Limit CPU time of process group

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Is there a way to limit the absolute CPU time (in CPU seconds) spend in a process group?

ulimit -t 10; ./my-process looks like a good option but if my-process forks then each process in the process group gets its own limit. The whole process group can use an arbitrary amount of time by forking every 9 seconds.

The accepted answer on a similar question is to use cgroups but doesn't explain how. However, there are other answers (Limit total CPU usage with cgroups) saying that this is not possible in cgroups and only relative cpu usage can be limited (for example, 0.2 seconds out of every 1 second).

Liran Funaro suggested using a long period for cpu.cfs_period_us (https://stackoverflow.com/a/43660834/892961) but the parameter for the quota can be at most 1 second. So even with a long period I don't see how to set a CPU time limit of 10 seconds or an hour.

If ulimit and cgroups cannot do this, is there another way?

cgroups ulimit limit time linux
3 Answers

This not directly answer the question but refers to the discussion on the actual need of the OP.

If your competition ignores everything except CPU time, it may be fundamentally flawed. One can simply, for example, cache results in the primary storage device. Since you do not count storage access time, it may have the least CPU cycles, but the worse actual performance. A perfect crime would be to simply send the data via the Internet to another computer, which calculate the task then return the answer. This would finish the task with what appear to be zero cycles. You actually want to measure "real" time and give this process the highest priority in your system (or actually running it secludedly).

When checking students' homework, we simply used an unrealistic time limit (e.g., 5 minutes for what should be a 10 seconds program), then killing the process if it has not finished in time and failing this submission.

If you want to pick a winner, then simply re-run the best competitors multiple times to ensure the validity of their results.

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