https://bugs.kde.org/show_bug.cgi?id=474828
Bug ID: 474828
Summary: The system monitor shows a zigzag (sawtooth) profile
when downloading at a constant rate.
Classification: Applications
Product: plasma-systemmonitor
Version: unspecified
Platform: openSUSE
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: general
Assignee: ksysguard-b...@kde.org
Reporter: ttk...@gmail.com
CC: ahiems...@heimr.nl, plasma-b...@kde.org
Target Milestone: ---
Created attachment 161840
--> https://bugs.kde.org/attachment.cgi?id=161840&action=edit
screenshot shows zigzag profile of the network traffic
SUMMARY
***
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.
***
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).
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