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The Department of Computer Science & Engineering
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STUART C. SHAPIRO: CSE
305
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Idea: Evaluate all the expressions concurrently, each in its own
process. Give each of them access to a global boolean
flag, initialized to true. When a process terminates, if
its result was false, it sets flag to false. As soon as
a process notices that flag is false, it terminates.
A main process is needed to spawn the subprocesses, give each of
them an argument expression to work on, and to return the final answer
to the caller. The main process should return false as soon as it
notices that flag is false.
Should the main process terminate the subprocesses when it is ready
to return false? Not if each subprocess terminates itself when
flag is false.
How could the main process ever return true? It must know that all
the subprocesses have terminated, even though flag is
still true. It can do this with a global
childProcessCount, initialized to the number of
subprocesses. When a subprocess terminates, it should decrement the
childProcessCount. Then, when the main process notices
that childProcessCount is zero, it can terminate and
return true. There are then no subprocesses left to terminate.
The global variable childProcessCount is shared among
the processes, and must be protected, so we don't get a sequence such
as: access1 decrement1 access2 decrement2 store2 store1.
The global variable flag is acted on by all the
processes, but needn't be protected because what's stored into
flag does not depend on a previous value.
ACL provides mp:process-run-function:
(mp:process-run-function
(list :name <a name for this process>
:resume-hook <a 0-argument function that's run whenever the process resumes>)
<a function the process runs>
<a sequence of arguments for the function>)
mp:make-process-lock
(mp:make-process-lock &key name)
mp:with-process-lock:
(mp:with-process-lock (<lock>) <body>)
mp:process-kill
(mp:process-kill <process>)
mp:*current-process*
and mp:process-wait
"This function suspends the current process (the value of *current-process*) until applying function to arguments yields true." [Allegro CL version 6.2 documentation](mp:process-wait <string> <function> &rest <arguments>)
See eager-beaver and program.
