Kernel Korner - What's New in the 2.6 Scheduler?

When large SMP systems started spending more time scheduling processes than running them, it was time for a change. The new Linux scheduler works well on systems with many processors.
The 2.6 Scheduler

Thus, the stage was set for the introduction of the O(1) scheduler in 2.6, which boasts that the time to select the best task and get it on a processor is constant, regardless of the load on the system or the number of CPUs for which it is scheduling. Instead of one queue for the whole system, one active queue is created for each of the 140 possible priorities for each CPU. As tasks gain or lose priority, they are dropped into the appropriate queue on the processor on which they'd last run. It is now a trivial matter to find the highest priority task for a particular processor. A bitmap indicates which queues are not empty, and the individual queues are FIFO lists. Therefore, you can execute an efficient find-first-bit instruction over a set of 32-bit bitmaps and then take the first task off the indicated queue every time.

As tasks complete their timeslices, they go into a set of 140 parallel queues per processor, named the expired queues. When the active queue is empty, a simple pointer assignment can cause the expired queue to become the active queue again, making turnaround quite efficient.

It's not possible to draw the 140 queues now present for each CPU without resorting to mere dots. But, Figure 2 offers an approximation and drives home the major difference between the 2.4 and the 2.6 schedulers. Except on a heavily loaded system, most queues are empty. Those that are not empty have their best selection at the head of the queue, so searching for the next task to run has become easy.

Figure 2. The 2.6 scheduler has 140 queues per processor, making it easy to search for the next runnable task.

There's one shortcoming of this 2.6 method. Once a task lands on a processor, it might use up its timeslice and get put back on a prioritized queue for rerunning—but how might it ever end up on another processor? In fact, if all the tasks on one processor exit, might not one processor stand idle while another round-robins three, ten or several dozen other tasks? To address this basic issue, the 2.6 scheduler must, on occasion, see if cross-CPU balancing is needed. It also is a requirement now because, as mentioned previously, it's possible for one CPU to be busy while another sits idle. Waiting to balance queues until tasks are about to complete their timeslices tends to leave CPUs idle too long. Instead, 2.5 and 2.6 leverage the process accounting, which is driven from clock ticks, to inspect the queues regularly. Every 200ms a processor checks to see if any other processor is out of balance and needs to be balanced with that processor. If the processor is idle, it checks every 1ms so as to get started on a real task as soon as possible.

This is not to say that the scheduler is fixed now and all work on it has stopped. Some workloads and architectures provide some interesting scenarios that the scheduler still doesn't deal with well.

Current and Future Work

The goals of a successful scheduler can be stated simply, even if they always can't be attained simply.

  1. Minimize the time spent scheduling, so as to maximize the time spent executing.

  2. On multiple CPUs, keep the load spread around so it is easier to share the processors fairly.

  3. Provide good response to interactive programs.

In addition, the philosophy of the Linux scheduler is that it should be mostly right all of the time rather than perfect much of the time. Even though different workloads exhibit different behaviors and place different stresses on the system, the scheduler should be sufficiently general and robust so that all workloads are handled at least adequately, without additional tuning being necessary.

Interactivity

Most of the scheduler tweaking in 2.6 has been done in an attempt to improve interactive response. Originally, this was interactive in the traditional sense of dragging windows across a monitor or typing on a keyboard. An interactive task was meant to define a task that utilized a lot of human interaction. But it gradually has been expanded to mean “tasks that should receive high priority upon waking up from self-imposed sleeps”. This includes the previous set of tasks but also now includes tasks for which a delay is noticeable by humans, such as delays in music players. Because this is a subjective evaluation, it might never be resolved to everyone's satisfaction. General agreement from testers, however, is the situation is better now with the 2.6 scheduler.

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