Concurrency #

Is the ability of a system to manage multiple tasks through simultaneous execution or time-sharing.

Threads #

A unit of work.

A thread of execution.

Order of Thread Execution #

When threads are spawned they are ran in a non-deterministic manner.

This is due to its managing environment system (ie. the OS or a virtual machine) preemptively stopping it to let other threads have a chance at progressing.

Problems With Threads #

Race Conditions #

A race condition is a problem that occurs when multiple entities (ie. threads) access the same data.

The problem occurs because the operations within an execution does not work atomically ie. does not consider that all operations must be successful for it to proceed.

Data that is accessed by multiple entities is called the critical section.

In order to fix race conditions we must introduce the notion of locks which each entity holds when it is executing on a critical section.

Enforcing execution order this way is called serialization.

Deadlocks #

A dead lock is a problem that occurs when multiple entities cannot proceed due to one or more of them holding a lock that the other needs.

See Wikipedia: Deadlocks for more details.