Linux定义线程优先级范围在头文件<linux/sched.h>
/*
* Priority of a process goes from 0..MAX_PRIO-1, valid RT
* priority is 0..MAX_RT_PRIO-1, and SCHED_NORMAL/SCHED_BATCH
* tasks are in the range MAX_RT_PRIO..MAX_PRIO-1. Priority
* values are inverted: lower p->prio value means higher priority.
*
* The MAX_USER_RT_PRIO value allows the actual maximum
* RT priority to be separate from the value exported to
* user-space. This allows kernel threads to set their
* priority to a value higher than any user task. Note:
* MAX_RT_PRIO must not be smaller than MAX_USER_RT_PRIO.
*/
#define MAX_USER_RT_PRIO 100
#define MAX_RT_PRIO MAX_USER_RT_PRIO
#define MAX_PRIO (MAX_RT_PRIO + 40)
#define DEFAULT_PRIO (MAX_RT_PRIO + 20)Linux线程调度策略
Linux定义线程调度策略在头文件<linux/sched.h>
#define SCHED_NORMAL 0
#define SCHED_FIFO 1
#define SCHED_RR 2
#define SCHED_BATCH 3
/* SCHED_ISO: reserved but not implemented yet */
#define SCHED_IDLE 5-
SCHED_NORMAL即为SCHED_OTHER：采用分时调度策略。
-
SCHED_FIFO：采用实时调度策略，且先到先服务，一直运行直到有更高优先级任务到达或自己放弃。
-
SCHED_RR：采用实时调度策略，且时间片轮转，时间片用完，系统将重新分配时间片，并置于就绪队列尾。
-
SCHED_BATCH：针对批处理进程。
-
SCHED_IDLE：使用此调度器的进程的优先级最低。在实现CFS时引入。
shell下查看线程优先级
-
top命令
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1 root 20 0 2804 1576 1192 S 0.0 0.3 0:00.97 init
2 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kthreadd
3 root RT 0 0 0 0 S 0.0 0.0 0:00.00 migration/0
4 root 20 0 0 0 0 S 0.0 0.0 0:00.02 ksoftirqd/0
5 root RT 0 0 0 0 S 0.0 0.0 0:00.00 watchdog/0
6 root 20 0 0 0 0 S 0.0 0.0 0:00.06 events/0PR为线程优先级，范围[0, 40)，实时优先级的线程显示为RT。值越小优先级越高 NI为Nice值，范围[-20, 20)，PR值为初始线程优先级加上Nice值。PR 和 NI 值都是从 proc文件系统中读取得到 “/proc/[pid]/stat”
cat /proc/1/stat
1 (init) S 0 1 1 0 -1 4202752 4680 154640 23 343 6 136 154 151 20 0 ...
cat /proc/3/stat
3 (migration/0) S 2 0 0 0 -1 2216730688 0 0 0 0 0 0 0 0 -100 0 ...通过man proc 查看stat的第18项和第19项的说明如下
priority %ld
(18) (Explanation for Linux 2.6) For processes running a real-time scheduling policy (policy below; see sched_setscheduler(2)),
this is the negated scheduling priority, minus one; that is, a number in the range -2 to -100, corresponding to real-time priorities 1 to 99.
For processes running under a non-real-time scheduling policy, this is the raw nice value (setpriority(2)) as represented in the kernel.
The kernel stores nice values as numbers in the range 0 (high) to 39 (low), corresponding to the user-visible nice range of -20 to 19.
Before Linux 2.6, this was a scaled value based on the scheduler weighting given to this process.
nice %ld
(19) The nice value (see setpriority(2)), a value in the range 19 (low priority) to -20 (high priority).-
ps命令
c@c-desktop:~$ ps -el
F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD
4 S 0 1 0 0 80 0 - 702 poll_s ? 00:00:01 init
1 S 0 2 0 0 80 0 - 0 kthrea ? 00:00:00 kthreadd
1 S 0 3 2 0 -40 - - 0 migrat ? 00:00:00 migration/0
1 S 0 4 2 0 80 0 - 0 ksofti ? 00:00:00 ksoftirqd/0
5 S 0 5 2 0 -40 - - 0 watchd ? 00:00:00 watchdog/0
1 S 0 6 2 0 80 0 - 0 worker ? 00:00:00 events/0这里的PRI为什么和top的PR不一样呢，通过查看procps源代码找到了原因
In linux procps, the column labeled "PRI" in ps -l is -o opri
c@c-desktop:~$ ps -e -o pid,pri,opri,intpri,priority,ni,cmd
PID PRI PRI PRI PRI NI CMD
1 19 80 80 20 0 /sbin/init
2 19 80 80 20 0 [kthreadd]
3 139 -40 -40 -100 - [migration/0]
4 19 80 80 20 0 [ksoftirqd/0]
5 139 -40 -40 -100 - [watchdog/0]
6 19 80 80 20 0 [events/0]procps/output.c中含有具体说明
// "priority" (was -20..20, now -100..39)
// "intpri" and "opri" (was 39..79, now -40..99)
// "pri_foo" -- match up w/ nice values of sleeping processes (-120..19)
// "pri_bar" -- makes RT pri show as negative (-99..40)
// "pri_baz" -- the kernel's ->prio value, as of Linux 2.6.8 (1..140)
// "pri" (was 20..60, now 0..139)
// "pri_api" -- match up w/ RT API (-40..99)shell下修改线程优先级
-
nice命令
nice命令可以指定线程加载时的Nice值，来改变线程优先级。nice -n [priority] [command] 输入的优先级超过19时取19，小于-20时取-20。
root@c-desktop:~/Code# nice -n -20 ./a.out &
root@c-desktop:~/Code# ps -o pid,priority,ni,cmd
PID PRI NI CMD
2138 0 -20 ./a.out
root@c-desktop:~/Code# nice -n 19 ./a.out &
root@c-desktop:~/Code# ps -o pid,priority,ni,cmd
PID PRI NI CMD
2145 39 19 ./a.out-
renice命令
renice命令可以改变运行中线程的Nice值，进而改变线程的优先级。renice -n [priority] -p [pid] 输入的优先级超过19时取19，小于-20时取-20。
root@c-desktop:~/Code# ps -o pid,priority,ni,cmd
PID PRI NI CMD
2145 39 19 ./a.out
root@c-desktop:~/Code# renice -n -20 -p 2145
2145: old priority 19, new priority -20
root@c-desktop:~/Code# ps -o pid,priority,ni,cmd
PID PRI NI CMD
2145 0 -20 ./a.out