This paper proposes and evaluates single-ISA heterogeneous multi-core architectures as a mechanism to reduce processor power dissipation. Our design incorporates heterogeneous cores representing different points in the power/performance design space; during an application 's execution, system software dynamically chooses the most appropriate core to meet specific performance and power requirements.

Power density in high-performance processors continues to increase with technology generations as scaling of current, clock speed, and device density outpaces the downscaling of supply voltage and thermal ability of packages to dissipate heat. Power density is characterized by localized chip hot spots that can reach critical temperatures and cause failure. Previous architectural approaches to power density have used global clock gating, fetch toggling, dynamic frequency scaling, or resource duplication to either prevent heating or relieve overheated resources in a superscalar processor. Previous approaches also evaluate design technologies where power density is not a major problem and most applications do not overheat the processor. Future processors, however, are likely to be chip multiprocessors (CMPs) with simultaneously-multithreaded (SMT) cores. SMT CMPs pose unique challenges and opportunities for power density. SMT and CMP increase throughput and thus on-chip heat, but also provide natural granularities for managing power-density. This paper is the first work to leverage SMT and CMP to address power density. We propose heat-and-run SMT thread assignment to increase processor-resource utilization before cooling becomes necessary by co-scheduling threads that use complementary resources. We propose heat-and-run CMP thread migration to migrate threads away from overheated cores and assign them to free SMT contexts on alternate cores, leveraging availability of SMT contexts on alternate CMP cores to maintain throughput while allowing overheated cores to cool. We show that our proposal has an average of 9 % and up to 34 % higher throughput than a previous superscalar technique running the same number of threads.

Chip-level power and thermal implications will continue to rule as one of the primary design constraints and performance limiters. The gap between average and peak power actually widens with increased levels of core integration. As such, if per-core control of power levels (modes) is possible, a global power manager should be able to dynamically set the modes suitably. This would be done in tune with the workload characteristics, in order to always maintain a chip-level power that is below the specified budget. Furthermore, this should be possible without significant degradation of chip-level throughput performance. We analyze and validate this concept in detail in this paper. We assume a per-core DVFS (dynamic voltage and frequency scaling) knob to be available to such a conceptual global power manager. We evaluate several different policies for global multi-core power management. In this analysis, we consider various different objectives such as prioritization and optimized throughput. Overall, our results show that in the context of a workload comprised of SPEC benchmark threads, our best architected policies can come within 1 % of the performance of an ideal oracle, while meeting a given chip-level power budget. Furthermore, we show that these global dynamic management policies perform significantly better than static management, even if static scheduling is given oracular knowledge. 1

Power density continues to increase exponentially with each new technology generation, posing a major challenge for thermal management in modern processors. Much past work has examined microarchitectural policies for reducing total chip power, but these techniques alone are insufficient if not aimed at mitigating individual hotspots. The industry’s current trend has been toward multicore architectures, which provide additional opportunities for dynamic thermal management. This paper explores various thermal management techniques that exploit the distributed nature of multicore processors. We classify these techniques in terms of core throttling policy, whether that policy is applied locally to a core or to the processor as a whole, and process migration policies. We use Turandot and a HotSpot-based thermal simulator to simulate a variety of workloads under thermal duress on a 4-core PowerPC TM processor. Using benchmarks from the SPEC 2000 suite we characterize workloads in terms of instruction throughput as well as their effective duty cycles. Among a variety of options we find that distributed controltheoretic DVFS alone improves throughput by 2.5X under our test conditions. Our final design involves a PI-based core thermal controller and an outer control loop to decide process migrations. This policy avoids all thermal emergencies and yields an average of 2.6X speedup over the baseline across all workloads. 1.

Within-die process variation causes individual cores in a Chip Multiprocessor (CMP) to differ substantially in both static power consumed and maximum frequency supported. In this environment, ignoring variation effects when scheduling applications or when managing power with Dynamic Voltage and Frequency Scaling (DVFS) is suboptimal. This paper proposes variation-aware algorithms for application scheduling and power management. One such power management algorithm, called LinOpt, uses linear programming to find the best voltage and frequency levels for each of the cores in the CMP — maximizing throughput at a given power budget. In a 20core CMP, the combination of variation-aware application scheduling and LinOpt increases the average throughput by 12–17 % and reduces the average ED 2 by 30–38 % — all relative to using variation-aware scheduling together with a simple extension to Intel’s Foxton power management algorithm.

