Bug 474828

Created attachment 161840 [details]
screenshot shows zigzag profile of the network traffic

SUMMARY
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The system monitor shows a zigzag (sawtooth) profile when downloading at a constant rate.

Note, this only seems to happen on systems where the underlying kernel network driver doesn't allow adjusting the refresh frequency of the device transmission statistics.
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STEPS TO REPRODUCE
1. download at a constant rate, e.g. wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.5.5.tar.xz
2. open the KDE system monitor and observe the network diagram


OBSERVED RESULT
zigzag profile, where the max is twice the download rate

EXPECTED RESULT
constant profile

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: openSUSE 15.4 and 15.5
KDE Plasma Version: 
KDE Frameworks Version: 5.102.0
Qt Version: 5.15.8

ADDITIONAL INFORMATION

The underlying problem seems to be with ksystemstats5 which calculates the download/upload rates using a hard-coded interval of 0.5s:

https://github.com/KDE/ksystemstats/blob/master/plugins/network/NetworkManagerBackend.cpp#L20
                // Update rate in ms
                static const int UpdateRate = 500;

However, not all kernel network device driver support updating the refresh frequency of the received/sent byte statistics:
        # ethtool -c eth0 | grep stats-block-usecs
        stats-block-usecs: 0

        # ethtool -C eth0 stats-block-usecs 500000
        netlink error: Invalid argument

        # lspci | grep Ethernet
        04:00.0 Ethernet controller: Broadcom Inc. and subsidiaries NetLink BCM57780 Gigabit Ethernet PCIe (rev 01)

The update-frequency of the statistics of my eth0 device seems to be about 1s (checked via cat /proc/net/dev), which explains the zigzag profile (first 0.5s no change in received bytes --> 0 Mb/s, second 0.5s all bytes received in the whole second --> twice as much Mb/s).