Due to its high CPU usage i want to limit Chromium web browser by cpulimit and use terminal to run:
cpulimit -l 30 -- chromium --incognito
but it does not limit CPU usage as expected (i.e. to maximum to 30%). It again uses 100%. Why? What am I doing wrong?
Yeah, chromium doesn't care much when you stop one of its threads.
cpulimit is, in 2021, really not the kind of tool that you want to use, especially not with interactive software: it "throttles" processes (or unsuccessfully tries to, in your case) by stopping and resuming them via signals. Um. That's a terrible hack, and it leads to unreliability you really don't want in a modern browser that might well be processing audio and video, or trying to scroll smoothly.
Good news is that you really don't need it. Linux has cgroups, and these can be used to limit the resource consumption of any process, or group of processes (if you, for example, don't want chromium, skype and zoom together to consume more than 50% of your overall CPU capacity). They can also be used to limit other things, like storage access speed and network transfer.
In the case of your browser, that'd boil down to (top of head, not tested):
# you might need to create the right mountpoints first
sudo mkdir /sys/fs/cgroup/cpu
sudo mount -t cgroup -o cpu cpu /sys/fs/cgroup/cpu
# Create a group that controls `cpu` allotment, called `/browser`
sudo cgcreate -g cpu:/browser
# Create a group that controls `cpu` allotment, called `/important`
sudo cgcreate -g cpu:/important
# allocate few shares to your `browser` group, and many shares of the CPU time to the `important` group.
sudo cgset -r cpu.shares=128 browser
sudo cgset -r cpu.shares=1024 important
cgexec -g cpu:browser chromium --incognitio
cgexec -g cpu:important make -j10 #or whatever
The trick is usually giving your interactive session (e.g. gnome-session) a high share, and other things a lower one.
Note that this guarantees shares; it doesn't take away, unless necessary. I.e. if your CPU can't do anything else in that time (because nothing else is running, or because everything with more shares is blocked, for example by waiting for hard drives), it will still be allocated to the browser process. But that's usually what you want: It has no downsides (it doesn't make the rest of the system run any slower, the browser is just quicker "done" with what it has to do, which on the upside probably even saves energy on the average: when multiple CPU cores are just done, then things can be clocked down/suspended automatically).
I think it might be useful to focus on finding why it uses this much CPU, using the Taskmanager built-in to chromium to find which tab is using it, or a profiler like perf and flame graphs.
But if you really want to slow down the browser you should consider the modern built-in solution cgroups, see for example: https://forums.gentoo.org/viewtopic-t-1010870-start-0.html
Alternatively consider using (re)nice to lower the priority of chromium, using AdBlockers or upgrading your hardware.
Why cpulimit is not working as expected might be due to multiple threads, child processes not stopping when sigstop stops the parent proces but I'm not sure.
Update: this https://github.com/opsengine/cpulimit/issues/39 and this: https://stackoverflow.com/questions/31623697/limit-the-percentage-of-cpu-a-process-tree-is-allowed-to-use confirm my suspicion that you might not be limiting all child processes. Newer versions of cpulimit provide the option: --include-children or: -i for that.
