mjhjmtu

I am trying to test out the renice command on my Mac, it seems to be having no effect. This is how I'm testing it:

I have a dual core CPU with hyper threading, so I run 6 instances of node -e 'while(1)' which, combined, max out my CPU. (Alternatively, run while true; do echo running > /dev/null ; done if you dont have node.)

I then get a PID of one of the node processes, either by the activity monitor application or pgrep, and then run sudo renice -n 19 -p $PID.

By looking at the activity monitor application or htop, the CPU usage of all of the node processes are still equivalent:

My understanding was that the process that I reniced would have a low priority, and so would not use a significant amount of CPU time when there are other processes using it.

Am I misunderstanding something about renice? Or is this a bug in Mac OS?

Running some rudimentary tests I’m seeing this too on 10.13.4.

I wrote a small C program called looper which did some basic maths in an infinite loop:

#include <stdio.h>

int main()
 unsigned long long prev = 0;
 unsigned long long current = 1;
 unsigned long long total = 0;

 while(1)
 total = current + prev;
 prev = current;
 current = total;
 printf("%llun", total);
 

I ran multiple copies of this on my FreeBSD box and examined them using htop(1). Using renice(8) I could see the Nice value being incremented, the Priority value being incremented, and the CPU percentage decreasing - expected behavior.

I ran the same program on macOS 10.13.4 and repeated the process. While I could see the Nice value incrementing, the Priority value and CPU percentage remained consistent.

I wonder if it is a bug, I've not had cause to look this closely at Nice values in macOS before - the behavior has been close enough to what I expected that I've not been inclined to investigate.

If you have a developer account it would be worth reporting this to Apple.