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

Question

The man page for nice says "nice() 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);
}

Answer 1

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.

Answer 2

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).

Answer 3

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