# L-15 MCS 572 Mon 30 Sep 2024 : worksteal.jl

"""
Illustrates the work stealing idea to
processing job queues by many threads.
At the command prompt, type
julia -t 4 worksteal.jl
to run with four threads.
"""

using Base.Threads

nt = nthreads()
println("the number of threads : ", nt)

nbr = 10 # number of jobs in each queue
# allocate memory for all job queues
jobs = [zeros(nbr) for i=1:nt]

# every worker generates its own job queue
# even indexed queues have light work loads
@threads for i=1:nt
    if i % 2 == 0
        jobs[i] = rand((1, 2, 3), nbr)
    else
        jobs[i] = rand((4, 5, 6), nbr)
    end
    println("Worker ", threadid(), " has jobs ", jobs[i], " ", sum(jobs[i]))
end
# every job queue has its own index to the next job
jobidx = [Atomic{Int}(1) for i=1:nt]
@threads for i=1:nt
    while true
        myjob = atomic_add!(jobidx[i], 1)
        if myjob > length(jobs[i])
            break
        end
        println("Worker ", threadid(),
                " spends ", jobs[i][myjob], " seconds",
                " on job ", myjob, " ...")
        sleep(jobs[i][myjob])
        jobs[i][myjob] = threadid()
    end
    println("Worker ", threadid(), " will steal jobs ...")
    more2steal = true
    while more2steal
        more2steal = false
        for j=1:threadid()-1
            myjob = atomic_add!(jobidx[j], 1)
            if myjob <= length(jobs[j])
                println("Worker ", threadid(),
                        " spends ", jobs[j][myjob], " seconds",
                        " on job ", myjob, " of ", j, " ...")
                sleep(jobs[j][myjob])
                jobs[j][myjob] = threadid()
            end
            more2steal = (myjob < length(jobs[j]))
        end
        for j=threadid()+1:nt
            myjob = atomic_add!(jobidx[j], 1)
            if myjob <= length(jobs[j])
                println("Worker ", threadid(),
                        " spends ", jobs[j][myjob], " seconds",
                        " on job ", myjob, " of ", j, " ...")
                sleep(jobs[j][myjob])
                jobs[j][myjob] = threadid()
            end
            more2steal = (myjob < length(jobs[j]))
        end
    end
end
for i=1:nt
    println("Jobs done : ", jobs[i])
end