def peak_memory_mb() -> float:
    """
    Get peak memory usage for this process, as measured by
    max-resident-set size:

    https://unix.stackexchange.com/questions/30940/getrusage-system-call-what-is-maximum-resident-set-size

    Only works on OSX and Linux, returns 0.0 otherwise.
    """
    if resource is None or sys.platform not in ('linux', 'darwin'):
        return 0.0

    # TODO(joelgrus): For whatever, our pinned version 0.521 of mypy does not like
    # next line, but later versions (e.g. 0.530) are fine with it. Once we get that
    # figured out, remove the type: ignore.
    peak = resource.getrusage(resource.RUSAGE_SELF).ru_maxrss  # type: ignore

    if sys.platform == 'darwin':
        # On OSX the result is in bytes.
        return peak / 1_000_000

    else:
        # On Linux the result is in kilobytes.
        return peak / 1_000