Understanding CPU Usage on Cumulus Linux


My Cumulus Linux switch is sitting idle with just a few ports active, no routing config, no traffic. When looking at the output of top, switchd is using between 15-25% of the CPU. Should I be concerned about routing or switching performance when my network gets loaded with traffic?


Before we start to understand CPU usage, first let’s clarify what we can be talking about when describing routing and switching performance.

Performance is a word used for many different types of measurements, including:

Max Traffic Rate (data plane)

  • Maximum traffic rate (Mbps or Gbps) in and out between two ports on a switch or router.
  • Maximum aggregate traffic rate in and out of all ports on a switch or router simultaneously.

Routing and Switching Table Size (control plane)

  • Maximum number of L3 subnet prefixes that can be stored in the routing tables and L3 hardware switching tables.
  • Maximum number of L2 MAC entries that can be stored in the switch hardware tables.

Stability under Stress (control and data plane)

  • How the switch/router performs and how fast it converges back to a steady state when a large amount of steady state traffic is flowing (data plane) and routing instability is introduced (control plane).

It is important to understand the separation of the control plane from the data plane. The data plane is the place where packets are switched through the box. In all modern routers/switches, this occurs in hardware by the switching ASICs. Nearly all modern switches can switch packets at essentially line rate, and most can switch at maximum rate through all the ports simultaneously.

So, when we look at CPU statistics, we are looking at how the switch is handling control plane activities, not data plane switching.

For the control plane, the primary concerns, as mentioned above, are routing and switching table sizes and stability under stress.

The routing and switching table sizes are limited by the amount of memory in the switching ASICs. For Cumulus Linux boxes, these numbers are shown in the Cumulus Networks hardware guides, which you can find on the hardware compatibility list.

Stability under stress: The amount of network churn that a box is capable of handling well depends much more on your particular network design than anything else. A solid network design and choice of routing protocols to match your requirements reduces the amount of protocol processing required during a network event, and will increase the stability of your network. All modern switches of the same throughput category use CPUs of essentially the same performance category. Also, Cumulus Linux uses control plane policing and QoS-based buffering to protect the CPU and help ensure that critical control plane traffic is processed during times of network instability.

What Is switchd Doing?

The primary task of switchd is to watch for L2 and L3 routing changes, update the routing/switching tables if needed, and then update the switching ASICs of any changes. It also updates switching counters from the data stored on the ASICs and does error checking. So it spends idle time doing housekeeping activities.

Linux CPU Usage Basics

CPU measurement is an imprecise science, especially at low CPU usage. Basically it is a measurement of work that is done divided over a time interval and then converted to a percentage. There are several pages on the Web describing how this calculation is done.

The Linux OS round-robins through all the active processes. When each process wakes up, it looks to see if it has work to do. If it does, it essentially has the CPU for a period of time to process all that work. If it has a lot of work to do, the CPU will run essentially 100% until the work is complete. If a process shows up in top as 20% usage, that means it ran at about 100% for 1/5 of the time measurement interval. Some of that work is attributable to overhead associated with the wake-up/sleep processing. switchd has to wake up fairly regularly and run through its maintenance.

15-25% CPU is the basic overhead of switchd when “nothing” is happening. If work comes along, then CPU usage will increase for a short amount of time to process that work. The main thing you need to remember is that switchd can scale to a large amount of changes. A 100% load is much more than 5 times the idle state of 20% load.

Multiple CPUs

If you are looking at top to watch the CPU, you generally are given statistics per CPU, so theoretically you only need to be concerned when the total activity starts approaching 200% (or 2.0 in the load average at the top). A switchd usage of 20% is the measurement of the percentage used on only one CPU, not an average of the two.

If you press the 1 key while watching top it will toggle the top display to show both CPUs on the output. In an idle router, you can see that only one CPU is getting any appreciable activity.

What to Watch for

If the CPU jumps up occasionally to some amount over 50%, but returns back to normal within a few seconds, there is nothing to be concerned about. This just indicates that some work came along and the process took time to handle that work.

However, if the CPU runs hot for a while (steady state), that would indicate that something is changing or unstable in the network, or there is some other cause of a large amount of control plane traffic. At that point, look for those processes that are receiving the most activity and try to determine what network events are creating that activity and fix the root cause, or adjust your network configuration accordingly.

Cumulus Linux uses systemd to monitor system health, including high CPU. When CPU usage reaches critical levels for a period of time, systemd will alert you of this condition.