Tukukkk

Edit3: I made a graph for my Edit2,

Edit2: In short, for my question I adopt the definitions on Wikipedia, and for the definition of the terms:

• Parallel: Two threads, run independently, at any physical instant. So one thread won't interrupt the other, at any instant.


• Concurrent: Two threads, run independently, interleavedly is allowed, i.e. not restricted to parallel, and one can interrupt the other.


• In short, for me and Wikipedia writers, Concurrent includes Parallel. Thanks.

Edit: Just to be clear, for me parallelism means true parallelism, I add a "true" for it because people I talked to tend to think parallel==concurrent. (See my second link)

Is it true that on modern processor, "true" parallelism is possible on a single core? I asked elsewhere but didn't get a confirming answer. What I want to know is e.g. whether at t=0, two instructions are fetched and executed. At the same physical instant.

My question came from here:

parallel computing is impossible on a (one-core) single processor, as only one computation can occur at any instant (during any single clock cycle).

On some processors this is (sometimes) possible. Since different instructions use different processor resources (ALU, floating point, load, store, etc), it's sometimes possible to parallelize some of them. For example, see here for details on how that works on an Ivy Bridge (x86) CPU: https://dendibakh.github.io/blog/2018/03/21/port-contention

It is indeed possible to have parallelism on a superscalar processor. A superscalar processor can execute multiple instructions at the same time by using multiple execution units.

There are certain limitations depending on the architecture. It is not true parallelism. If you have to calculate
$$A = B + C,$$
$$D = A + 3,$$
you cannot execute both instructions at the same time. However you can execute
$$A = B + C,$$
$$D = D + 3,$$
simultaneously by utilizing two ALUs.

So as an answer to your question, you can have a certain level of parallelism on a single core, as long as your instructions do not use the same hardware resources.

There are lots of different types of parallelism.

Instruction level parallelism is a feature of any superscalar processor. Multiple instructions are in progress at any point in time. However, those instructions are from the same thread of control.

Thread level parallelism within a single core is possible with hyperthreading -- two separate threads using different core resources at the same time. One thread can use the integer ALU while another is executing a load or store.

Data level parallelism is also a type of parallelism. SIMD units can execute the same instructions on multiple registers at the same time. For instance, if you need to apply the same blur transformation to every pixel in an image, you might be able to do that 8 pixels in parallel, but within the same thread of control.