Apple - Can I manually limit the %CPU used by a process?

cputhrottle is the tool you need. You can install it with Homebrew.

You can monitor a series of processes by name by running the Bash script below. I'm not quite sure how to turn this into a login item since cputhrottle requires superuser permissions. Run it as a script, in an Automator workflow, whatever:

# Get the Process/App names from Activity Monitor and put them here
apps=("AppOne" "AppTwo" "AppThree")
# Set the respective limits here
limits={30 40 50)

while true; do
  for app in ${apps}; do
    for limit in ${limits}; do
      for pid in $(pgrep ${app}); do
        sudo /path/to/cputhrottle ${pid} ${limit}
      done
    done
  done
done

[Edited]

I've added a different version for this script (a bash script), which might be useful for people looking for limiting the CPU for multiple applications.

This new script also allows you to specify a list containing the application name and the CPU limit for it.

The main difference is that you can add cpu limit per application, and it will run only once per application. I've also added the option for killing all cputhrottle processes.

The script assumes that both cputhrottle and pidof are installed before running it.

#!/bin/bash

if [[ $EUID > 0 ]]; then
  echo "Please run this script as root/sudo"
  exit 1
fi

# Pass --kill as argument to kill all running cputhrottles
if [ $1 = "--kill" ]; then  
  echo "Looking for running cputhrottles..."
  pids=`pidof cputhrottle`
  for pid in ${pids}; do
    echo "> Killing PID ${pid}"
    sudo kill ${pid}
  done
  echo "Done!"
  exit 0
fi

declare -a applications

# Syntax='application;max-cpu'
applications[0]='Chrome;40'
applications[1]='Firefox;50'
applications[2]='pycharm;40'
applications[3]='webstorm;40'
applications[4]='Safari;35'

for i in "${applications[@]}"; do
  app=(${i//;/ })
  app_name=${app[0]}
  cpu_limit=${app[1]}

  printf "\nLooking for ${app_name}...\n"
  pids=`pidof ${app}`
  for pid in ${pids}; do
    echo "> PID=${pid}, CPU=${cpu_limit}"
    sudo cputhrottle ${pid} ${cpu_limit} &
  done
done

printf "\nDone!\n"
echo "Run this script passing '--kill' as argument to remove all cputhrottles."

Source:

• https://gist.github.com/golimpio/692145a19666582f84e0e66bb5dd2b2b

You can indeed! There's CPUThrottle, which allows to specify a PID to restrict.

Note, they're trying to use that much for a reason, it's a useful tool but whether it'll make it better or worse for you on a day to day will be something you discover.

Although not a direct answer to the OP's question, if you're having an issue with a particular process taking up too much of your CPU time, and making your computer unusable, and you don't mind how long that process takes to finish the task it's working on, you can use the renice to alter the priority of that process, making it behave nicely (hence the name).

First, you need to find the PID of the process that's using up the CPU resources. You can either do that in Activity Monitor, or in Terminal.app with the ps command - e.g. to find the PID of the Safari browser, type:

MacBook:~  ps -ef | grep Safari
  501 17452   263   0 11:36pm ??         4:15.60 /Applications/Safari.app/Contents/MacOS/Safari

The second line above is the output, and the PID is 17452 in this particular case.

Then, the next task is to change the priority of the process (let's say it's Safari we want to make behave nicely). To do this, in Terminal.app type:

MacBook:~  renice -n 10 -p 17452

The -n option changes the nice level by adding 10 to the current value (0 by default). The range of values are -20 to 20, with lowest value meaning highest priority. As an ordinary user, you can use values 0 to 20. To assign a negative value, you need to have root privileges (e.g. use sudo command). Read more about nice and renice by typing man nice and man renice in Terminal.app.

nice and renice don't limit the percentage of the CPU available to a given application per se, they do however allow to change the scheduling priority, or in other words how much of the CPU time a process will get. This is all relative to the CPU load on your system, so if the system is under utilised, you most likely won't see any difference.