Difference Between Concurrency and Parallelism

When you learn multithreading, you will encounter a term: concurrency.This is a vocabulary in the operating system. It is necessary to understand the difference between concurrency and parallelism. I have collected several expressions from the Internet to help understand.
One:
Concurrency means that a processor handles multiple tasks at the same time.
Parallelism refers to the simultaneous processing of multiple different tasks by multiple processors or multi-core processors.
Concurrency is logically simultaneous (simultaneous), while parallelism is physically simultaneous.
Let's take an analogy: Concurrency means that one person eats three steamed buns at the same time, while parallelism means that three people eat three steamed buns at the same time.
2:
Parallel (parallel): refers to the simultaneous execution of multiple instructions on multiple processors at the same time.It's as if two people are digging a hole with a shovel each, and after an hour, each person will have a big hole.So no matter from a micro or macro point of view, the two are executed together.

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Concurrency: It means that only one instruction can be executed at the same time, but multiple process instructions are rapidly executed in turn, so that it has the effect of multiple processes executing at the same time in the macro, but not at the same time in the micro, just divide the time into several segments, so that multiple processes can be executed alternately quickly.It's like two people using the same shovel and taking turns digging a hole. After an hour, they each dig a smaller hole. If they want to dig two larger holes, it will definitely take two hours.
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Parallelism exists in multiprocessor systems, and concurrency can exist in both uniprocessor and multiprocessor systems. Concurrency can exist in uniprocessor systems because concurrency is an illusion of parallelism. Parallelism requires programs to be able to simultaneouslyMultiple operations are performed, while concurrency simply requires the program to pretend to perform multiple operations simultaneously (one operation per small time slice, multiple operations switching execution quickly).
Three:
When there are multiple threads in operation, if the system has only one CPU, it is impossible to perform more than one thread at the same time. It can only divide the CPU running time into several time periods.The time period is then allocated to each thread for execution. When the thread code of a period of time is running, other threads are in a suspended state. This method is called Concurrent.

When the system has more than one CPU, the operations of threads may be non-concurrent. When one CPU executes one thread, the other CPU can execute another thread. The two threads do not preempt each other's CPU resources and can be performed simultaneously.We call this method Parallel.
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