High Performance Hybrid Memory Systems with 3D-stacked DRAM

The bandwidth of traditional DRAM is pin limited and so does not scale well with the increasing demand of data intensive workloads. 3D-stacked DRAM can alleviate this problem providing substantially higher bandwidth to a processor chip. However, the capacity of 3D-stacked DRAM is not enough to replace the bulk of the memory and therefore it is used together with off-chip DRAM in a hybrid memory system, either as a DRAM cache or as part of a flat address space with support for data migration. The performance of both above alternative designs is limited by their particular overheads. This thesis proposes new designs that improve the performance of hybrid memory systems. It does so first by alleviating the overheads of current approaches and second, by proposing a new design that combines the best attributes of DRAM caching and data migration while addressing their respective weaknesses. The first part of this thesis focuses on improving the performance of DRAM caches. Besides the unavoidable DRAM access to fetch the requested data, tag access is in the critical path adding significant latency and energy costs. Existing approaches are not able to remove these overheads and in some cases limit DRAM cache design options. To alleviate the tag access overheads of DRAM caches this thesis proposes Decoupled Fused Cache (DFC), a DRAM cache design that fuses DRAM cache tags with the tags of the on-chip Last Level Cache (LLC) to access the DRAM cache data directly on LLC misses. Compared to current state-of-the-art DRAM caches, DFC improves system performance by 11% on average. Finally, DFC reduces DRAM cache traffic by 25% and DRAM cache energy consumption by 24.5%. The second part of this thesis focuses on improving the performance of data migration. Data migration has significant performance potential, but also entails overheads which may diminish its benefits or even degrade performance. These overheads are mainly due to the high cost of swapping data between memories which also makes selecting which data to migrate critical to performance. To address these challenges of data migration this thesis proposes LLC guided Data Migration (LGM). LGM uses the LLC to predict future reuse and select memory segments for migration. Furthermore, LGM reduces the data migration traffic overheads by not migrating the cache lines of memory segments which are present in the LLC. LGM outperforms current state-of-the art data migration, improving system performance by 12.1% and reducing memory system dynamic energy by 13.2%. DRAM caches and data migration offer different tradeoffs for the utilization of 3D-stacked DRAM but also share some similar challenges. The third part of this thesis aims to provide an alternative approach to the utilization of 3D-stacked DRAM combining the strengths of both DRAM caches and data migration while eliminating their weaknesses. To that end, this thesis proposes Hybrid2, a hybrid memory system design which uses only a small fraction of the 3D-stacked DRAM as a cache and thus does not deny valuable capacity from the memory system. It further leverages the DRAM cache as a staging area to select the data most suitable for migration. Finally, Hybrid2 alleviates the metadata overheads of both DRAM caches and migration using a common mechanism. Depending on the system configuration, Hybrid2 on average outperforms state-of-the-art migration schemes by 6.4% to 9.1%, compared to DRAM caches Hybrid2 gives away on average only 0.3%, to 5.3% of performance offering up to 24.6% more main memory capacity

Main memory in HPC: do we need more, or could we live with less?

An important aspect of High-Performance Computing (HPC) system design is the choice of main memory capacity. This choice becomes increasingly important now that 3D-stacked memories are entering the market. Compared with conventional Dual In-line Memory Modules (DIMMs), 3D memory chiplets provide better performance and energy efficiency but lower memory capacities. Therefore, the adoption of 3D-stacked memories in the HPC domain depends on whether we can find use cases that require much less memory than is available now. This study analyzes the memory capacity requirements of important HPC benchmarks and applications. We find that the High-Performance Conjugate Gradients (HPCG) benchmark could be an important success story for 3D-stacked memories in HPC, but High-Performance Linpack (HPL) is likely to be constrained by 3D memory capacity. The study also emphasizes that the analysis of memory footprints of production HPC applications is complex and that it requires an understanding of application scalability and target category, i.e., whether the users target capability or capacity computing. The results show that most of the HPC applications under study have per-core memory footprints in the range of hundreds of megabytes, but we also detect applications and use cases that require gigabytes per core. Overall, the study identifies the HPC applications and use cases with memory footprints that could be provided by 3D-stacked memory chiplets, making a first step toward adoption of this novel technology in the HPC domain.This work was supported by the Collaboration Agreement between Samsung Electronics Co., Ltd. and BSC, Spanish Government through Severo Ochoa programme (SEV-2015-0493), by the Spanish Ministry of Science and Technology through TIN2015-65316-P project and by the Generalitat de Catalunya (contracts 2014-SGR-1051 and 2014-SGR-1272). This work has also received funding from the European Union’s Horizon 2020 research and innovation programme under ExaNoDe project (grant agreement No 671578). Darko Zivanovic holds the Severo Ochoa grant (SVP-2014-068501) of the Ministry of Economy and Competitiveness of Spain. The authors thank Harald Servat from BSC and Vladimir Marjanovi´c from High Performance Computing Center Stuttgart for their technical support.Postprint (published version

