How to limit web services so they don’t kill CPU

Hi! As any developer, I’ve encountered a small problem when working with PHP under Apache. It’s all good until you have to test the “real life” situations. For example – if you use imagemagick library in your application. It can get quite heavy on server load. So if you want to read a solution for it, read on.

First of all, what do I mean by “quite heavy”. Well, imagemagick as my example is great image manipulation tool, but it takes quite a lot of CPU power compared to gd library that PHP uses. And if you have a site that has something around 50 users at the same moment who use imagemagick in the same time, with poor server they’ll mots probably create a mini DDOS attack on you, non intentionally. A solution for Linux based servers is “CPU limit”.

About CPU limit

CPU limit is open source Linux based solution that gives you option to limit each process’s (by PID) CPU power consumption in percentage. So for example, you can always limit your apache to 2% of CPU, and MySQL to let’s say 10%. Its pretty much straightforward, so you can fit your needs.

Installation process on Ubuntu

There are few steps to complete this tasks, so here they are in right order:

First, 2 packages are needed, and you can install those with following commands:

sudo apt-get install cpulimit
sudo apt-get install gawk

Next step is to create Daemon for CPU limit:

#!/bin/bash
# ==============================================================
# CPU limit daemon - set PID's max. percentage CPU consumptions
# ==============================================================

# Variables
CPU_LIMIT=20 # Maximum percentage CPU consumption by each PID
DAEMON_INTERVAL=3 # Daemon check interval in seconds
BLACK_PROCESSES_LIST= # Limit only processes defined in this variable. If variable is empty (default) all violating processes are limited.
WHITE_PROCESSES_LIST= # Limit all processes except processes defined in this variable. If variable is empty (default) all violating processes are limited.

# Check if one of the variables BLACK_PROCESSES_LIST or WHITE_PROCESSES_LIST is defined.
if [[ -n "$BLACK_PROCESSES_LIST" && -n "$WHITE_PROCESSES_LIST" ]] ; then # If both variables are defined then error is produced.
echo "At least one or both of the variables BLACK_PROCESSES_LIST or WHITE_PROCESSES_LIST must be empty."
exit 1
elif [[ -n "$BLACK_PROCESSES_LIST" ]] ; then # If this variable is non-empty then set NEW_PIDS_COMMAND variable to bellow command
NEW_PIDS_COMMAND="top -b -n1 -c | grep -E '$BLACK_PROCESSES_LIST' | gawk '$9>CPU_LIMIT {print $1}' CPU_LIMIT=$CPU_LIMIT"
elif [[ -n "$WHITE_PROCESSES_LIST" ]] ; then # If this variable is non-empty then set NEW_PIDS_COMMAND variable to bellow command
NEW_PIDS_COMMAND="top -b -n1 -c | gawk 'NR>6' | grep -E -v '$WHITE_PROCESSES_LIST' | gawk '$9>CPU_LIMIT {print $1}' CPU_LIMIT=$CPU_LIMIT"
else
NEW_PIDS_COMMAND="top -b -n1 -c | gawk 'NR>6 && $9>CPU_LIMIT {print $1}' CPU_LIMIT=$CPU_LIMIT"
fi
 
# Search and limit violating PIDs
while sleep $DAEMON_INTERVAL
do
NEW_PIDS=$(eval "$NEW_PIDS_COMMAND") # Violating PIDs
LIMITED_PIDS=$(ps -eo args | gawk '$1=="cpulimit" {print $3}') # Already limited PIDs
QUEUE_PIDS=$(comm -23 < (echo "$NEW_PIDS" | sort -u) <(echo "$LIMITED_PIDS" | sort -u) | grep -v '^$') # PIDs in queue for i in $QUEUE_PIDS do cpulimit -p $i -l $CPU_LIMIT -z & # Limit new violating processes done done 

If you wish to auto-start the Daemon on PC start, you have to do the following 3 steps: Step 1:

 sudo chmod 700 /usr/bin/cpulimit_daemon.sh 

Step 2: Now save this to /etc/init.d/cpulimit:

 #!/bin/sh # # Script to start CPU limit daemon # set -e case "$1" in start) if [ $(ps -eo pid,args | gawk '$3=="/usr/bin/cpulimit_daemon.sh" {print $1}' | wc -l) -eq 0 ]; then nohup /usr/bin/cpulimit_daemon.sh >/dev/null 2>&1 &
ps -eo pid,args | gawk '$3=="/usr/bin/cpulimit_daemon.sh" {print}' | wc -l | gawk '{ if ($1 == 1) print " * cpulimit daemon started successfully"; else print " * cpulimit daemon can not be started" }'
else
echo " * cpulimit daemon can't be started, because it is already running"
fi
;;
stop)
CPULIMIT_DAEMON=$(ps -eo pid,args | gawk '$3=="/usr/bin/cpulimit_daemon.sh" {print $1}' | wc -l)
CPULIMIT_INSTANCE=$(ps -eo pid,args | gawk '$2=="cpulimit" {print $1}' | wc -l)
CPULIMIT_ALL=$((CPULIMIT_DAEMON + CPULIMIT_INSTANCE))
if [ $CPULIMIT_ALL -gt 0 ]; then
if [ $CPULIMIT_DAEMON -gt 0 ]; then
ps -eo pid,args | gawk '$3=="/usr/bin/cpulimit_daemon.sh" {print $1}' | xargs kill -9 # kill cpulimit daemon
fi
 
if [ $CPULIMIT_INSTANCE -gt 0 ]; then
ps -eo pid,args | gawk '$2=="cpulimit" {print $1}' | xargs kill -9 # release cpulimited process to normal priority
fi
ps -eo pid,args | gawk '$3=="/usr/bin/cpulimit_daemon.sh" {print}' | wc -l | gawk '{ if ($1 == 1) print " * cpulimit daemon can not be stopped"; else print " * cpulimit daemon stopped successfully" }'
else
echo " * cpulimit daemon can't be stopped, because it is not running"
fi
;;
restart)
$0 stop
sleep 3
$0 start
;;
status)
ps -eo pid,args | gawk '$3=="/usr/bin/cpulimit_daemon.sh" {print}' | wc -l | gawk '{ if ($1 == 1) print " * cpulimit daemon is running"; else print " * cpulimit daemon is not running" }'
;;
esac
exit 0

Step 3:

sudo chown root:root /etc/init.d/cpulimit
//Set the ownership
sudo chmod 755 /etc/init.d/cpulimit
//Set the privileges
sudo update-rc.d cpulimit defaults
//Add script to boot

And only thing left to do is test it out. You can do that with following commands (which are self-explanatory):

sudo service cpulimit status
sudo service cpulimit start
sudo service cpulimit stop
sudo service cpulimit restart

Conclusion

Well, I not the author of those scripts, but I think it is quite useful, and I wanted to share it with all of you. So have fun with limiting your processes! 😀