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On a single core CPU, each process runs in the OS, and the CPU jumps around from one process to another to best utilize itself. A process can have many threads, in which case the CPU runs through these threads when it is running on the respective process.
Now, on a multiple core CPU:
Do the cores run in every process together, or can the cores run separately in different processes at one particular point of time? For instance, you have program A running two threads. Can a dual core CPU run both threads of this program? I think the answer should be yes if we are using something like OpenMP. But while the cores are running in this OpenMP-embedded process, can one of the cores simply switch to other process?
For programs that are created for single core, when running at 100%, why is the CPU utilization of each core distributed? (e.g. A dual core CPU of 80% and 20%. The utilization percentage of all cores always add up to 100% for this case.) Do the cores try to help each other by running each thread, of each process, in some ways?
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Cores increase the amount of work accomplished at a time, whereas threads improve throughput, computational speed-up. Cores is an actual hardware component whereas thread is a virtual component that manages the tasks. Cores use content switching while threads use multiple CPUs for operating numerous processes.
Threads are the virtual components or codes, which divides the physical core of a CPU into virtual multiple cores. A single CPU core can have up-to 2 threads per core. For example, if a CPU is dual core (i.e., 2 cores) it will have 4 threads.
Hyper-threading is a process by which a CPU divides up its physical cores into virtual cores that are treated as if they are actually physical cores by the operating system. These virtual cores are also called threads [1]. Most of Intel's CPUs with 2 cores use this process to create 4 threads or 4 virtual cores.
A single process or thread only runs on a single core at a time. If there are more threads requesting CPU time than available cores (generally the case), the operating system scheduler will move threads on and off cores as needed.
Cores (or CPUs) are the physical elements of your computer that execute code. Usually, each core has all necessary elements to perform computations, register files, interrupt lines etc.
Most operating systems represent applications as processes. This means that the application has its own address space (== view of memory), where the OS makes sure that this view and its content are isolated from other applications.
A process consists of one or more threads, which carry out the real work of an application by executing machine code on a CPU. The operating system determines, which thread executes on which CPU (by using clever heuristics to improve load balance, energy consumption etc.). If your application consists only of a single thread, then your whole multi-CPU-system won't help you much as it will still only use one CPU for your application. (However, overall performance may still improve as the OS will run other applications on the other CPUs so they don't intermingle with the first one).
Now to your specific questions:
1) The OS usually allows you to at least give hints about on which core you want to execute certain threads. What OpenMP does is to generate code that spawns a certain amount of threads to distribute shared computational work from loops of your program in multiple threads. It can use the OS's hint mechanism (see: thread affinity) to do so. However, OpenMP applications will still run concurrently to others and thus the OS is free to interrupt one of the threads and schedule other (potentially unrelated) work on a CPU. In reality, there are many different scheduling schemes you might want to apply depending on your situation, but this is highly specific and most of the time you should be able to trust your OS doing the right thing for you.
2) Even if you are running a single-threaded application on a multi-core CPU, you notice other CPUs doing work as well. This comes a) from the OS doing its job in the meantime and b) from the fact that your application is never running alone -- each running system consists of a whole bunch of concurrently executing tasks. Check Windows' task manager (or ps/top on Linux) to check what is running.
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Note also that the OS doesn't much care which process the threads are from. It will usually schedule threads to processors / cores regardless of which process the thread is from. This could lead to four threads from one process running at the same time, as easily as one thread from four processes running at the same time.
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