[linux] How to create a CPU spike with a bash command

I want to create a near 100% load on a Linux machine. It's quad core system and I want all cores going full speed. Ideally, the CPU load would last a designated amount of time and then stop. I'm hoping there's some trick in bash. I'm thinking some sort of infinite loop.

Utilizing ideas here, created code which exits automatically after a set duration, don't have to kill processes --

#!/bin/bash
echo "Usage : ./killproc_ds.sh 6 60  (6 threads for 60 secs)"

# Define variables
NUM_PROCS=${1:-6} #How much scaling you want to do
duration=${2:-20}    # seconds

function infinite_loop {
endtime=$(($(date +%s) + $duration))
while (($(date +%s) < $endtime)); do
    #echo $(date +%s)
    echo $((13**99)) 1>/dev/null 2>&1
    $(dd if=/dev/urandom count=10000 status=none| bzip2 -9 >> /dev/null) 2>&1 >&/dev/null
done
echo "Done Stressing the system - for thread $1"
}


echo Running for duration $duration secs, spawning $NUM_PROCS threads in background
for i in `seq ${NUM_PROCS}` ;
do
# Put an infinite loop
    infinite_loop $i  &
done

I went through the Internet to find something like it and found this very handy cpu hammer script.

#!/bin/sh

# unixfoo.blogspot.com

if [ $1 ]; then
    NUM_PROC=$1
else
    NUM_PROC=10
fi

for i in `seq 0 $((NUM_PROC-1))`; do
    awk 'BEGIN {for(i=0;i<10000;i++)for(j=0;j<10000;j++);}' &
done

Using examples mentioned here, but also help from IRC, I developed my own CPU stress testing script. It uses a subshell per thread and the endless loop technique. You can also specify the number of threads and the amount of time interactively.

#!/bin/bash
# Simple CPU stress test script

# Read the user's input
echo -n "Number of CPU threads to test: "
read cpu_threads
echo -n "Duration of the test (in seconds): "
read cpu_time

# Run an endless loop on each thread to generate 100% CPU
echo -e "\E[32mStressing ${cpu_threads} threads for ${cpu_time} seconds...\E[37m"
for i in $(seq ${cpu_threads}); do
    let thread=${i}-1
    (taskset -cp ${thread} $BASHPID; while true; do true; done) &
done

# Once the time runs out, kill all of the loops
sleep ${cpu_time}
echo -e "\E[32mStressing complete.\E[37m"
kill 0

One core (doesn't invoke external process):

while true; do true; done

Two cores:

while true; do /bin/true; done

The latter only makes both of mine go to ~50% though...

This one will make both go to 100%:

while true; do echo; done

To load 3 cores for 5 seconds:

seq 3 | xargs -P0 -n1 timeout 5 yes > /dev/null

This results in high kernel (sys) load from the many write() system calls.

If you prefer mostly userland cpu load:

seq 3 | xargs -P0 -n1 timeout 5 md5sum /dev/zero

If you just want the load to continue until you press Ctrl-C:

seq 3 | xargs -P0 -n1 md5sum /dev/zero

cat /dev/urandom > /dev/null

I combined some of the answers and added a way to scale the stress to all available cpus:

#!/bin/bash

function infinite_loop { 
    while [ 1 ] ; do
        # Force some computation even if it is useless to actually work the CPU
        echo $((13**99)) 1>/dev/null 2>&1
    done
}

# Either use environment variables for DURATION, or define them here
NUM_CPU=$(grep -c ^processor /proc/cpuinfo 2>/dev/null || sysctl -n hw.ncpu)
PIDS=()
for i in `seq ${NUM_CPU}` ;
do
# Put an infinite loop on each CPU
    infinite_loop &
    PIDS+=("$!")
done

# Wait DURATION seconds then stop the loops and quit
sleep ${DURATION}

# Parent kills its children 
for pid in "${PIDS[@]}"
do
    kill $pid
done

to increase load or consume CPU 100%

sha1sum /dev/zero &

then you can see CPU uses by typing command

top

to release the load

killall sha1sum

Dimba's dd if=/dev/zero of=/dev/null is definitely correct, but also worth mentioning is verifying maxing the cpu to 100% usage. You can do this with

ps -axro pcpu | awk '{sum+=$1} END {print sum}'

This asks for ps output of a 1-minute average of the cpu usage by each process, then sums them with awk. While it's a 1 minute average, ps is smart enough to know if a process has only been around a few seconds and adjusts the time-window accordingly. Thus you can use this command to immediately see the result.

I've used bc (binary calculator), asking them for PI with a big lot of decimals.

$ for ((i=0;i<$NUMCPU;i++));do
    echo 'scale=100000;pi=4*a(1);0' | bc -l &
    done ;\
    sleep 4; \
    killall bc

with NUMCPU (under Linux):

$ NUMCPU=$(grep $'^processor\t*:' /proc/cpuinfo |wc -l)

This method is strong but seem system friendly, as I've never crashed a system using this.

