How can I produce high CPU load on a Linux server?

Question

I’m currently in the process of debugging a Cacti installation and want to create CPU load to debug my CPU utilization graphs.

I tried to simply run cat /dev/zero > /dev/null, which works great but only utilizes 1 core:

Is there a better method of testing/maxing-out system resources under load?

Related: How can I produce high CPU load on Windows?

Solution 1

Try stress It's pretty much an equivalent of the Windows consume.exe:

oliver$ ./stress --cpu 3
stress: info: [18472] dispatching hogs: 3 cpu, 0 io, 0 vm, 0 hdd

Solution 2

No need to install any extra package, your good old shell is able to do it alone.

This one-liner will load your four cores¹ at 100%:

for i in 1 2 3 4; do while : ; do : ; done & done

How it works is quite simple, it starts four endless loops. Each of them is repeating the null instruction (:). Each loop is able to load a CPU core at 100%.

If you use bash, ksh93 and other shells supporting ranges, (i.e. not dash or older ksh), you can use this non portable syntax:

for i in {1..4}; do ...

Replace 4 with the number of CPUs you'd like to load if different from 4.

Assuming you had no background job already running when you launched one of these loops, you can stop the load generation with that command:

for i in 1 2 3 4; do kill %$i; done

---

Answering @underscore_d's comment, here is an enhanced version that simplify a lot stopping the load and that also allow specifying a timeout (default 60 seconds.) A Control-C will kill all the runaway loops too. This shell function works at least under bash and ksh.

# Usage: lc [number_of_cpus_to_load [number_of_seconds] ]
lc() {
  (
    pids=""
    cpus=${1:-1}
    seconds=${2:-60}
    echo loading $cpus CPUs for $seconds seconds
    trap 'for p in $pids; do kill $p; done' 0
    for ((i=0;i<cpus;i++)); do while : ; do : ; done & pids="$pids $!"; done
    sleep $seconds
  )
}

¹_{Note that with CPUs supporting more than one thread per core (Hyper-threading), the OS will dispatch the load to all virtual CPUs. In that case, the load behavior is implementation dependent (each thread might be reported as 100% busy or not).}.

Solution 3

One alternative way would be

openssl speed -multi $(grep -ci processor /proc/cpuinfo)

or (if nproc is present)

openssl speed -multi $(nproc --all)

OpenSSL is almost always present on nowadays distros, so no extra packages needed.

Solution 4

I made a simple python script which does the same. You can control the number of cpu cores you want to load. The good thing about this is that it won't consume any other resource besides the cpu. (I think mark johnson's idea would consume a lot of I/O resources, which is undesired here.)

from multiprocessing import Pool

def f(x):
    # Put any cpu (only) consuming operation here. I have given 1 below -
    while True:
        x * x

# decide how many cpus you need to load with.
no_of_cpu_to_be_consumed = 3

p = Pool(processes=no_of_cpu_to_be_consumed)
p.map(f, range(no_of_cpu_to_be_consumed))

Just run this script from the terminal $ python temp1.py. You need to kill the script when you are done.

Here, is my cpu consumption output when I load 3 of my cores.

Solution 5

Start two

sha1sum /dev/zero &

commands for every core in your system.

To stop

killall sha1sum

or

kill sha1sum

View more solutions

Comments

[Nate Koppenhaver]

is it possible to run multiple instances of cat simultaneously?

[Oliver Salzburg]

@NateKoppenhaver: Yes, that seems to be possible when wrapping them in screen sessions. But I would prefer a more sophisticated solution if possible.

[吴环宇]

Heh, I always used cat /dev/random > /dev/null. Guess /dev/zero works too. :-)

[Luc]

@oKtosiTe What Rich Homolka said is right, but it's not just that it's a bad thing to do, it's also useless because it'll block almost immediately and stop consuming cpu.

[Luc]

@Stefan Interesting, I didn't know that tool existed. Still, it isn't installed by default on any Debian (based) distribution that I know of.

[Wildcard]

I've been using matho-primes all >/dev/null & but it's undoubtedly not the BEST answer for everyone.

[jftuga]

What program do you used to display CPU usage like this? It reminds me of top, but I don't recall the CPU 'charts'.

[BoppreH]

@jftuga probably htop, top's prettier brother.

[jlliagre]

This would be simpler: while : ; do : ; done

[吴环宇]

Wouldn't that make terminating the processes a bit of a hassle?

[ott--]

@jlliagre Yours wön't go above loadavg 1.

[jlliagre]

Your loop isn't primarily loading the CPU but more filling the memory. It will eventually crash with an out of memory error.

[ott--]

@jlliagre Mine fills memory and swap (if present), thus producing a load of 3 before it killed because it runs out of memory.

[jlliagre]

That's the problem. You aren't answering the question asked which is how to produce a high CPU load on a server. Your script is quickly making a system unresponsive and then crashes. There are much more reliable ways to get a loadavg of 3. eg: for i in 1 2 3; do while : ; do : ; done & ; done

[Christian Mann]

killall cat should do it.

[吴环宇]

Depending on whether you have other cat processes running (I usually do).

[Pushpak Dagade]

yes its htop. Best real time, colorful interactive process viewer for linux - htop.sourceforge.net

[mmdemirbas]

Thanks, but & causes a command to run in a separate thread or separate core? I am confused.

[jlliagre]

@mmdemirbas: The ampersand causes the command to run as a separate process. The scheduler is then dispatching all the active processes to all available cores .

[Derrick]

Wasn't paying attention and ran this on a Windows box. Very bad things...

[ben]

on ubuntu, you can install with sudo apt-get install stress

[enapupe]

on debian wheezy too.

[Pedro Melo]

On Fedora, sudo yum install stress

[das_j]

Arch: sudo pacman -S stress

[Redi]

As a reminder you can stop this test by issuing killall bash - just make sure you don't have any other important script running at the time.

[jlliagre]

@acoder Thanks for suggesting a way to end the loop. I would however avoid killall bash. Answer edited to add a safer method to end the load generation.

[Christian Long]

brew install stress on OS X

[DavidPostill]

Please read How do I recommend software for some tips as to how you should go about recommending software. At the very least you should provide more than just/at least a link, for example some additional information about the software itself, and how it can be used to solve the problem in the question.

[phil294]

or simply end them with fg ctrl+c, 4 times.

[Akira Yamamoto]

+1 for the lc shell function

[markdsievers]

From source: wget http://people.seas.harvard.edu/~apw/stress/stress-1.0.4.tar.‌​gz && tar -xvf stress-1.0.4.tar.gz && cd stress-1.0.4 && ./configure && make && sudo make install See README.

[Shadi]

On Amazon Linux, sudo amazon-linux-extras install epel -y && sudo yum install stress -y (ref)

[Marinaio]

I used this in a script for testing!

[Shadi]

The link to the stress package is dead

[Amber.G]

Thanks for the response. I tried 'for i in 1 2 3 4; do while : ; do : ; done & done'. My question is how to verify/check the CPU load accordingly? I used 'top', however, the % load per each thread is not 100%...Please advise.

[jlliagre]

@Amber.G You can run htop as the OP or mpstat to see per CPU load.

[oneworld]

This one worked on Amazon linux EC2 --- sudo yum install dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.r‌​pm and then sudo yum install stress -y

[Jack Kinsella]

Succinctly, if no other BG jobs are running - kill $(jobs -p)

[Pavan Kumar]

My choice is the other one-line solution below which does not require any installation: superuser.com/a/443633/451797

[Gatonito]

Like this one. Short & sweet and didn't need to download anything.