A million qubit-scale quantum computer is essential to realize the quantum supremacy. Modern large-scale quantum computers integrate multiple quantum computers located in dilution refrigerators (DR) to overcome each DR's unscaling cooling budget. However, a large-scale multi-DR quantum computer introduces its unique challenges (i.e., slow and erroneous inter-DR entanglement, increased qubit scale), and they make the baseline error handling mechanism ineffective by increasing the number of gate operations and the inter-DR communication latency to decode and correct errors. Without resolving these challenges, it is impossible to realize a fault-tolerant large-scale multi-DR quantum computer.

Superconductor single-flux-quantum (SFQ) logic family has been recognized as a promising technology for cryogenic applications (e.g., quantum computing, astronomy, metrology) thanks to its ultra-fast and low-energy characteristics. Therefore, recent efforts in SFQ-based computing have focused on developing fast and low-power SFQ processors for cryogenic applications. However, there still has been little progress toward a convincing SFQ processor design due to the critical performance challenges originating from its extremely deep pipeline. In this paper, we propose a super-fast and low-power in-order SFQ processor by tackling the challenges from the deep pipeline. First, we develop a minimal-depth SFQ processor pipeline with novel architecture-level ideas. Next, we conduct in-depth performance analyses and identify three real performance bottlenecks in the deeply pipelined SFQ processors (i.e., stall/flush logic, RAW stall, fetch unit). Finally, we propose SuperCore, our super-fast SFQ-based processor architecture, with three SFQ-friendly solutions that effectively resolve the identified bottlenecks. With our solutions applied, SuperCore achieves 11 times speed-up over the SFQ processor baseline. In addition, SuperCore achieves six times speed-up and consumes up to 193 times less power compared to in-order CMOS processors running at 4K.

For datacenter architects, it is the most important goal to minimize the datacenter’s total cost of ownership for the target performance (i.e., TCO/performance). As the major component of a datacenter is a server farm, the most effective way of reducing TCO/performance is to improve the server’s performance and power efficiency. To achieve the goal, we claim that it is highly promising to reduce each server’s temperature to its most cost-effective point (or temperature scaling). In this article, we propose CoolDC , a novel and immediately applicable low-temperature cooling method to minimize the datacenter’s TCO. The key idea is to find and apply the most cost-effective sub-freezing temperature to target servers and workloads. For that purpose, we first apply the immersion cooling method to the entire servers to maintain a stable low temperature with little extra cooling and maintenance costs. Second, we define the TCO-optimal temperature for datacenter operation (e.g., 248K~273K (-25℃~0℃)) by carefully estimating all the costs and benefits at low temperatures. Finally, we propose CoolDC, our immersion-cooling datacenter architecture to run every workload at its own TCO-optimal temperature. By incorporating our low-temperature workload-aware temperature scaling, CoolDC achieves 12.7% and 13.4% lower TCO/performance than the conventional air-cooled and immersion-cooled datacenters, respectively, without any modification to existing computers.

A 10+K qubit Quantum-Classical Interface (QCI) is essential to realize the quantum supremacy. However, it is extremely challenging to architect scalable QCIs due to the complex scalability trade-offs regarding operating temperatures, device and wire technologies, and microarchitecture designs. Therefore, architects need a modeling tool to evaluate various QCI design choices and lead to an optimal scalable QCI architecture.

Datacenters rapidly evolve by adopting new features such as new hardware deployment and software patches. Adopting a new feature requires an accurate evaluation of its impact to minimize the risk to the multi-million dollar computing infrastructure. However, a comprehensive performance analysis of a datacenter is extremely challenging due to its cost and multitenancy. Evaluating the performance in a live datacenter is accurate but prohibitive to prevent any damage to production services. Using conventional load-testing benchmarks on small-scale testbeds is imprecise as they do not consider the effect of other co-located jobs.

Cryogenic computing, which runs a computer device at an extremely low temperature, is promising thanks to its significant reduction of wire resistance as well as leakage current. Recent studies on cryogenic computing have focused on various architectural units including the main memory, cache, and CPU core running at 77K. However, little research has been conducted to fully exploit the fast cryogenic wires, even though the slow wires are becoming more serious performance bottleneck in modern processors. In this paper, we propose a CPU microarchitecture which extensively exploits the fast wires at 77K. For this goal, we first introduce our validated cryogenic-performance models for the CPU pipeline and network on chip (NoC), whose performance can be significantly limited by the slow wires. Next, based on the analysis with the models, we architect CryoSP and CryoBus as our pipeline and NoC designs to fully exploit the fast wires. Our evaluation shows that our cryogenic computer equipped with both microarchitectures achieves 3.82 times higher system-level performance compared to the conventional computer system thanks to the 96% higher clock frequency of CryoSP and five times lower NoC latency of CryoBus.

10+K qubit quantum computer is essential to achieve a true sense of quantum supremacy. With the recent effort towards the large-scale quantum computer, architects have revealed various scalability issues including the constraints in a quantum control processor, which should be holistically analyzed to design a future scalable control processor. However, it has been impossible to identify and resolve the processor's scalability bottleneck due to the absence of a reliable tool to explore an extensive design space including microarchitecture, device technology, and operating temperature.

