books-futures-explained/reader-excercises.md

Reader excercises

So our implementation has taken some obvious shortcuts and could use some improvement. Actually digging into the code and try things yourself is a good way to learn. Here are som relatively simple and good excersises:

Avoid `thread::park`

The big problem using Thread::park and Thread::unpark is that the user can access these same methods from their own code. Try to use another method of telling the OS to supend our thread and wake it up again on our command. Some hints:

• Check out CondVars, here are two sources Wikipedia and the docs for CondVar
• Take a look at crates that help you with this exact problem like Crossbeam (specifically the Parker)

Avoid wrapping the whole `Reactor` in a mutex and pass it around

First of all, protecting the whole Reactor and passing it around is overkill. We're only interested in synchronizing some parts of the information it contains. Try to refactor that out and only synchonize access to what's really needed.

• Do you want to pass around a reference to this information using an Arc?
• Do you want to make this information global so it can be accessed from anywhere?

Next , using a Mutex as a synchonization mechanism might be overkill since many methods only reads data.

• Could an RwLock be more efficient some places?
• Could you use any of the synchonization mechanisms in Crossbeam?
• Do you want to dig into atomics in Rust and implement a synchonization mechanism of your own?

Creating a new Waker for every event

Right now we create a new instance of a Waker for every event we create. Is this really needed?

• Could we create one instance and then cache it (see this article from u/sjepang)?
  • Should we cache it in thread_local! storage?
  • Or should be cache it using a global constant?

Could we implement more methods on our executor?

What about CPU intensice tasks? Right now they'll prevent our executor thread from progressing an handling events. Could you create a thread pool and create a method to send such tasks to the threadpool instead and make it wake you up when done?

In both async_std and tokio this method is called spawn_blocking, a good place to start is to read the documentation and the code thy use to implement that.