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When looking for a new process to run on a CPU, the kernel has to consider only the runnable processes
(that is, the processes in the TASK_RUNNING state).
Earlier Linux version put all runnable processes in the same list called runqueue. Because it would be too
costly to maintain the list ordered according to process priorities, the earlier schedulers were compelled
to scan the whole list in order to select the best runnable process.
Linux 2.6 implements the runqueue differently. The aim is to allow the scheduler to select the best runnable
process in constant time, independently of the number of runnable processes. We will defer to Chapter 7 a
detailed description of this new kind of runqueue, and we will provide here only some basic information.
The trick used to achieve the scheduler speedup consists of spliting the runqueue in many lists of runnable
processes, one list per process priority. Each task_struct descriptor includes a run_list field of type list_head.
If the process priority is equal to k (a value ranging between 0 and 139), the run_list field links the process
descriptor into the list of runnable processes having priority k. Furthermore, on a multiprocessor system, each
CPU has its own runqueue, that is, its own set of lists of processes. This is a classic example of making a data
structures more complex to improve performance: to make scheduler operation more efficient, the runqueue
list has been split into 140 different lists!
ULK --- Chap3 Processes: Lists of Tasking_Running processes
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原文地址：http://www.cnblogs.com/miaoyong/p/4948920.html