Cryogenic computing, which runs a computer device at an extremely low temperature, is highly promising thanks to the significant reduction of the wire latency and leakage current. A recently proposed cryogenic DRAM design achieved the promising performance improvement, but it also reveals that it must reduce the DRAM’s dynamic power to overcome the huge cooling cost at 77 K. Therefore, researchers now target to reduce the cryogenic DRAM’s refresh power by utilizing its significantly increased retention time driven by the reduced leakage current. To achieve the goal, however, architects should first answer many fundamental questions regarding the reliability and then design a refresh-free, but still robust cryogenic DRAM by utilizing the analysis result.In this work, we propose a near refresh-free, but robust cryogenic DRAM (NRFC-DRAM), which can almost eliminate its refresh overhead while ensuring reliable operations at 77 K. For the purpose, we first evaluate various DRAM samples of multiple vendors by conducting a thorough analysis to accurately estimate the cryogenic DRAM’s retention time and reliability. Our analysis identifies a new critical challenge such that reducing DRAM’s refresh rate can make the memory highly unreliable because normal memory operations can now appear as row-hammer attacks at 77 K. Therefore, NRFC-DRAM requires a cost-effective, cryogenic-friendly protection mechanism against the new row-hammer-like "faults" at 77 K.To resolve the challenge, we present CryoGuard, our cryogenic-friendly row-hammer protection method to ensure the NRFC-DRAM’s reliable operations at 77 K. With CryoGuard applied, NRFC-DRAM reduces the overall power consumption by 25.9 % even with its cooling cost included, whereas the existing cryogenic DRAM fails to reduce the power consumption.

Cryogenic computing, which is to run a computer at extremely low temperatures (e.g., 77K), is a highly promising solution to dramatically improve the computer's performance and power efficiency thanks to the significantly reduced leakage power and wire resistance. However, computer architects are facing fundamental challenges in developing and deploying cryogenic-optimal architectural units due to the lack of understanding about its cost-effectiveness and feasibility (e.g., device and cooling costs vs. speedup, energy and area saving) and thus how to architect such cryogenic-optimal units.

Cryogenic computing can achieve high performance and power efficiency by dramatically reducing the device's leakage power and wire resistance at low temperatures. Recent advances towards cryogenic computing focus on developing cryogenic-optimal cache and memory devices to overcome memory capacity, latency, and power walls. However, little research has been conducted to develop a cryogenic-optimal core architecture despite its high potentials in performance, power, and area efficiency. Once a cryogenic-optimal core becomes available, it will also take full advantage of the cryogenic-optimal cache and memory devices, which leads to a cryogenic-optimal computer.In this paper, we first develop CryoCore-Model (CC-Modet), a cryogenic processor modeling framework which can accurately estimate the maximum clock frequency of processor models running at 77K. Next, driven by the modeling tool, we design CryoCore, a 77K-optimal core microarchitecture to maximize the core's performance and area efficiency while minimizing the cooling cost. The key idea of CryoCore is to architect a core in a way to reduce the size and number of cooling-unfriendly microarchitecture units and maximize the potential of a voltage and frequency scaling at 77K. Finally, we propose two halfsized, but differently voltage-scaled CryoCore designs aiming for either the maximum performance or power efficiency. With both conventional and our design integrated with cryogenic memories, our high-performance CryoCore design achieves 41% higher single-thread performance for the same power budget and 2x higher multi-thread performance for the same die area. Our lowpower CryoCore design reduces the power cost by 38% without sacrificing the single-thread performance.

Superconductor single-flux-quantum (SFQ) logic family has been recognized as a highly promising solution for the post-Moore's era, thanks to its ultra-fast and low-power switching characteristics. Therefore, researchers have made a tremendous amount of effort in various aspects to promote the technology and automate its circuit design process (e.g., low-cost fabrication, design tool development). However, there has been no progress in designing a convincing SFQ-based architectural unit due to the architects' lack of understanding of the technology's potentials and limitations at the architecture level. In this paper, we present how to architect an SFQ-based architectural unit by providing design principles with an extreme-performance neural processing unit (NPU). To achieve the goal, we first implement an architecture-level simulator to model an SFQ-based NPU accurately. We validate this model using our die-level prototypes, design tools, and logic cell library. This simulator accurately measures the NPU's performance, power consumption, area, and cooling overheads. Next, driven by the modeling, we identify key architectural challenges for designing a performance-effective SFQ-based NPU (e.g., expensive on-chip data movements and buffering). Lastly, we present SuperNPU, our example SFQ-based NPU architecture, which effectively resolves the challenges. Our evaluation shows that the proposed design outperforms a conventional state-of-the-art NPU by 23 times. With free cooling provided as done in quantum computing, the performance per chip power increases up to 490 times. Our methodology can also be applied to other architecture designs with SFQ-friendly characteristics.

Modern computer architectures suffer from lack of architectural innovations, mainly due to the power wall and the memory wall. That is, architectural innovations become infeasible because they can prohibitively increase power consumption and their performance impacts are eventually bounded by slow memory accesses. To address the challenges, making computer systems run at ultra-low temperatures (or cryogenic computer systems) has emerged as a highly promising solution as both power consumption and wire resistivity are expected to significantly reduce at ultra-low temperatures. However, cryogenic computers have not been yet realized as computer architects do not fully understand the behaviors of existing computer systems and their cost effectiveness at such ultra-low temperatures.