Concurrency and Asynchronous Programming: a Detailed Overview 6 example, when an operation only required some parts of the CPU, an instruction could be run on the ALU simultaneously. This became the start of hyper-threading. Y our computer today, for example, may have 6 cores and 12 logical cores.. This is exactly where hyper-threading comes in. It “simulates” two cores on the same core by using unused parts of the CPU to drive progress on thread 2 and simultaneously running the code on thread 1. It does this by using a number of smart tricks (such as the one with the ALU). Now, using hyper-threading, we could actually offload some work on one thread while keeping the UI interactive by responding to events in the second thread even though we only had one CPU core, thereby utilizing our hardware better. Y ou might wonder about the performance of hyper-threading It turns out that hyper-threading has been continuously improved since the 90s. Since you're not actually running two CPUs, there will be some operations that need to wait for each other to finish. The performance gain of hyper-threading compared to multitasking in a single core seems to be somewhere close to 30% but it largely depends on the workload. Multicore processors As most know, the clock frequency of processors has been flat for a long time. Processors get faster by improving caches, branch prediction, and speculative execution, and by working on the processing pipelines of the processors, but the gains seem to be diminishing. On the other hand, new processors are so small that they allow us to have many on the same chip. Now, most CPUs have many cores and most often, each core will also have the ability to perform hyper-threading. Do you really write synchronous code? Like many things, this depends on your perspective. From the perspective of your process and the code you write, everything will normally happen in the order you write it. From the operating system's perspective, it might or might not interrupt your code, pause it, and run some other code in the meantime before resuming your process. From the perspective of the CPU, it will mostly execute instructions one at a time. * It doesn't care who wrote the code, though, so when a hardware interrupt happens, it will immediately stop and give control to an interrupt handler. This is how the CPU handles concurrency.