Limiting CPU usage can be important to prevent your server from crashing.

This is especially useful for any custom scripts you might be running in a crontab.

First, we will spin up a Ubuntu 12.10 x64 droplet:

Install cpulimit

apt-get -y install cpulimit

Usage Syntax

Usage: cpulimit TARGET [OPTIONS...]
   TARGET must be exactly one of these:
      -p, --pid=N        pid of the process
      -e, --exe=FILE     name of the executable program file
      -P, --path=PATH    absolute path name of the executable program file
   OPTIONS
      -b  --background   run in background
      -l, --limit=N      percentage of cpu allowed from 0 to 100 (mandatory)
      -v, --verbose      show control statistics
      -z, --lazy         exit if there is no suitable target process, or if it dies
      -h, --help         display this help and exit

Benchmark CPU Usage

Lets benchmark our CPU usage without cpulimit.

Here is an example of how to utilize your CPU with an application:

md5sum /dev/zero &

This 'forks' the md5sum process into background. You can now see CPU usage with top:

As you can see it is consuming almost 100% of CPU resources (given that we have one CPU core on this droplet).

We can bring up this process to foreground using fg and cancel it with CTRL+C:

Limit CPU usage with cpulimit

Now we can test cpulimit to see if it actually does what it is supposed to.

Lets limit our CPU usage to 40% and run same command:

cpulimit -l 40 md5sum /dev/zero &

Sure enough, it is limited to 40%:

Multi-core Droplets

On droplets with multiple processors, you would need to limit CPU usage on each process.

Here is a script that forks 4 processes without any restrictions and lets them run concurrently on your server:

for j in `seq 1 4`; do md5sum /dev/zero & done

Each CPU core is using almost 100% of resources:

top - 23:29:28 up 7 days, 13:54,  1 user,  load average: 0.80, 1.08, 0.53
Tasks:  77 total,   5 running,  72 sleeping,   0 stopped,   0 zombie
%Cpu0  :100.0 us,  0.0 sy,  0.0 ni,  0.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu1  : 93.2 us,  6.8 sy,  0.0 ni,  0.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu2  : 95.0 us,  5.0 sy,  0.0 ni,  0.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu3  : 98.3 us,  1.7 sy,  0.0 ni,  0.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:   8178228 total,   380196 used,  7798032 free,    28136 buffers
KiB Swap:        0 total,        0 used,        0 free,   251708 cached

  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND                                                                                                                                                                        
 8400 root      20   0  7172  612  520 R 101.7  0.0   0:03.10 md5sum                                                                                                                                                                         
 8401 root      20   0  7172  612  520 R 101.7  0.0   0:03.10 md5sum                                                                                                                                                                         
 8399 root      20   0  7172  616  520 R  98.4  0.0   0:03.06 md5sum                                                                                                                                                                         
 8402 root      20   0  7172  612  520 R  98.4  0.0   0:03.09 md5sum                                                                                                                     

To use cpulimit for each process, place it in front of the command:

for j in `seq 1 4`; do cpulimit -l 40 md5sum /dev/zero & done

Now each process uses at most 40% of each thread, and does not overload the server:

top - 23:31:03 up 7 days, 13:55,  1 user,  load average: 2.68, 1.72, 0.82
Tasks:  81 total,   5 running,  76 sleeping,   0 stopped,   0 zombie
%Cpu0  : 39.4 us,  0.7 sy,  0.0 ni, 59.6 id,  0.0 wa,  0.0 hi,  0.0 si,  0.3 st
%Cpu1  : 38.7 us,  1.7 sy,  0.0 ni, 59.6 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu2  : 39.4 us,  1.3 sy,  0.0 ni, 59.3 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu3  : 39.4 us,  1.7 sy,  0.0 ni, 58.9 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:   8178228 total,   380452 used,  7797776 free,    28144 buffers
KiB Swap:        0 total,        0 used,        0 free,   251708 cached

  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND                                                                                                                                                                        
 8442 root      20   0  7172  616  520 R  40.4  0.0   0:06.10 md5sum                                                                                                                                                                         
 8440 root      20   0  7172  612  520 R  40.0  0.0   0:06.09 md5sum                                                                                                                                                                         
 8435 root      20   0  7172  616  520 R  39.7  0.0   0:06.09 md5sum                                                                                                                                                                         
 8436 root      20   0  7172  612  520 R  39.7  0.0   0:06.10 md5sum                                                                                          

And you are all done!

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