CBC Multithreading Time Limit

[ebongo1]

I have been reading through a few online discussions on GitHub and other sources (https://groups.google.com/g/pulp-or-discuss/c/IKE--kT6c5U/m/Y0OUJCqoBQAJ, https://github.com/coin-or/pulp/issues/463) about using multiple threads with CBC, but I haven’t been able to find a direct answer to my question. So I wanted to reach out to you directly about it. Does the time limit correlate to the number of threads passed in to the solver? So for instance when I set “threads=4” and “time limit=3600”, does that mean each thread gets a time limit of 3600/4 seconds? I.e. does CBC use CPU time or wall time for the time limit?

[pchtsp]

As you suspect, CBC uses CPU time for the time limit. So, it will indeed use more or less 3600/4 seconds for each thread (more or less because for the root relaxation it uses only one thread).

Maybe @tkralphs can complement the answer here. In fact, this question could be also moved / copied to the cbc github here: https://github.com/coin-or/Cbc/

[tkralphs]

This could be a question better asked in the Cbc forum except for the fact that this is something that can be controlled by parameter. So if the default of cpu is not what you want (and it probably isn't what you want), you can change it within PuLP when you call Cbc so there is more uniformity across solvers. Just add -timeMode elapsed to the Cbc command-line. I'm not too sure if users can do this themselves or not.

[pchtsp]

ah! Good to know that, thanks @tkralphs. I think we will probably set this configuration by default in CBC from PuLP, so it matches the defaults for GUROBI and CPLEX. In the meantime and as a workaround, it is indeed possible to pass arbitrary parameters to CBC from PuLP so this should work right now:

import pulp
solver = pulp.PULP_CBC_CMD(timeLimit=3600, threads=4, options=["timeMode elapsed"])

(I did a quick test in my current model and it apparently passed the parameter correctly to CBC).

[ebongo1]

Oh ok that is very helpful to know! I still will move my original question over to the CBC github, but thank you both!

[pchtsp]

Thanks to @tkralphs comment, I've added the timeMode="elapsed" by default for CBC in PuLP. So now, if you re-install pulp (v2.5.1) timeLimit is measured in walltime by default:

import pulp
# this counts wall-time and stops after 3600 seconds.
solver = pulp.PULP_CBC_CMD(timeLimit=3600, threads=4)

You can change that to cpu time by doing the following:

import pulp
# this sums the time in each cpu, so stops roughly after 3600/4 seconds, as before the change
solver = pulp.PULP_CBC_CMD(timeLimit=3600, threads=4, timeMode='cpu')