The Threadpool

The threadpool is where we'll process the CPU intensive tasks and the I/O tasks which can't be reasonably handled by epoll, kqueue or IOCP. One of these tasks are file system operations.

The reason for doing file I/O in the thread pool is complex, but the main takeaway is that due to how files are cached and how the hard drive works, most often the file I/O will be Ready almost immediately, so waiting for that in a event queue has very little effect in practice.

The second reason is that while Windows does have a completion based model, Linux and macOS don't. Reading a file into a buffer which your process controls can take some time, and if we do that in our main loop it will block a little bit which we really try to avoid.

By processing the tasks in a thread pool we make sure that these operations won't block our main event loop and only notify us once the data is ready for us in memory.

The code we need to add to process events from the thread pool is short an simple:

fn process_threadpool_events(&mut self, thread_id: usize, callback_id: usize, data: Js {
    self.callbacks_to_run.push((callback_id, data));

We take thread_id, callback_id and data as arguments. We get this through the channel we have shared with our threadpool threads.

Once we have that information we push our callbacks into the queue of callbacks_to_run which will run on the next call to run_callbacks we went through in the previous chapter.

Last we take the id of the thread that sent the finished task and put it into our pool of available threads.