With the technology scaling, multicore and many-core processors introduced themselves as an alternative to offset performance demands. Power and thermal constraints make remarkable area of chip underutilized, which is called the dark silicon in the literature. In this paper, we model a multicore processor based on Amdahl's Law and meanwhile consider both reliability issues and memory overheads. Then based on the accurate empirical results, we suggest effective voltage and frequency scaling under power constraints and different amount of dark silicon. Since power-efficient small cores result in more active cores in the multicore architecture, we attempt to improve the total performance by introducing more power-efficient multicore architectures along with decreasing the dark silicon percentage. According to the results, voltage scaling of the processor has negligible effect on performance of memory-intensive applications. However, performance of CPU-intensive applications is sensitive to voltage scaling, where for parallel and serial applications performance is improved and diminished, respectively. Energy and performance per watt of all applications are improved by voltage scaling.