Green Data Centers: A Survey, Perspectives, and Future Directions <s> D. Challenges and Research Issues <s> Previous studies have demonstrated the advantages of single-ISA heterogeneous multi-core architectures for power and performance. However, none of those studies examined how to design such a processor; instead, they started with an assumed combination of pre-existing cores. This work assumes the flexibility to design a multi-core architecture from the ground up and seeks to address the following question: what should be the characteristics of the cores for a heterogeneous multi-processor for the highest area or power efficiency? The study is done for varying degrees of thread-level parallelism and for different area and power budgets. The most efficient chip multiprocessors are shown to be heterogeneous, with each core customized to a different subset of application characteristics — no single core is necessarily well suited to all applications. The performance ordering of cores on such processors is different for different applications; there is only a partial ordering among cores in terms of resources and complexity. This methodology produces performance gains as high as 40%. The performance improvements come with the added cost of customization. <s> BIB001 </s> Green Data Centers: A Survey, Perspectives, and Future Directions <s> D. Challenges and Research Issues <s> This keynote paper: presents a 21st century vision of computing; identifies various computing paradigms promising to deliver the vision of computing utilities; defines Cloud computing and provides the architecture for creating market-oriented Clouds by leveraging technologies such as VMs; provides thoughts on market-based resource management strategies that encompass both customer-driven service management and computational risk management to sustain SLA-oriented resource allocation; presents some representative Cloud platforms especially those developed in industries along with our current work towards realising market-oriented resource allocation of Clouds by leveraging the 3rd generation Aneka enterprise Grid technology; reveals our early thoughts on interconnecting Clouds for dynamically creating an atmospheric computing environment along with pointers to future community research; and concludes with the need for convergence of competing IT paradigms for delivering our 21st century vision. <s> BIB002 </s> Green Data Centers: A Survey, Perspectives, and Future Directions <s> D. Challenges and Research Issues <s> Although most current multicore processors are homogeneous, microarchitects are now proposing heterogeneous core implementations, including systems in which heterogeneity is introduced at runtime. This article shows that operating system schedulers must consider dynamic heterogeneity or suffer significant power-efficiency and performance losses. <s> BIB003 </s> Green Data Centers: A Survey, Perspectives, and Future Directions <s> D. Challenges and Research Issues <s> Temperature-induced reliability issues are among the major challenges for multicore architectures. Thermal hot spots and thermal cycles combine to degrade reliability. This research presents new reliability-aware job scheduling and power management approaches for chip multiprocessors. Accurate evaluation of these policies requires a novel simulation framework that can capture architecture-level effects over tens of seconds or longer, while also capturing thermal interactions among cores resulting from dynamic scheduling policies. Using this framework and a set of new thermal management policies, this work shows that techniques that offer similar performance, energy, and even peak temperature can differ significantly in their effects on the expected processor lifetime. <s> BIB004 </s> Green Data Centers: A Survey, Perspectives, and Future Directions <s> D. Challenges and Research Issues <s> We consider preemptive online scheduling algorithms to minimize the total weighted/unweighted flow time plus energy for speed-scalable heterogeneous multiprocessors. We show that the well-known priority scheduling algorithms Highest Density First, Weighted Shortest Elapsed Time First, and Weighted Late Arrival Processor Sharing, are not O(1)-speed O(1)-competitive for the objective of weighted flow even in the special case of fixed variable speed processors (aka the related machines setting). This illustrates that scheduling heterogeneous multiprocessors is a different, and algorithmically more challenging problem, than scheduling homogeneous multiprocessors. ::: ::: We then show that a variation of the non-clairvoyant algorithm Late Arrival Processor Sharing coupled with a non-obvious speed scaling algorithm is scalable for the objective of unweighted flow plus energy on speed-scalable multiprocessors. This is the first provably scalable non-clairvoyant algorithm on heterogeneous multi-processors, even in the related machines setting, for the objective of total (unweighted) flow time. <s> BIB005 </s> Green Data Centers: A Survey, Perspectives, and Future Directions <s> D. Challenges and Research Issues <s> Certain data-communication protocols hog the spotlight, but all of them have a lot in common. Computer Networking: A Top-Down Approach Featuring the Internet explains the engineering problems that are inherent in communicating digital information from point to point. The top-down approach mentioned in the