Process vs thread with example

Question

I read articles on processes vs threads, but I am still not clear on the difference.

Suppose a process is using the CPU/Processor, doing some big calculation that takes 10 minutes. How will another process run at the same time in parallel? In a single core vs a dual core processor?

Same thing for threads, how will another thread run in parallel when the CPU/Processor is engaged with another thread?

How is context switching different for threads and for processes? I mean both process and threads use the same RAM memory, so what's the difference?

Answer 1

From my vague memory of Operating Systems I can offer you a little bit of help. First you have to know the difference between concurrent and simultaneous. They are not the same thing; simultaneous means both things occur at the same time and concurrent means they appear to be running simultaneously but in reality they're switching so fast you can't tell.

Processes and threads can be considered similar, but a big difference is that a process is much larger than a thread. For that reason, it is not good to have switching between processes. There is too much information in a process that would have to be saved and reloaded each time the CPU decides to switch processes.

A thread on the other hand is smaller and so it is better for switching. A process may have multiple threads that run concurrently, meaning not at the same exact time, but run together and switch between them. The context switching here is better because a thread won't have as much information to store/reload.

If you only have a single core then you can only do concurrent execution, for the most part. Once you have multiple cores you can have threads run on both cores and thus have simultaneous execution. It is up to the Operating System to schedule when threads run, when processes get to run, when to switch, how to switch them, etc. The Operating System gives you the illusion that work is being done simultaneously when this is not always the case.

If you have more confusion feel free to comment.

Answer 2

A process is a thing very related to the Operating System (OS). The thread is in the simplest terms, is an executing program. One or more threads run in the context of the process. The Java Virtual Machine (JVM) is a process in your OS.

And inside the JVM you can have multiple threads running concurrently.

The processor is a resource of your machine, like the memory. Your OS let your process to share the available resources, in our simple case processors and memory.

When you develop in Java, all processor in your machine are available resources.

When you develop your solution, you can have even multiple Java processes (i.e. multiple JVM) running a single or multiple thread each. But this mostly depends by your problem.

The real difference between a process and a thread is that both have an executing program, but threads share the same memory. This let your threads to theoretically work on the same data, but you have pay the complexity of concurrency and synchronisation.

Answer 3

Each CPU only runs one thread in a process at a time. However the OS can stop and save a thread and load and run another quickly (as little as 0.0001 seconds) This gives the illusion that many threads are running at once, even though only one is running.