Abstract—Performance and power are two primary design issues for systems ranging from server computers to handhelds. Performance is affected by both temperature and supply voltage because of the temperature and voltage dependence of circuit delay. Furthermore, as semiconductor technology scales down, leakage power’s exponential dependence on temperature and supply voltage becomes significant. Therefore, future design studies call for temperature and voltage aware performance and power modeling. In this paper, we study microarchitecture-level temperature and voltage aware performance and power modeling. We present a leakage power model with temperature and voltage scaling, and show that leakage and total energy vary by 38 % and 24%, respectively, between 65 C and 110 C. We study thermal runaway induced by the interdependence between temperature and leakage power, and demonstrate that without temperature-aware modeling, underestimation of leakage power may lead to the failure of thermal controls, and overestimation of leakage power may result in excessive performance penalties of up to 5.24%. All of these studies underscore the necessity of temperature-aware power modeling. Furthermore, we study optimal voltage scaling for best performance with dynamic power and thermal management under different packaging options. We show that dynamic power and thermal management allows designs to target at the common-case thermal scenario among benchmarks and improves performance by 6.59 % compared to designs targeted at the worst case thermal scenario without dynamic power and thermal management. Additionally, the optimal for the best performance may not be the largest allowed by the given packaging platform, and that advanced cooling techniques can improve throughput significantly. Index Terms—Floorplan, leakage power, microarchitecture, temperature, thermal management. I.

We propose to modify a conventional single-chip multicore so that a sequential program can migrate from one core to another automatically during execution. The goal of execution migration is to take advantage of the overall onchip cache capacity. We introduce the affinity algorithm, a method for distributing cache lines automatically on several caches. We show that on working-sets exhibiting a property called "splittability", it is possible to trade cache misses for migrations. Our experimental results indicate that the proposed method has a potential for improving the performance of certain sequential programs, without degrading significantly the performance of others.

Under current worst-case design practices, manufacturers specify conservative values for processor frequencies in order to guarantee correctness. To recover some of the lost performance and improve single-thread performance, this paper presents the Paceline leader-checker microarchitecture. In Paceline, a leader core runs the thread at higher-than-rated frequency, while passing execution hints and prefetches to a safely-clocked checker core in the same chip multiprocessor. The checker redundantly executes the thread faster than without the leader, while checking the results to guarantee correctness. Leader and checker cores periodically swap functionality. The result is that the thread improves performance substantially without significantly increasing the power density or the hardware design complexity of the chip. By overclocking the leader by 30%, we estimate that Paceline improves SPECint and SPECfp performance by a geometric mean of 21 % and 9%, respectively. Moreover, Paceline also provides tolerance to transient faults such as soft errors. 1

High performance multi-core processors are becoming an industry reality. Although multi-cores are suited for multithreaded and multi-programmed workloads, many applications are still mono-thread and multi-core performance with a single thread workload is an important issue. Furthermore, recent studies suggest that performance, power and temperature considerations of future multi-cores may necessitate activity-migration between cores. Motivated by the above, this paper investigates the performance implications of single thread migration on a multi-core. Specifically, the study considers the influence on the performance of a single thread of the following migration and multi-core parameters: frequency of migration, core warm-up modes, subset of resources that are warmed-up, number of cores, and cache hierarchy organization. The results of this study can provide insight to architects on how to design performance-efficient power and thermal strategies for a multi-core chip. The experimental results, for the benchmarks and microarchitectures used in this study, show that the performance loss due to activity migration on a multi-core with private L1s and a shared L2 can be minimized if: (a) a migrating thread continues its execution on a core that was previously visited by the thread, and (b) cores remember their predictor state since their previous activation (all other core resources can be cold). The data also show that the transfer of the register state between two cores can be slow, latency of several 100s of cycles, without limiting performance. 1

Simultaneous multithreading (SMT) and chip multiprocessing (CMP) both allow a chip to achieve greater throughput, but their relative energy-efficiency and thermal properties are still poorly understood. This paper uses Turandot, PowerTimer, and HotSpot to explore this design space for a POWER4/POWER5-like core. For an equalarea comparison with this style of core, we find CMP to be superior in terms of performance and energy-efficiency for CPU-bound benchmarks, but SMT to be superior for memory-bound benchmarks due to a larger L2 cache. Although both exhibit similar peak operating temperatures and thermal management overheads, the mechanism by which SMT and CMP heat up are quite different. More specifically, SMT heating is primarily caused by localized heating in certain key structures, CMP heating is mainly caused by the global impact of increased energy output. Because of this difference in heat up machanism, we found that the best thermal management technique is also different for SMT and CMP. Indeed, non-DVS localized thermal-management can outperform DVS for SMT. Finally, we show that CMP and SMT will scale differently as the contribution of leakage power grows, with CMP suffering from higher leakage due to the second core’s higher temperature and the exponential temperature-dependence of subthreshold leakage. 1.