Multicores and mulithreads

How is process-based multitasking achieved by using multi-threading in each process?

For example, consider when an operating system is running with two background process. Each process supports internally multi-threading features. Now, how does time slicing happen between and inside these processes, and 开发者_StackOverflowhow does time slicing happen between threads?

The scheduler typically works at the thread level. In simplest terms the scheduler gives each runnable thread its timeslice in turn.

So a process with two threads will get twice as much CPU time as a process with one thread.

From:

http://msdn.microsoft.com/en-us/library/ms684259(VS.85).aspx

"A multitasking operating system divides the available processor time among the processes or threads that need it. The system is designed for preemptive multitasking; it allocates a processor time slice to each thread it executes. The currently executing thread is suspended when its time slice elapses, allowing another thread to run. When the system switches from one thread to another, it saves the context of the preempted thread and restores the saved context of the next thread in the queue.

The length of the time slice depends on the operating system and the processor. Because each time slice is small (approximately 20 milliseconds), multiple threads appear to be executing at the same time. This is actually the case on multiprocessor systems, where the executable threads are distributed among the available processors. However, you must use caution when using multiple threads in an application, because system performance can decrease if there are too many threads."

Also check out This link for when to use multi-tasking

The operating system decides when and for how long each thread exectues. For Microsoft operating systems, there is no way to determine or predict which thread in which process will execute next. Each thread also has a priority that it runs at. Higher priority threads tend to get more time than lower This priority can be changed by the user or by a program. See this link for more info.

"Now, how does time slicing happen between and inside these processes, and how does time slicing happen between threads?"

That's entirely up to the operating system to decide, really. A really basic OS might not do time-slicing at all, and just let each process run through to completion on a first-come, first-serve basis.

However, most modern operating systems will use some flavor of scheduling algorithm to decide which thread gets to execute on which core and for how long, and perform the context-switching necessary to save and restore per-thread state when swapping out one thread for another.