Inside the Windows NT Scheduler, Part 1

Windows NT is a preemptive multithreading operating system. That is, NT lets several programs run simultaneously and switches among them often enough to create the illusion that each program is the only program running on the machine. Well, that's the idea anyway. How to smoothly share one CPU (or multiple CPUs) among many threads of control is a complicated problem. Solving this problem dynamically many times per second is the job of the NT scheduler. The NT scheduler must honor the relative priorities that the application's programmers designate for each thread and attempt to provide responsiveness to user-interactive threads.

In this first part of a two-part series about the algorithms NT's scheduler employs, I'll introduce basic information about the NT scheduler. (For an overview of how NT schedules applications to run, see Christa Anderson, "Foreground Application Handling in NT 4.0," June 1997.) You'll find out about the priority levels that NT assigns to threads, how Win32 programs specify priorities for their threads, the situations that invoke the scheduler, and the algorithms NT uses on uniprocessors in those situations. I'll wrap up with a discussion of some advanced features of the scheduler, including priority boosting and starvation prevention. Next month, I'll provide an in-depth tour of how the NT scheduler implements multiprocessor scheduling.

Threads and Priorities
The basic scheduling unit in NT is a thread. A thread is a point of control within a process. Processes consist of a virtual address space that includes executable instructions, a set of resources such as file handles, and one or more threads that execute within its address space. Typical applications consist of only one process, so program and process are often used synonymously. Most programs today are single-threaded, which means they run as one process with one thread. However, multithreaded programs are becoming more commonplace. An example of a multithreaded program is a program that lets a user sort a list, with an option to cancel. One thread in the program's process might perform the CPU-intensive sorting task while another thread in the process displays a how-to-cancel message to the user and waits for a response. The scheduler does not differentiate between threads of different processes. Instead, the scheduler examines the priorities of all the threads ready to run at a given instant to pick which thread to execute.

NT assigns each thread a priority number from 1 to 31, where higher numbers signal higher priorities. (NT uses priority 0 for the system idle thread, which executes when no other thread is able to.) NT reserves priorities 16 through 31 (realtime priorities) for use by time-critical operations. Only a user with Administrator privileges can direct the system to execute threads in this range. NT uses priorities 1 through 15 (dynamic priorities) for the program threads of typical applications (e.g., Notepad, Word, Lotus Notes).

The NT kernel provides functions that let you set a thread to any of the 31 priority levels, but the Win32 API is more indirect. In Win32, specifying a thread's priority is a two-step process. You must first set the priority class of the process; then, you can apply a relative priority to individual threads.

A process priority class is a priority level around which NT lets the process' threads execute. The Win32 API defines four priority classes: realtime, high, normal, and idle. These names correspond to priority levels 24, 13, 8, and 4. Setting a process priority class causes all the threads of that process to begin executing at priorities within ±2 of the class priority. This scheme is shown in Figure 1, page 168. New processes inherit the priority class of their parent. Process threads start at the priority level associated with their process' priority class.

The relative priorities that can change a thread's priority from its process class priority are highest, above normal, normal, below normal, and lowest. Highest adds 2 to the thread's priority, above normal adds 1, normal adds 0, below normal adds -1, and lowest adds -2. Figure 2, page 168, shows the relative priorities applied to the Normal priority class range.

The Win32 API includes two special-case priority modifiers: time-critical and idle. Time-critical moves a dynamic thread's priority to the top of the dynamic range (15), and idle moves it to the bottom (1). Similarly, time-critical and idle move realtime threads to the top (31) and bottom (16) of the realtime range.

Whose Turn Is It?
Threads must take turns running on the CPU so that one thread doesn't prevent other threads from performing work. One of the scheduler's jobs is to assign units of CPU time (quantums) to threads. A quantum is typically very short in duration, but threads receive quantums so frequently that the system appears to run smoothly--even when many threads are performing work. One difference between NT Server and NT Workstation is the length of a user thread's quantum. On most x86 systems running NT Server, a quantum is 120 milliseconds (ms). On x86 systems running NT Workstation, a quantum can be 20ms, 40ms, or 60ms, depending on your system settings and whether the thread is a background or foreground application thread (more on this topic later).

The scheduler must make a CPU scheduling decision every time one of three situations occurs:

* A thread's quantum on the CPU expires.

* A thread waits for an event to occur.

* A thread becomes ready to execute.

When a thread's quantum expires, the scheduler executes the FindReadyThread algorithm to decide whether another thread needs to take over the CPU. If a higher-priority thread is ready to execute, it replaces (or preempts) the thread that was running.

In many cases, threads perform processing and then wait for an event to occur. For example, a client/server application might have a server thread that performs processing when it receives client requests and then waits for more requests. A waiting (or blocked) thread gives up its quantum early, and the scheduler must execute the FindReadyThread algorithm to find a new thread to run.

When a new thread or a blocked thread becomes ready to execute (e.g., when the client/server application server thread receives another client request), the scheduler executes the ReadyThread algorithm. This algorithm determines whether the ready thread will take over the CPU immediately or be placed in an eligible-to-execute list.

FindReadyThread and ReadyThread are the key algorithms the NT scheduler uses to determine how threads take turns on the CPU. The uniprocessor versions of FindReadyThread and ReadyThread are straightforward algorithms. Let's examine how FindReadyThread and ReadyThread work.

FindReadyThread. FindReadyThread locates the highest-priority thread that's ready to execute. The scheduler keeps track of all ready-to-execute threads in the Dispatcher Ready List. The Dispatcher Ready List contains 31 entries, each of which corresponds to a priority level and a queue of threads assigned to that priority level. The FindReadyThread algorithm scans the Dispatcher Ready List and picks the front thread in the highest-priority nonempty queue. Figure 3 shows an example Dispatcher Ready List with three ready threads--two at priority 10 and one at priority 7. FindReadyThread directs the scheduler to choose the first thread in priority 10's queue as the next thread to run.

ReadyThread. ReadyThread is the algorithm that places threads in the Dispatcher Ready List. When ReadyThread receives a ready-to-execute thread, it checks to see whether the thread has a higher priority than the executing thread. If the new thread has a higher priority, it preempts the current thread and the current thread goes to the Dispatcher Ready List. Otherwise, ReadyThread places the ready-to-execute thread in the appropriate Dispatcher Ready List. At the front of the queue, ReadyThread places threads that the scheduler pulls off the CPU before they complete at least one quantum; all other threads (including blocked threads) go to the end of the queue.