirity function