How do I set the `brms` arguments "threads" and "cores" correctly?

Interfaces brms
2

My understanding is that the argument cores refers to the maximum number of MCMC chains to run in parallel, and the argument threads refers to the number of threads to use within an MCMC chain. So the actual number of CPU cores used will be cores * threads.

In your case, you should set cores = 4 and threads = threading(4).

You can verify this yourself by looking at parts of the brms source code to see how the brms arguments are passed to the cmdstanr backend. Here’s a relevant snippet of code:

    c(args) <- nlist(
      iter_sampling = iter - warmup,
      iter_warmup = warmup,
      chains, thin,
      parallel_chains = cores,
      show_messages = silent < 2,
      show_exceptions = silent == 0,
      fixed_param = algorithm == "fixed_param"
    )
    if (use_threading) {
      args$threads_per_chain <- threads$threads
    }

So the brms arguments chains, cores, and threads correspond to chains, parallel_chains, and threads_per_chain respectively in the sample method of cmdstanr. You can then check the cmdstanr documentation to verify my explanation above.

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