# Parallelism

## About

Is when two tasks can run each on a separate physical core, meaning they will run at the same **time**.

{% hint style="warning" %}
They will run in parallel unless they share the same resource. (e.g file, printer, etc)

Multiple threads handling the same resource at the same time is messy (may lead to **race conditions**), so this is where **Mutex** comes to help.
{% endhint %}

## Race Condition

Is when multiple threads compete to access the same resource.

Without knowing which will access first, it can lead to non-deterministic or unexpected results.

## Mutex (Locks)

Is a flag to signal exclusivity on a resouce.

The thread that access first will signal the resource to be "used", so that subsequent threads access will wait until the resource is "free".

### Dead Locks

Are when threads forget or don't handle the removal of the "used" flag from the resources.

So they will forever wait the flag clearance.

### Blocking I/O

Is when a thread will stay running until it finishes its operation involving an I/O resource. (e.g writing to disk, or using network connections)

**Assynchronous I/O** deals with these blocks over I/O. *(An event is emitted on the I/O finish, so this way the thread doesn't need to synchronouly wait for it)*