Is there a POSSIBILITY for context switching to occur on a multi-core processor or does it ONLY HAPP-CodePudding

https://en.wikipedia.org/wiki/Context_switch

In computing, a context switch is the process of storing the state of a process or thread, so that it can be restored and resume execution at a later point, and then restoring a different, previously saved, state.[1] This allows multiple processes to share a single central processing unit (CPU), and is an essential feature of a multitasking operating system.

The precise meaning of the phrase "context switch" varies. In a multitasking context, it refers to the process of storing the system state for one task, so that task can be paused and another task resumed. A context switch can also occur as the result of an interrupt, such as when a task needs to access disk storage, freeing up CPU time for other tasks. Some operating systems also require a context switch to move between user mode and kernel mode tasks. The process of context switching can have a negative impact on system performance.[2]: 28

and the second question 2):

If I understand correctly, on a single-core processor ONLY ONE thread can be executed AT A TIME (that's why context switching is INEVITABLE), so there is virtual parallelism.

So, is it completely SAFE not to use locks (like mutex, etc) to access shared resources (variables) on single-core processors (there are almost no such processors nowadays but take it as a "theoretical" question)? Thanks

CodePudding user response：

is it completely SAFE not to use locks (like mutex, etc) to access shared resources (variables) on single-core processors?

Probably not. It can be safe, if the code is running under the regime of cooperative multitasking, and if the programmer takes care to ensure that no thread executes any yield point while it has shared variables in some invalid state. But, Most operating systems these days use preemptive multitasking, in which the OS can take the CPU away from one thread and give it to another at any time, and with no warning.

When writing multi-threaded code for a single-CPU system (see below, for more about that) one need not worry so much about the system's memory model, as when programming for an SMP architecture or a NUMA architecture, but one still must take care to prevent the threads from interfering with each other.

(there are almost no such processors nowadays...)

Ha! Try telling that to an embedded software developer (E.g., myself.) There are single-CPU computers embedded in all manner of different things these days. Your microwave oven, your thermostat, a CPAP machine, a bluettooth headset... Your car might contain dozens of them. So might a mobile robot or a complex, automated factory assembly line.

CodePudding user response：

Yes, context switches occur on multicore processors, for the same reasons as on single core ones.

No, of course it's not always safe to have multiple threads access shared resources without locks. Doesn't matter how many cores you have. (Only maybe if you use very, very restricted definitions of what "safe" and "shared resource" mean.)

If you have two threads running code like the following with the same shared variable:

read variable
mutate value
write result back to variable

Then if a context switch happens in the middle of this sequence, and you have no mutex lock on the variable, you'll get inconsistent results. "Inconsistent" could easily include behavior that would cause memory leaks or crash the program: imagine if the variable is part of a data structure like a linked list or tree. Nothing about this needs a separate core.