Process Scheduing
Operating Systems ultimately control these types of thread scheduling.
Windows systems are priority-based and so will allow a process to consume more resources that others. This is why your machine can 'hang', if a process has been escalated to a high priority. Priorities are ranged between 1-31 as far as I know.
Mac OS / Linux / Unix are time-based, allowing all processes to have equal amounts of CPU time. Therefore loading more processes will slow your system down as they all share a smaller slice of execution time.
Uniprocessor Environment
The OS is ultimately responsible for this but switching processes involves (I cannot guarantee accuracy here, but its just an indication):
- Halting a process / thread
- Storing the current stack (code location)
- Storing the current registers of the CPU
- Asking the kernel for the next process/thread to run
- Kernel indicates which one has to be run
- OS reloads the registers from the cache
- OS reloads the current stack for the next application.
- Resumes the process
Obviously the more threads and processes you have running, the slower it will become. The problem is that the time taken to switch processes can actually take longer than the time allowed to execute the process.
Threads are just child processes of a single process. For a single processor, it just looks like additional work.
Multi-processor Environment
Multi-processor environments work differently as the cache is shared amongst processors. I believe these are called L1 (Level) and L2 caches. So the difference is that processor A can reload the state stored by processor B without conflicts. 'Hyper-threading' also has the same approach, although this is processor specific. The difference here is that a processor could solely control a specific process - this is called 'CPU Affinity' Its not encouraged for every process, but it does allow an application to have a dedicated processor to work off.