[Java Interview] The interview is over! A friend who has worked for 5 years was hanged in an interview today and asked me, what is the difference between parallelism and concurrency?

"What is the difference between parallelism and concurrency?"

Regarding this question, many students who have worked for more than 5 years cannot answer it.

In other words, I have a certain understanding, but I don't know how to express it.

Hi, my name is Mic, a Java programmer who has been working for 14 years.

What is the interviewer looking for in this question?

Problem Analysis

Parallelism and concurrency actually described the concepts in Java concurrent programming.

What they emphasize is the ability of the CPU to handle the task.

In simple terms, concurrency is the number of tasks that the CPU can process at the same time, and for applications, there will be no lag.

Parallelism means that multiple tasks are allowed to execute at the same time. In a multi-core CPU architecture, the number of tasks to be executed at the same time is determined by the number of cores. For example, in a CPU with 4 cores and 4 threads, only 4 threads can be executed at the same time.

These two concepts look similar, but in fact, the latitude of description is different. Concurrency describes the perspective of program processing power.

Parallelism describes the perspective of how the CPU handles tasks, one at the macro level and the other at the micro level.

The two of them complement each other. The ability of the CPU to execute tasks in parallel can also improve the concurrent processing performance of the program.

So the performance of multi-core CPU is better than that of single-core CPU.

Of course, if it is a single-core CPU, the concurrency capability can also be improved by switching time slices.

Joe Armstrong, the father of Erlang, explained the difference between parallelism and concurrency with a picture.

Concurrency is when two queues use one coffee machine alternately, and parallelism is when two queues use two coffee machines at the same time.

Therefore, in my opinion, this interview question can be a good test of the applicant's understanding of Java concurrent programming.

There are many articles on the Internet that try to explain this concept, but these explanations make the problem more and more complicated.

I think that only a deep understanding of the underlying principles of threads can answer this question well.

Expert:

Parallelism and concurrency are concepts in Java concurrent programming.

Parallelism refers to the ability to execute multiple threads at the same time at the same time under the multi-core CPU architecture.

In a single-core CPU architecture, only one thread can run at a time.

In a CPU architecture with 4 cores and 4 threads, 4 threads can run at the same time, and these 4 threads are executed in parallel.

Concurrency refers to the number of tasks that the CPU can process at the same time, which can also be understood as the concurrency capability of the CPU.

In the single-core CPU architecture, the operating system improves the concurrency capability of the CPU through the CPU time slice mechanism

In the multi-core CPU architecture, the concurrency capability of the CPU is improved based on the parallel execution capability of tasks and the capability of CPU time slice switching.

So, in general, concurrency is a macro concept, which refers to the amount of pressure the CPU can carry, and parallelism is a micro concept, which describes the ability of the CPU to perform multiple tasks at the same time.

Summarize

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