P = parpool(4); creates four processes. All four of them start executing the same block of code, with nearly the only difference between the processes being that if they call labIndex they will get different results from 1 to 4.
All four of the processes will begin executing at the same time, without waiting for anything. They will not wait for input.
As soon as any of the processes has nothing left to do, it will go into a "waiting to exit" state and will not participate in any further work except to help collect the outputs when the last of the labs exits. This applies even if another lab specifically does labSend() to it.
If your workers interact, then it is common to write the smpd block in terms of "for" or "while" loops. You can use either design pattern:
spmd
for K = 1 : whatever
if labIndex == 1
something
elseif labIndex == 2
something else
end
end
end
or
spmd
if labIndex == 1
for K = 1 : whatever
something
end
elseif labIndex == 2
for K = 1 : whatever
something else
end
end
end
The second of those is more common, as it permits the different labs to have different varieties of work.
The way to get a lab to wait for work is to have the lab execute labReceive() or labBarrier()
There is no clock and no enable port being used here. Each of the labs is a different process that runs as quickly as it can, and the different labs do not try to synchronize until you use labSend() / labReceive(0 or labBarrier(), or until they finish.
