Raspberry pi 4 4gb, raspbian buster, stalls every minute using external wifi dongle

Question

I am having a strange issue with my Pi 4. Every minute around second 47, it 'stalls' for a few seconds, like some process is going on. The CPU spikes and pings to the router are delayed. I am running Kodi and Snapserver/librespot.

Kodi playback is not interupted, the kodi remote does not respond anymore but recovers. But especially librespot does not like this and crashes due to this issue when it tries to play a new song during this period.

This of course suggests that there is a process running every minute, but I can't find it.

Any clue how to find the culprit?

Update: output of top command during hickup, sorted by CPU time:

Upon closer inspection, I also see the RTW_CMD_THREAD having a little load, where it normally is just doing almost nothing.

Something happening exactly every minute looks like a cron job. — Dmitry Grigoryev, May 26 '20 at 12:26
Yes, but there are no cron jobs. Besides, I'd expect the process to start running at 0 seconds in the minute, but it's not @DmitryGrigoryev — jberculo, May 26 '20 at 14:23
@jberculo I can see that top with 1.57 seconds is displayed above systemd with 9 seconds, so the sorting is not correct. — Dmitry Grigoryev, May 26 '20 at 14:42
I fixed the problem with switching off the power save mode of the wlan. Thanks for helping out! — jberculo, May 26 '20 at 16:16

score 3 · Accepted Answer · answered May 26 '20 at 16:15

After focussing on top (ht @coderMike/@DmitryGrigoryev), I noticed RTW_CMD_THREAD spiking at the same moment as the stall occuring. I then remembered that the power save mode of the RP-wifi could cause similar problems, but this is switched off by default nowadays. But as I am using a wifi dongle instead of the onboard wifi, i checked, and the power save mode was on. I used Make "iw wlan0 set power_save off" permanent to disable it, and the problem vanished!

Raspberry pi 4 4gb, raspbian buster, stalls every minute using external wifi dongle

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