is nice() used to change the thread priority or the process priority?

The man page for nice says "nice()开发者_运维技巧 adds inc to the nice value for the calling process. So, can we use it to change the nice value for a thread created by pthread_create?

EDIT: It seems that we can set the nice value per thread.

I wrote an application, setting different nice values for different threads, and observed that the "nicer" thread has been scheduled with lower priority. Checking the output, I found that the string "high priority ................" gets outputted more frequently.

void * thread_function1(void *arg)
{
  const pid_t tid = syscall(SYS_gettid);

  int ret = setpriority(PRIO_PROCESS, tid, -10);
  printf("tid of high priority thread %d , %d\n", tid ,getpriority(PRIO_PROCESS, tid));
  while(1)
  {
    printf("high priority ................\n");
  }
}


void * thread_function(void *arg)
{
  const pid_t tid = syscall(SYS_gettid);
  int ret = setpriority(PRIO_PROCESS, tid, 10);
  printf("tid of low priority thread %d , %d \n", tid ,getpriority(PRIO_PROCESS, tid));
  while(1)
  {
    printf("lower priority\n");
  }
}


int main()
{
  pthread_t id1;
  pthread_t id2;

  pid_t pid = getpid();
  pid_t tid = syscall(SYS_gettid);

  printf("main thread : pid = %d , tid = %d \n" , pid, tid);
  pthread_create(&id1, NULL, thread_function1,  NULL);
  pthread_create(&id2, NULL,thread_function,   NULL);

  pthread_join(id1, NULL);
  pthread_join(id2, NULL);
}

The pthreads man page says:

POSIX.1 also requires that threads share a range of other attributes (i.e., these attributes are process-wide rather than per-thread):

[...]

• nice value (setpriority(2))

So, theoretically, the "niceness" value is global to the process and shared by all threads, and you should not be able to set a specific niceness for one or more individual threads.

However, the very same man page also says:

LinuxThreads

The notable features of this implementation are the following:

[...]

• Threads do not share a common nice value.

NPTL

[...]

NPTL still has a few non-conformances with POSIX.1:

• Threads do not share a common nice value.

So it turns out that both threading implementations on Linux (LinuxThreads and NPTL) actually violate POSIX.1, and you can set a specific niceness for one or more individual threads by passing a tid to setpriority() on these systems.

According to the man page for setpriority, a lower nice value (nice values are in the range of -20 to 20) means higher priority in scheduling. It looks like your program works as expected (nice = -10 gives this thread higher priority).

I wanted to test how changing these values really affects the thread's priority, so I modified your snippet to this benchmark:

• Running on default SCHED_OTHER scheduling policy
• Created 12 low priority threads to make sure they compete on resources - on Red hat 7 with 8 cores. (cat /proc/cpuinfo)
• Modified the thread_function() to do some "number crunching work"

When setting to edge priorities you can definitely see with top -H that the high priority thread runs more often, but no starvation occurs to other threads. relevant fields are NI and TIME+

From top man page:

NI -- Nice Value The nice value of the task. A negative nice value means higher priority, whereas a positive nice value means lower priority. Zero in this field simply means priority will not be adjusted in determining a task's dispatch-ability.

TIME -- CPU Time Total CPU time the task has used since it started.

#include<cstdio> 
#include<pthread.h> 
#include<unistd.h> 
#include<sys/syscall.h> 
#include<sys/resource.h>
#include <stdio.h>
#include <stdlib.h>
#define NUM_THREADS 12
 
struct ThreadParams{
 const char* priority;
 const int niceLevel;
};

void * thread_function(void *arg)
{
  const pid_t tid = syscall(SYS_gettid);


  struct ThreadParams* params = (ThreadParams*)arg; 

  int ret = setpriority(PRIO_PROCESS, tid, params->niceLevel);
  printf("tid of %s priority thread %d , %d\n", params->priority, tid ,getpriority(PRIO_PROCESS, tid));
  long long int count = 0;
  while(1)
  {
      count++;
      if(count == 10000000000) //10^10 iterations
      {
          printf("%s priority ................\n", params->priority);
          count = 0;
      }
  }
}



int main()
{
  pthread_t tIdHigh;
  pthread_t tIdsLow[NUM_THREADS];

  pid_t pid = getpid();
  pid_t tid = syscall(SYS_gettid);

  printf("main thread : pid = %d , tid = %d \n" , pid, tid);
  
  struct ThreadParams highParams = {"High", -20};
  struct ThreadParams lowParams  = {"Low", 19};

  for(int i=0; i < NUM_THREADS ; i++)
  {
    pthread_create(&(tIdsLow[i]), NULL,thread_function,  &lowParams);
  }
  
  pthread_create(&tIdHigh, NULL, thread_function,  &highParams);
  
  for(int i=0; i < NUM_THREADS ; i++)
  {
    pthread_join(tIdsLow[i], NULL);
  }
  pthread_join(tIdHigh, NULL);
  return 0;

}

Compiled with g++ <FILE_NAME>.cpp -lpthread.

Run top -H -p $(pidof <PROCESS_NAME>) to enable Threads-mode and get information for specific process