Why the CPU utilization of SRX branch devices is high even when there is no traffic?

The Branch solution devices in the SRX family show high CPU utilization even when there is no traffic on the device.
The information below explains how CPU utilization is shown in J-Web and CLI, and why CPU utilization is shown as high, with examples.

Note: This article applies to Junos OS 10.0 and earlier. After Junos OS 10.0, there is a separate display for the control core and data core on the J-Web dashboard.

On the J-Web Dashboard tab, CPU utilization shows over 90% like below even there are no sessions and no traffics.

Why the CPU utilization of SRX branch devices is high even when there is no traffic?

The “top” command on the shell or “show system processes extensive no-forwarding” commands on CLI report that CPU utilization is over 100% like below.

1. Why is the CPU utilization of SRX branch devices shown high on J-Web?

  • J-Web displays the instantaneous CPU utilization of the complete system, which includes control core and the data core.
  • It shows always high because the data core utilization is nearly 100%. If so, refer to the last bullet in this step.
  • To check control core, enter the CLI command “show chassis routing-engine”:
  • To check each data core, enter the command “top –H” in the shell.

2. Why is the utilization of data core always high even there are is no traffic and sessions?

  • The data core runs in a tight loop always to process as soon as a packet arrives instead of using interrupts to process packets whenever it arrives.

3. Why is the “top” and “show system processes extensive no-forwarding” command reporting that CPU utilization is over 100%?

  • The forwarding daemon runs nearly 100% in a tight loop in data cores (1..n) and the JUNOS accounts time spent in both cores against forwarding daemon.
  • To check per core utilization, use “top –H” instead of using “top”.

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