Polling a resource / poor-mans spinlock

[Krastanov]

Is there a "proper" way to atomically check whether a resource is available in a non-blocking manner?

I want to write something along the lines of

while true:
    if isavailable(resource)
        @yield request(resource)
        break
    else
        @yield timeout(waittime)
    end
end

I am currently thinking about using resource.level == 0 but this seems like an internal implementation detail, not a real API.

[hdavid16]

That would be the way to do it currently. You could add the function though to check the level and determine if available.

Why don't you just let it get in the queue though if it's not available? Are you checking to add it to the resource that is available (assuming you have more than one resource)?

[Krastanov]

@hdavid16 , yes, it is because I am trying to avoid locking too many resources. However, I should admit that I am not experienced in this type of simulations, so maybe I am doing something wrong.

Basically, I have a chain of resources and I have a process for each pair of resources. If I naively do the following:

@resumable function establish_link(env, r1, r2)
    @yield request(r1) & request(r2)
    @yield timeout(longwaittime)
    release(r1)
    release(r2)
end
resources = [Resource() for i in 1:100]
for i in 1:99
    @process link(env, resources[i], resources[i+1])
end

then establishing all links will take about 99*longwaittime units of time, because each iteration of the loop needs to wait for the resources of the previous iteration. But if the request was in the form from my first post, then establishing all links will take just 2*longwaittime. Is there a better way to write this?

[hdavid16]

Ok. So basically you decrease the simulation time because you end up running every other link simultaneously and then you do the ones that were skipped, also simultaneously.

Your approach of checking the availability is more robust than doing two loops, one for the odd i and the other for the even ones. Especially if your service times are not the same.

[Krastanov]

We now have islocked and isready from #93 . Closing as complete.