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.

As work began on the 2.5 Linux kernel tree back in December 2001, there was a lot of talk in the community about scaling. Linux had begun to appear in some of the roles traditionally filled by larger servers, and several vendors were offering versions of Linux suitable for symmetric multi-processing (SMP). Although commercial interest in that area seemed to be growing, there also was a growing realization that even SMP Linux wasn't scaling as well as it should. If, say, two single-processor desktop machines could outperform a single four-processor machine, who'd want to use (or buy) the four-way?

One of the first areas of the kernel that required attention was the scheduler. It became apparent that as the load and the number of CPUs increased, the scheduler worked harder and harder and ended up taking more and more time away from the processes it was scheduling. In the worst case, nearly the entire system was consumed trying to decide what to run next.

When we speak of the Linux scheduler, we're not referring to a specific task that handles all the scheduling. Rather, each task itself does a little bit of the scheduling each time it acquires or yields the processor, by calling a scheduler function within the kernel. So when we speak about the scheduler doing this or that, we really mean the scheduler function and its related routines in the context of some other task.

The 2.4 Scheduler

The original 2.4 scheduler was quite simple. All tasks on the system were already on a global list called tasklist, and these were assigned a goodness rating. Goodness was determined by:

  • How many clock ticks you might have left: when a task is given the processor, the task is allocated a certain amount of time to use it before that task is interrupted involuntarily and replaced by another task. If it gives up the processor voluntarily—to wait for I/O, for example—then the task's generosity would be rewarded by being at a higher priority to regain the processor once the I/O job was complete.

  • CPU affinity: by using another system call, it is possible to advise the scheduler that you wish to remain on a particular processor, even if another processor should free up first.

  • Nice or user-set priority: if the user is root, it's possible for the user to increase or decrease the priority of a task within a fairly substantial range.

  • Whether the task was a real-time task: a task that has been designated a real-time task has a higher priority than all tasks that are not real time.

So when a processor came free, the 2.4 scheduler would examine the tasklist, looking for the task with the highest goodness, and select that task for running next. Figure 1 demonstrates how the 2.4 scheduler worked. The tasklist was the runqueue, and because it wasn't ordered in any helpful way, each iteration of the scheduler would examine the tasklist completely, looking for the best candidate for the idle processor. In the case of multiple processors, it was a matter of chance if you ended up on the same processor twice in a row, even if you were the only runnable task.

Figure 1. The 2.4 scheduler was shared among processors and wasn't ordered in any helpful way.

This model had the advantage of being quite simple to implement and fairly simple to debug. The tasklist was guarded by a single read/write spinlock. This allowed multiple tasks to examine it in parallel while still providing the mechanism for obtaining exclusive access for the comparatively rare event of changing it.

Unfortunately, these same features also were the model's disadvantages. Instrumentation of the then-current 2.4 scheduler began to zero-in on the problem: the single read/write spinlock tended to become a point of contention on both busy systems and systems with four or more CPUs. Only a single queue was used for all processors, and it had to be examined completely for each reschedule. As the system got busier, the tasklist got longer; the linear search for the best task took longer as well. As a result, having decided which process to run, you waited longer to acquire the lock exclusively to remove that task from the runnable list and mark it running. If the wait was long enough, several processors might choose the same process only to learn that it already had been given to a different processor. The other processors then would have to go back to the linear search and find another task. As the system got busier, the scheduler consumed more CPU time, to the point where scheduling processes took more time than did running them. Changes needed to be made so that a loaded system didn't schedule itself into a standstill.