Linux priority

Linux priority 基本概念可參考此篇介紹，摘錄如下：

In Linux system priorities are 0 to 139 in which 0 to 99 for real time and 100 to 139 for users. nice value range is -20 to +19 where -20 is highest, 0 default and +19 is lowest. 

The relation between nice value and priority is :
PR = 20 + NI
so , the value of PR = 20 + (-20 to +19) is 0 to 39 that maps 100 to 139.

Linux schedule policy 分成兩大類，如下：

• normal (只能修改 nice value 進行調整, 調整後的 priortiy 介於 100~139)

• SCHED_OTHER

• SCHED_BATCH

• SCHED_IDLE

• real-time (調整後的 priortiy 介於 0~99)

• SCHED_FIFO

• SCHED_RR

其優先權如下
SCHED_FIFO = SCHED_RR > SCHED_OTHER ~ SCHED_BATCH > SCHED_IDLE

我們可藉著設定 process schedule policy ，更改 process priority，舉例如下

1. 建立 thread 時，指定 priority
struct sched_param param;
param.sched_priority = 1;
s = pthread_attr_setinheritsched(&attr, PTHREAD_EXPLICIT_SCHED);
if (s != 0)
   handle_error_en(s, "pthread_attr_setinheritsched");
s = pthread_attr_setschedpolicy(&attr, SCHED_RR);
if (s != 0)
   handle_error_en(s, "pthread_attr_setschedpolicy");
param.sched_priority = sched_get_priority_min(SCHED_RR);
s = pthread_attr_setschedparam(&attr, &param);
if (s != 0)
   handle_error_en(s, "pthread_attr_setschedparam");
 
2. 當 thread 建立之後，動態改變 priority
struct sched_param param;
param.sched_priority = 1;
pthread_setschedparam(pthread_self(), SCHED_RR, &param);


另外當程式執行之後，可以使用下列命令，得知各個 Thread 的 priority

$ ps H -C a.out -o 'pid tid pri nice cmd comm'
  PID   TID PRI  NI CMD                         COMMAND
16062 16062  19   0 ./a.out 123                 a.out
16062 16063  41   - ./a.out 123                 ./a_client

注意：此處 PRI 跟使用 top 看到的 priority 會有差距，其換算公式為 priority = 39 - PRI

Reference:

• http://man7.org/linux/man-pages/man7/sched.7.html
• https://askubuntu.com/questions/656771/process-niceness-vs-priority