Use of CPUQuota in systemd

Question

I am trying to put a hard limit in CPU usage for a dd command . I have created the following unit file

[Unit] Description=Virtual Distributed Ethernet  [Service] ExecStart=/usr/bin/ddcommand CPUQuota=10%  [Install] WantedBy=multi-user.target 

which call the following simple script

#!/bin/sh dd if=/dev/zero of=/dev/null bs=1024k 

As I have seen in this guide: http://www.freedesktop.org/software/systemd/man/systemd.resource-control.html The CPU usage for my dd service should not exceed the 10%. But when I run the system-cgtop command the usage is about 70-75% .

Any ideas of what am I doing wrong and how can I fix it?

P.S. When I execute systemctl show dd I get the following results regarding CPU

CPUShares=18446744073709551615 StartupCPUShares=18446744073709551615 CPUQuotaPerSecUSec=100ms LimitCPU=18446744073709551615 

Answer 1

I accidentally ran across another valid answer given on https://unix.stackexchange.com/questions/213903/linux-cgroups-limit-cpu-usage-in-absolute-values-which-do-not-depend-on-cpu-spe

If you want a hard limit on CPU bandwidth, you can use cpu.cfs_quota_us and cpu.cfs_period_us. From the Kernel's CFS docs:

The bandwidth allowed for a group is specified using a quota and period. Within each given "period" (microseconds), a group is allowed to consume only up to "quota" microseconds of CPU time. When the CPU bandwidth consumption of a group exceeds this limit (for that period), the tasks belonging to its hierarchy will be throttled and are not allowed to run again until the next period.

Answer 2

Alternate ways to limit cpu usage: 1. Use taskset command. 2. Use control groups. 3. Use docker and limit cpu utilization by using cpuset. 4. Reduce the number of threads in the application. Some calculation is required with this approach.