Scheduling and Synchronization in Multicore CPU and in Single core CPU-CodePudding

From what I am understanding from the top answers of this post ( https://stackoverflow.com/questions/16116952/can-multithreading-be-implemented-on-a-single-processor-system#:~:text=Yes, you can have multiple,one thing at a time.),

If I am only running one multithreaded program that creates 4 threads in a multicore CPU with 4 cores, there is no need for scheduling as all 4 threads of my program will be running in individual cores( or microprocessors). But there maybe a need for synchronization since all 4 threads access the memory of the program( or a process) that is stored in the same space in the main memory.

On the other hand, In a single core CPU computer. If I run the same program that creates 4 threads, I will need both synchronization and Scheduling since all threads must utilize the same Core ( or a microprocessor).

Please correct my understanding if it is wrong.

CodePudding user response：

there is no need for scheduling as all 4 threads of my program will be running in individual cores

This is not true in practice. The OS scheduler operate in both cases. Unless you pin threads to core, threads can migrate from one core to another. In fact, even if you pin them, there are generally few other threads that can be ready on the machine (eg. ssh daemon, tty session, graphical programs, kernel threads, etc.) so the OS has to schedule them. There will be context-switches though the number will be much lower than with a single processor.

there maybe a need for synchronization since all 4 threads access the memory of the program( or a process) that is stored in the same space in the main memory.

This is true. Note that threads can also work on different memory area (so that there is no need for synchronization except when they are joined). Note also that "main memory" includes CPU caches here.

In a single core CPU computer. If I run the same program that creates 4 threads, I will need both synchronization and Scheduling since all threads must utilize the same Core ( or a microprocessor).

Overall, yes. That being said, the term "scheduling" is unclear. THere are multiple kind of scheduling: preemptive VS cooperative scheduling. Here, as a programmer, you do not need to do anything special since the scheduling is done by the OS. Thus, it is a bit unexpected to say that you "need" scheduling. The OS will schedule the threads on the same core using preemption (by allocating different time-slices for each threads on the same core).