Linux Scheduler – CFS and Nice

This article explains how Nice (user level process priority) affects Linux Scheduler(CFS).

What is nice?
In simple words, nice is way to influence how process is scheduled in Linux.

What is possible range of nice?
Possible nice value range is: -20 to 19

How does nice value affect priority?
More the value of nice, more nice is the process to other processes!.
In the sense, higher the nice value, lower is the priority.
So, process that has a nice value of -20 is very high priority and 19 has least priority.

How can I see nice value of a process?
Two options
(a) Use ps command.
ps -o pid,comm,nice -p Will display PID, Command and Nice value of the process.
(b) Use top command
top displays a column NI, indicating nice value.

What is default nice value of a process?
The default nice value is zero

How can one change nice value of a process
There are two options command line and system calls.

Which commands can I use?
nice to change the priority when issuing a new command
nice -n

renice to change the priority of an existing process
renice -p

which system calls?
nice() system call
int nice(int inc);
nice() adds inc to the nice value for the calling process.

Nice and Weights of a task
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How is nice value mapped to weight?
weight is roughly equivalent to 1024 / (1.25)^ ^(nice)

Some examples please
weight is 1024 for nice value 0
weight is 820 for nice value 1
weight is 1277 for nice value -1

If weight is indicative of how much time a process gets on CPU, nice seems to have non-linear effect on time consumed!
Yes, that’s right.
Nice value has exponential effect!

Is there a easy way to understand this effect?
Whole scheme has been designed to implement the below idea
nice value down by 1 – 10% more CPU
nice value up by 1 – 10% less CPU

How does weight of a process affect its CPU availability?
In simple terms,
weight of Run Queue of all tasks = sum of weight of all tasks in the queue.
proportion of CPU allotted = weight(process)/weight(RunQueue)

Too much theory…some examples please…

Consider two processes, A and B with nice value 0 (default) and running on single CPU.
nice(A) = 0 => weight(A) = 1024
nice(B) = 0 => weight(B) = 1024.

Assuming single CPU and no other process running, run queue of CPU has two tasks.
weight(RunQueue) = weight(A) + weight(B) = 2048.

proportion of CPU allotted = wieght(process)/weight(RunQueue)
cpuPercentage(A) = weight(A)/weight(RunQueue) = 1024/2048 = 50%
Needless to say cpuPercentage(B) = 50%

What if A has nice value 1 and B has 0
Then
weight(A) with nice 1 is 820
weight(B) with nice 0 is 1024
weight(RunQueue) = weight(A) + weight(B) = 1844.
cpuPercentage(A) = weight(A)/weight(RunQueue) = 820/1844 = ~45%
cpuPercentage(B) = weight(B)/weight(RunQueue) = 1024/1844 = ~55%

So by increasing nice value of A to 1, its CPU availability is lower than nice value 0 process by 10%…

Experiments with NICE on real Linux Machine
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#Create two busy wait processes – Note taskset command forces the task to run in CPU specified. I have used CPU #5 in my system.
taskset -c 5 dd if=/dev/zero of=/dev/null &
taskset -c 5 dd if=/dev/zero of=/dev/null &

# See the CPU loading using top – Note pressing ‘1’ when top is running shows individual CPU loads.
# Showed that processes are using 50% CPU each
top

#Change the nice value of one of the processes. 10211 was PID of the one of the processes. Now CPU usage changed to 55% for one and 44% for another as expected!
renice -n 1 10211
top

#Change the nice value of one of the processes. 10211 was PID of the one of the processes. Now CPU usage changed to 75% for one and 25% for another as expected!
renice -n 5 10211
top