The Difference Between Concurrency and Parallelism

[japananh]

The Difference Between Concurrency and Parallelism

Concurrency is a property of the code; parallelism is a property of the running program. - Concurrency in Go
Katherine Cox-Buday

The chunks of our program may appear to be running in parallel, but really they’re executing in a sequential manner faster than is distinguishable. The CPU context switches to share time between different programs, and over a coarse enough granularity of time, the tasks appear to be running in parallel. If we were to run the same binary on a machine with two cores, the program’s chunks might actually be running in parallel.

This reveals a few interesting and important things. The first is that we do not write parallel code, only concurrent code that we hope will be run in parallel. Once again, parallelism is a property of the runtime of our program, not the code.

The second interesting thing is that we see it is possible—maybe even desirable—to be ignorant of whether our concurrent code is actually running in parallel. This is only made possible by the layers of abstraction that lie beneath our program’s model: the concurrency primitives, the program’s runtime, the operating system, the platform the operating system runs on (in the case of hypervisors, containers, and virtual machines), and ultimately the CPUs. These abstractions are what allow us to make the distinction between concurrency and parallelism, and ultimately what give us the power and flexibility to express ourselves. We’ll come back to this.

The third and final interesting thing is that parallelism is a function of time, or context. Remember in “Atomicity” where we discussed the concept of context? There, context was defined as the bounds by which an operation was considered atomic. Here, it’s defined as the bounds by which two or more operations could be considered parallel.

For example, if our context was a space of five seconds, and we ran two operations that each took a second to run, we would consider the operations to have run in parallel. If our context was one second, we would consider the operations to have run sequentially.