#!/bin/bash
while [ 1 ]
do
        #Your code goes here
done

:(){ :|:& };:

This fork bomb will cause havoc to the CPU and will likely crash your computer.

Here is a program that you can download Here

Install easily on your Linux system

./configure
make
make install

and launch it in a simple command line

stress -c 40

to stress all your CPUs (however you have) with 40 threads each running a complex sqrt computation on a ramdomly generated numbers.

You can even define the timeout of the program

stress -c 40 -timeout 10s

unlike the proposed solution with the dd command, which deals essentially with IO and therefore doesn't really overload your system because working with data.

The stress program really overloads the system because dealing with computation.

#!/bin/bash
duration=120    # seconds
instances=4     # cpus
endtime=$(($(date +%s) + $duration))
for ((i=0; i<instances; i++))
do
    while (($(date +%s) < $endtime)); do :; done &
done

I think this one is more simpler. Open Terminal and type the following and press Enter.

yes > /dev/null &

To fully utilize modern CPUs, one line is not enough, you may need to repeat the command to exhaust all the CPU power.

To end all of this, simply put

killall yes

The idea was originally found here, although it was intended for Mac users, but this should work for *nix as well.

This does a trick for me:

bash -c 'for (( I=100000000000000000000 ; I>=0 ; I++ )) ; do echo $(( I+I*I )) & echo $(( I*I-I )) & echo $(( I-I*I*I )) & echo $(( I+I*I*I )) ; done' &>/dev/null

and it uses nothing except bash.

I use stress for this kind of thing, you can tell it how many cores to max out.. it allows for stressing memory and disk as well.

Example to stress 2 cores for 60 seconds

stress --cpu 2 --timeout 60

Although I'm late to the party, this post is among the top results in the google search "generate load in linux".

The result marked as solution could be used to generate a system load, i'm preferring to use sha1sum /dev/zero to impose a load on a cpu-core.

The idea is to calculate a hash sum from an infinite datastream (eg. /dev/zero, /dev/urandom, ...) this process will try to max out a cpu-core until the process is aborted. To generate a load for more cores, multiple commands can be piped together.

eg. generate a 2 core load: sha1sum /dev/zero | sha1sum /dev/zero

An infinite loop is the idea I also had. A freaky-looking one is:

while :; do :; done

(: is the same as true, does nothing and exits with zero)

You can call that in a subshell and run in the background. Doing that $num_cores times should be enough. After sleeping the desired time you can kill them all, you get the PIDs with jobs -p (hint: xargs)

To enhance dimba's answer and provide something more pluggable (because i needed something similar). I have written the following using the dd load-up concept :D

It will check current cores, and create that many dd threads. Start and End core load with Enter

#!/bin/bash

load_dd() {
    dd if=/dev/zero of=/dev/null
}

fulload() {
    unset LOAD_ME_UP_SCOTTY
    export cores="$(grep proc /proc/cpuinfo -c)"
    for i in $( seq 1 $( expr $cores - 1 ) )
      do
    export LOAD_ME_UP_SCOTTY="${LOAD_ME_UP_SCOTTY}$(echo 'load_dd | ')"
  done
        export LOAD_ME_UP_SCOTTY="${LOAD_ME_UP_SCOTTY}$(echo 'load_dd &')"
    eval ${LOAD_ME_UP_SCOTTY}
}

echo press return to begin and stop fullload of cores
  read
  fulload
  read
  killall -9 dd

Just paste this bad boy into the SSH or console of any server running linux. You can kill the processes manually, but I just shutdown the server when I'm done, quicker.

Edit: I have updated this script to now have a timer feature so that there is no need to kill the processes.

read -p "Please enter the number of minutes for test >" MINTEST && [[ $MINTEST == ?(-)+([0-9]) ]]; NCPU="$(grep -c ^processor /proc/cpuinfo)";  ((endtime=$(date +%s) + ($MINTEST*60))); NCPU=$((NCPU-1)); for ((i=1; i<=$NCPU; i++)); do while (($(date +%s) < $endtime)); do : ; done & done

I would split the thing in 2 scripts :

infinite_loop.bash :

#!/bin/bash
while [ 1 ] ; do
    # Force some computation even if it is useless to actually work the CPU
    echo $((13**99)) 1>/dev/null 2>&1
done

cpu_spike.bash :

#!/bin/bash
# Either use environment variables for NUM_CPU and DURATION, or define them here
for i in `seq ${NUM_CPU}` : do
    # Put an infinite loop on each CPU
    infinite_loop.bash &
done

# Wait DURATION seconds then stop the loops and quit
sleep ${DURATION}
killall infinite_loop.bash