Thanks to the Internet of Things (IoT), fog computing, and cloud computing, the number of IoT-based applications are growing more and more. An IoT-based application can decompose into a number of services where each of them has some specific characteristics such as a predefined deadline, computing resource requirements, and size. Since fog computing devices in a fog-cloud environment have different delay and power consumption profiles, the placement of IoT services in such an environment is a challenging task. Motivated by this, in this research, we propose an efficient policy, called MinRE, for service placement problem in fog-cloud computing systems. To provide both Quality of Service (QoS) for IoT services and energy efficiency for Fog Service Providers (FSPs), we classify applications into two categories: critical applications and normal ones. For critical applications, we propose MinRes, which aims to minimize response time, and for normal applications, we propose MinEng, whose goal is reducing the energy consumption of fog environment. We first run MinRes to host critical applications on fog nodes as far as possible. We conduct extensive simulation experiments to evaluate the performance of the proposed policy. The results show that our policy outperforms others in terms of percentage of deadline satisfied services and energy efficiency.
