Concurrency vs Parallelism


Hey everyone! This is a short article on concurrency, parallelism, and the differences between the two.

Performance and Concurrency

What is performance? Is it how fast a machine can calcuate prime numbers? Is it how many things a machine can do at one time? As always, it depends on the workload.

As a rule of thumb, in terms of serial execution, there is an inverse relationship between language dynamism and performance. The more dynamic a language is, the slower it will perform. C is very fast, but good luck getting it to do:

3.times { puts "This isn't confusing at all!" }

Concurrency and Parallelism

The difference between these two things is important to know, but its often confusing to people.

Concurrency is not parallelism. Concurrency means that more than one thing happens in some time slice. Parallelism means two things happening simultaneously. Parallelism is one way to achieve concurrency, but not the only way.

Let’s say we have two functions, unicorn() and prance(). In a non-parallel, non-concurrent system, execution looks like this:

unicorn() -> unicorn() does work -> unicorn() finishes -> prance() -> prance() does work -> prance() finishes

In a concurrent system, it might look like this:

unicorn() -> unicorn() does work -> prance() -> prance() does work -> unicorn() finishes -> prance() finishes See how unicorn started, then prance started?

In a parallel system, it would look like this: unicorn() -> unicorn() does work -> unicorn() finishes prance() -> prance() does work -> prance() finishes

Think of each line as code running on a processor. In the concurrent example, one CPU still does all the work. In the parallel example, two processors do all the work.

The more you can ~parallel~ ~parralell~ screw it, the more you can subdivide your workload into independent pieces, the more you can leverage multiple cores to increase performance.

Notes on OS processes, OS threads, and green/micro/lightweight threads

Another concept that often engenders confusion is the differences between processes, threads, microthreads, greenthreads, etc etc. These are important to know about and understand how they impact performance.

OS Processes

Any application lives as some sort of code on some sort of storage somewhere. When you open an application, that code is read from its storage and into the RAM of the machine to begin running. This is all done by the operating system, and when it has finished, you have a process with a unique ID. When you run a python program, a process containing the Python virtual machine is created, for example.

You can think of the OS processe as the most heavy-weight of the three and slowest to create. The key difference to remember is:

Processes do not share memory by default


Threads are created by the operating system, belong to a Process and are lighter weight. In contrast to processes:

Threads do share memory by default

Greenthreads, Microthreads, et article

These are often implemeneted and managed by the language runtime and are very light weight. The key thing to remember is:

The operating system cannot even see these threads

Heavyweight? Lightweight? What?

This refers to the amount of resources (RAM, CPU) it takes to create and manage. Processes require the most, green threads the least.

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