Introduction to Concurrency and Threads

Thus far, you have seen the development of the basic abstractions that the OS performs. You have seen how to take a single physical CPU and turn it into multiple virtual CPUs, thus enabling the illusion of multiple programs running at the same time. You have also seen how to create the illusion of a large, private virtual memory for each process; this abstraction of the address space enables each program to behave as if it has its own memory when indeed the OS is secretly multiplexing address spaces across physical memory (and sometimes, disk).

More than one point of execution

In this note, you will now be introduced to a new abstraction for a single running process: that of a thread. Instead of our classic view of a single point of execution within a program (i.e., a single PC where instructions are being fetched from and executed), a multi-threaded program has more than one point of execution (i.e., multiple PCs, each of which is being fetched and executed from). Perhaps another way to think of this is that each thread is very much like a separate process, except for one difference: they share the same address space and thus can access the same data.

The state of a single thread is thus very similar to that of a process. It has a program counter (PC) that tracks where the program is fetching instructions from. Each thread has its own private set of registers it uses for computation; thus, if there are two threads that are running on a single processor; when switching from running one (T1) to running the other (T2), a context switch must take place. The context switch between threads is quite similar to the context switch between processes, as the register state of T1 must be saved and the register state of T2 restored before running T2. With processes, you save state to a process control block (PCB); now, you’ll need one or more thread control blocks (TCBs) to store the state of each thread of a process. There is one major difference, though, in the context switch performed between threads as compared to processes: the address space remains the same (i.e., there is no need to switch which page table we are using).

One other major difference between threads and processes concerns the stack. In our simple model of the address space of a classic process (which you can now call a single-threaded process), there is a single stack, usually residing at the bottom of the address space (See left of the below figure).