subtitle means that the book starts at the top of the protocol stack--at the application layer--and works its way down through the other layers, until it reaches bare wire. The authors, for the most part, shun the well-known seven-layer Open Systems Interconnection (OSI) protocol stack in favor of their own five-layer (application, transport, network, link, and physical) model. It's an effective approach that helps clear away some of the hand waving traditionally associated with the more obtuse layers in the OSI model. The approach is definitely theoretical--don't look here for instructions on configuring Windows 2000 or a Cisco router--but it's relevant to reality, and should help anyone who needs to understand networking as a programmer, system architect, or even administration guru.The treatment of the network layer, at which routing takes place, is typical of the overall style. In discussing routing, authors James Kurose and Keith Ross explain (by way of lots of clear, definition-packed text) what routing protocols need to do: find the best route to a destination. Then they present the mathematics that determine the best path, show some code that implements those algorithms, and illustrate the logic by using excellent conceptual diagrams. Real-life implementations of the algorithms--including Internet Protocol (both IPv4 and IPv6) and several popular IP routing protocols--help you to make the transition from pure theory to networking technologies. --David WallTopics covered: The theory behind data networks, with thorough discussion of the problems that are posed at each level (the application layer gets plenty of attention). For each layer, there's academic coverage of networking problems and solutions, followed by discussion of real technologies. Special sections deal with network security and transmission of digital multimedia. <s> BIB006
The studies discussed above demonstrate that energyefficient mechanisms can assist in making data centers green and reducing energy usage. Furthermore, they reveal that there is a trade-off between performance and energy consumption. The aforementioned efforts also have demonstrated that a considerable amount of the energy consumed as a result of overprovisioning can be saved while still satisfying performance constraints. However, there are still several other issues to be addressed concerning the design of energy-efficient algorithms: • Most studies that have been conducted on energy efficiency thus far have focused on ideal power models. However, future investigations should consider scenarios that more closely resemble real systems. For example, some overhead is incurred when switching speeds because the processor must stop while the voltage is changing, and frequent changes in speed can also harm a processor's lifetime reliability BIB004 . An interesting question is how to incorporate these aspects of real systems into performance studies. • In classical parallel scheduling problems, the number of servers is fixed, and the scheduler primarily decides which task to process on which processor and determines the speed of each processor at a given time. However, data centers operate on a different model in which servers can be activated and released on demand BIB002 and in which it is necessary to exploit an appropriate number of servers to process the submitted tasks. Thus, the characteristics of data centers pose new challenges and open problems, of which the key underlying requirements are scalability, uncertainty and efficiency. • As mentioned earlier, the latency time and power consumption incurred in the process of rebooting means that the effects of turning servers or switches off and on cannot be overlooked. Moreover, these effects are more serious at the network level because communication data can be lost after a certain threshold latency time BIB006 . Fortunately, data center network architectures typically provide multiple paths for communication among their servers. The question of how to efficiently manage energy consumption in data center networks is still a major concern that is worthy of investigation. The well-structured topology of data center networks can be investigated to address this issue. A good mechanism design should minimize the effects on the network throughput, latency, congestion and packet loss while incorporating the structure of the data center network architecture. • Current results for multiple processors (servers) mostly focus on environments that consist of homogeneous collections of identical processors (servers). However, many computer architects believe that architectures based on heterogeneous processors will dominate future architectural design BIB001 , BIB003 . The primary advantage of heterogeneous architectures is that the design can include certain processors that are specialized for particular types of tasks, with the intent of assigning tasks to those processors that are best suited for them BIB005 . It is also natural to consider the possibility of heterogeneity in data centers, as the various servers contained in data centers often differ from each other in their power-performance relationships. At this time, few results for heterogeneous environments are extant in energy-efficient algorithm design. We believe that addressing the above challenges and research issues will lead to significant advances in the current level of energy efficiency of data centers.