Difference between multi-core and hyper threading

Question

I am trying to understand explicit parallelism in processors, but confused about how multi core processors(having multiple cores on a single chip) are different in concept from symmetric multi-threading(marketed as hyperthreading).

Answer 1

Hyper-Threading

Hyper-Threading allows only one processor to work on two commands at the same time. When only one processor is available it starts execution and realize that it need additional information then it sends command for new information while waiting for data to arrive it work on some other command.

Multi-Core

A multi-core processor has several physically separate processor cores located on the same chip. Here only one chip is used so it is cheap. Chip consist of additional I/O registers. Multi-core processors can also have Hyper-Threading.

Answer 2

The cores are what do the actual work. A thread is like a conveyor belt feeding instructions to the core to tell it what work to do.

As an analogy:

Each core is a human worker at a factory. He picks up a piece of paper (instruction) from a conveyor belt (the thread) to perform a task. If there are gaps between one paper and the next, he sits there doing nothing. This is a single core, with a single thread.

If he instead had 2 conveyor belts feeding him papers, if one is empty, he can pick up an instruction from the other. The worker is able to do more work because he has less idle time. This is 1 core with 2 threads. While this is better than a single conveyor belt, the worker still must perform the work himself, so he's the bottleneck.

But if he's joined by another human worker (adding another core) they can get twice as much work done. The new worker will always have their own conveyor belt (every core has at least 1 thread feeding it).

From least to best performance:

• 1 core, no hyperthreading (1 thread in total)
• 1 core, with hyperthreading (2 threads in total)
• 2 cores, no hyperthreading (2 threads in total)
• 2 cores, with hyperthreading (4 threads in total)

Note how 1 core with hyperthreading has the same # of threads as a regular dual-core, but the dual-core can get more work done.