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Asus router one CPU core 100% usage?

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Many Asus AC routers, including RT-AC68U, RT-AC86U, AC88U, etc. have an issue where one of the CPU cores seems stuck at 100% utilization, especially during transfers. The high CPU usage issue is present with many firmware versions, including the popular Merlin firmware spin off. Usually only one of the CPU cores seems stuck at 100%, but the router is still struggling.

To check the CPU usage - in the router's web admin interface, under "Network Map", click on the "Status" tab in the right-hand menu bar under "System Status." To fix the issue, follow the steps below:

Turn off Wireless "Auto channel selection" - this is by far the most common cause. The router constantly monitors channels to try and find less interference and causes unnecessary high CPU load. Login to your router's admin interface, navigate to "Wireless > 2.4GHz > Control Channel" and change it from "Auto" to an unused channel. Repeat for the 5GHz frequency band.

Turn off QoS - if you still see high CPU usage, under "Adaptive QoS > QoS", choose "Bandwidth Limiter", or turn off QoS.

USB flash drive issues - sometimes the USB flash drive used for shared storage may have issues, try unplugging it to see if that changes your CPU usage. Reformatting it and using the USB 2.0 port instead of USB 3.0 may help.

If you are still having issues, or you'd simply like to figure out exactly which process is causing the high load, login into the router with SSH. You will have to enable SSH in the "Administration > System" menu. Use the same admin username/password as with the web interface. Once you are logged in, try the "top" command, and see which process is using the CPU excessively.

Other possible solutions:
- upgrade to the latest firmware from Asus
- unplug router, wait a couple of seconds and plug back in
- reset router (hold down reset button for 10s while plugging in) - you will need to reconfigure everything, including wireless security, etc.

Wireless issues (most commonly auto channel switching) cause high CPU usage of this process: /usr/sbin/acsd
To find a good unused wireless channel for less interference, you can simply use an Android app like "Wifi Analyzer"
High RAM usage is not an issue under linux environments, as memory is being used for caching, this is quite normal.

See also: Asus router Traffic Analyzer - Statistic empty?

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