本文介绍Linux操作系统中的进程调度策略，包括实时进程和常规进程的调度机制。文章详细解释了Linux如何通过不同优先级和时间片来平衡系统响应时间和吞吐量，并讨论了避免进程饥饿的方法。
| 7.1 Scheduling Policy | |||||||||
| · fulfill several conflicting objectives: | |||||||||
| fast process response time | |||||||||
| good throughput for background jobs | |||||||||
| avoidance of process starvation | |||||||||
| reconciliation of the needs of low- and high-priority processes | |||||||||
| · when and how to select a new process to run is called scheduling policy | |||||||||
| · Linux scheduling is | |||||||||
| based on the time sharing technique | |||||||||
| based on ranking processes according to their priority | |||||||||
| process priority is dynamic | |||||||||
| · I/O-bound/CPU-bound | |||||||||
| · three classes of processes: | |||||||||
| Interactive processes | |||||||||
| Batch processes | |||||||||
| Real-time processes | |||||||||
| · Process Preemption | |||||||||
| TIF_NEED_RESCHED flag in the thread_info structure | |||||||||
| a preempted process is not suspended, because it remains in the TASK_RUNNING state | |||||||||
| · How Long Must a Quantum Last? | |||||||||
| The quantum duration is critical for system performance: | |||||||||
| it should be neither too long nor too short. | |||||||||
| If the average quantum duration is too short, the system overhead | |||||||||
| caused by process switches becomes excessively high | |||||||||
| In some cases, however, a very long quantum duration d | |||||||||
| egrades the responsiveness of the system | |||||||||
| · The rule of thumb adopted by Linux is choose a duration as long as possible, | |||||||||
| while keeping good system response time | |||||||||
| 7.2. The Scheduling Algorithm | |||||||||
| earlier versions of Linux | |||||||||
| the kernel scanned the list of runnable processes, | |||||||||
| computed their priorities, and selected the "best" process to run | |||||||||
| drawback:time spent in choosing the best process | |||||||||
| depends on the number of runnable processes | |||||||||
| · in Linux 2.6 | |||||||||
| scales well with the number of runnable processes, | |||||||||
| because it selects the process to run in constant time | |||||||||
| scales well with the number of processors | |||||||||
| because each CPU has its own queue of runnable processes | |||||||||
| does a better job of distinguishing interactive processes and batch processes | |||||||||
| · The scheduler always succeeds in finding a process to be executed; | |||||||||
| in fact, there is always at least one runnable process: the swapper process, | |||||||||
| which has PID 0 and executes only when the CPU cannot execute other processes | |||||||||
| · scheduling classes : | |||||||||
| SCHED_FIFO | |||||||||
| A First-In, First-Out real-time process | |||||||||
| leaves the process descriptor in its current position in the runqueue list | |||||||||
| SCHED_RR | |||||||||
| A Round Robin real-time process | |||||||||
| puts the process descriptor at the end of the runqueue list | |||||||||
| SCHED_NORMAL | |||||||||
| A conventional, time-shared process | |||||||||
| · Scheduling of Conventional Processes | |||||||||
| static priority,ranging from 100 (highest priority) to 139 (lowest priority) | |||||||||
| A new process always inherits the static priority of its parent | |||||||||
| a user can change the static priority of the processes | |||||||||
| nice( ) and setpriority( ) system calls | |||||||||
| · Base time quantum | |||||||||
| Base time quantum | = | (140 - static priority) * 20 if static priority < 120 | |||||||
| (milisecond) | (140 - static priority) * 5 if static priority >= 120 | ||||||||
| · Dynamic priority and average sleep time | |||||||||
| dynamic priority,ranging from 100 (highest priority) to 139 (lowest priority) | |||||||||
| The dynamic priority is the number actually looked up by the scheduler | |||||||||
| when selecting the new process to run | |||||||||
| dynamic priority = max (100, min ( static priority - bonus + 5, 139)) | |||||||||
| bonus :ranging from 0 to 10,depends on average sleep time of the process | |||||||||
| value less than 5 | penalty | ||||||||
| greater than 5 | premium | ||||||||
| the average sleep time is the average number of | |||||||||
| nanoseconds that the process spent while sleeping | |||||||||
| The average sleep time is also used by the scheduler to determine | |||||||||
| whether a given process should be considered interactive or batch | |||||||||
| interactive | |||||||||
| dynamic priority <= 3 * static priority / 4 + 28 | |||||||||
| equivalent to bonus - 5 >= static priority / 4 - 28 | |||||||||
| The expression static priority / 4 - 28 is called the interactive delta | |||||||||
| It should be noted that it is far easier for high priority than | |||||||||
| for low priority processes to become interactive | |||||||||
| · Active and expired processes | |||||||||
| To avoid process starvation | |||||||||
| when a process finishes its time quantum, it can be replaced by a lower priority | |||||||||
| process whose time quantum has not yet been exhausted. To implement | |||||||||
| this mechanism, the scheduler keeps two disjoint sets of runnable processes: | |||||||||
| Active Processes:These runnable processes have not yet | |||||||||
| exhausted their time quantum and are thus allowed to run | |||||||||
| Expired processes:These runnable processes have exhausted their time quantum | |||||||||
| and are thus forbidden to run until all active processes expire | |||||||||
| However, the general schema is slightly more complicated than this, because the scheduler tries to | |||||||||
| boost the performance of interactive processes. An active batch process that finishes | |||||||||
| Its time quantum always becomes expired. An active interactive process that finishes | |||||||||
| its time quantum usually remains active: the scheduler refills its time quantum and leaves | |||||||||
| it in the set of active processes. However, the scheduler moves an interactive process | |||||||||
| that finished its time quantum into the set of expired processes if the eldest expired process | |||||||||
| has already waited for a long time, or if an expired process has higher static priority (lower value) | |||||||||
| than the interactive process. As a consequence, the set of active processes will eventually | |||||||||
| become empty and the expired processes will have a chance to run. | |||||||||
| · Scheduling of Real-Time Processes | |||||||||
| ranging from 1 (highest priority) to 99 (lowest priority),are always considered active | |||||||||
| A real-time process is replaced by another process only | |||||||||
| when one of the following events occurs: | |||||||||
| · The process is preempted by another process having higher real-time priority. | |||||||||
| · The process performs a blocking operation, and it is put to sleep | |||||||||
| (in state TASK_INTERRUPTIBLE or TASK_UNINTERRUPTIBLE). | |||||||||
| · The process is stopped (in state TASK_STOPPED or TASK_TRACED), | |||||||||
| or it is killed (in state EXIT_ZOMBIE or EXIT_DEAD). | |||||||||
| · The process voluntarily relinquishes the CPU by invoking the sched_yield( ) system call | |||||||||
| · The process is Round Robin real-time (SCHED_RR), and it has exhausted its time quantum | |||||||||
| the duration of the base time quantum of Round Robin real-time processes does not | |||||||||
| depend on the real-time priority, but rather on the static priority of the process | |||||||||
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