def processes_with_env_psutil(env_name, env_value):
    """
    Find PIDs of processes having environment variable matching given one.

    Internally it uses `psutil` library.

    :param str env_name: name of environment variable to be found
    :param str env_value: environment variable value prefix
    :return: process identifiers (PIDs) of processes that have certain
             environment variable equal certain value
    :rtype: set
    """
    pids = set()

    for proc in psutil.process_iter():
        try:
            pinfo = proc.as_dict(attrs=['pid', 'environ'])
        except (psutil.NoSuchProcess, IOError):
            # can't do much if psutil is not able to get this process details
            pass
        else:
            penv = pinfo.get('environ')
            if penv and env_value in penv.get(env_name, ''):
                pids.add(pinfo['pid'])

    return pids