Hybrid2: Combining Caching and Migration in Hybrid Memory Systems

This paper considers a hybrid memory system composed of memory technologies with different characteristics; in particular a small, near memory exhibiting high bandwidth, i.e., 3D-stacked DRAM, and a larger, far memory offering capacity at lower bandwidth, i.e., off-chip DRAM. In the past,the near memory of such a system has been used either as a DRAM cache or as part of a flat address space combined with a migration mechanism. Caches and migration offer different tradeoffs (between performance, main memory capacity, data transfer costs, etc.) and share similar challenges related todata-transfer granularity and metadata management. This paper proposes Hybrid2 , a new hybrid memory system architecture that combines a DRAM cache with a migration scheme. Hybrid 2 does not deny valuable capacity from the memory system because it uses only a small fraction of the near memory as a DRAM cache; 64MB in our experiments.It further leverages the DRAM cache as a staging area to select the data most suitable for migration. Finally, Hybrid2 alleviates the metadata overheads of both DRAM caches and migration using a common mechanism. Using near to far memory ratios of 1:16, 1:8 and 1:4 in our experiments, Hybrid2 on average outperforms current state-of-the-art migration schemes by 7.9%, 9.1% and 6.4%, respectively. In the same system configurations, compared to DRAM caches Hybrid2 gives away on average only 0.3%, 1.2%, and 5.3% of performance offering 5.9%, 12.1%, and 24.6% more main memory capacity, respectively

성능과 용량 향상을 위한 적층형 메모리 구조

학위논문 (박사)-- 서울대학교 대학원 : 융합과학기술대학원 융합과학부(지능형융합시스템전공), 2019. 2. 안정호.The advance of DRAM manufacturing technology slows down, whereas the density and performance needs of DRAM continue to increase. This desire has motivated the industry to explore emerging Non-Volatile Memory (e.g., 3D XPoint) and the high-density DRAM (e.g., Managed DRAM Solution). Since such memory technologies increase the density at the cost of longer latency, lower bandwidth, or both, it is essential to use them with fast memory (e.g., conventional DRAM) to which hot pages are transferred at runtime. Nonetheless, we observe that page transfers to fast memory often block memory channels from servicing memory requests from applications for a long period. This in turn significantly increases the high-percentile response time of latency-sensitive applications. In this thesis, we propose a high-density managed DRAM architecture, dubbed 3D-XPath for applications demanding both low latency and high capacity for memory. 3D-XPath DRAM stacks conventional DRAM dies with high-density DRAM dies explored in this thesis and connects these DRAM dies with 3D-XPath. Especially, 3D-XPath allows unused memory channels to service memory requests from applications when primary channels supposed to handle the memory requests are blocked by page transfers at given moments, considerably increasing the high-percentile response time. This can also improve the throughput of applications frequently copying memory blocks between kernel and user memory spaces. Our evaluation shows that 3D-XPath DRAM decreases high-percentile response time of latency-sensitive applications by ∼30% while improving the throughput of an I/O-intensive applications by ∼39%, compared with DRAM without 3D-XPath. Recent computer systems are evolving toward the integration of more CPU cores into a single socket, which require higher memory bandwidth and capacity. Increasing the number of channels per socket is a common solution to the bandwidth demand and to better utilize these increased channels, data bus width is reduced and burst length is increased. However, this longer burst length brings increased DRAM access latency. On the memory capacity side, process scaling has been the answer for decades, but cell capacitance now limits how small a cell could be. 3D stacked memory solves this problem by stacking dies on top of other dies. We made a key observation in real multicore machine that multiple memory controllers are always not fully utilized on SPEC CPU 2006 rate benchmark. To bring these idle channels into play, we proposed memory channel sharing architecture to boost peak bandwidth of one memory channel and reduce the burst latency on 3D stacked memory. By channel sharing, the total performance on multi-programmed workloads and multi-threaded workloads improved up to respectively 4.3% and 3.6% and the average read latency reduced up to 8.22% and 10.18%.DRAM 제조 기술의 발전은 속도가 느려지는 반면 DRAM의 밀도 및 성능 요구는 계속 증가하고 있다. 이러한 요구로 인해 새로운 비 휘발성 메모리(예: 3D-XPoint) 및 고밀도 DRAM(예: Managed asymmetric latency DRAM Solution)이 등장하였다. 이러한 고밀도 메모리 기술은 긴 레이턴시, 낮은 대역폭 또는 두 가지 모두를 사용하는 방식으로 밀도를 증가시키기 때문에 성능이 좋지 않아, 핫 페이지를 고속 메모리(예: 일반 DRAM)로 스왑되는 저용량의 고속 메모리가 동시에 사용되는 것이 일반적이다. 이러한 스왑 과정에서 빠른 메모리로의 페이지 전송이 일반적인 응용프로그램의 메모리 요청을 오랫동안 처리하지 못하도록 하기 때문에, 대기 시간에 민감한 응용 프로그램의 백분위 응답 시간을 크게 증가시켜, 응답 시간의 표준 편차를 증가시킨다. 이러한 문제를 해결하기 위해 본 학위 논문에서는 저 지연시간 및 고용량 메모리를 요구하는 애플리케이션을 위해 3D-XPath, 즉 고밀도 관리 DRAM 아키텍처를 제안한다. 이러한 3D-톔소를 집적한 DRAM은 저속의 고밀도 DRAM 다이를 기존의 일반적인 DRAM 다이와 동시에 한 칩에 적층하고, DRAM 다이끼리는 제안하는 3D-XPath 하드웨어를 통해 연결된다. 이러한 3D-XPath는 핫 페이지 스왑이 일어나는 동안 응용프로그램의 메모리 요청을 차단하지 않고 사용량이 적은 메모리 채널로 핫 페이지 스왑을 처리 할 수 있도록 하여, 데이터 집중 응용 프로그램의 백분위 응답 시간을 개선시킨다. 또한 제안하는 하드웨어 구조를 사용하여, 추가적으로 O/S 커널과 유저 스페이스 간의 메모리 블록을 자주 복사하는 응용 프로그램의 처리량을 향상시킬 수 있다. 이러한 3D-XPath DRAM은 3D-XPath가 없는 DRAM에 비해 I/O 집약적인 응용프로그램의 처리량을 최대 39 % 향상시키면서 레이턴시에 민감한 응용 프로그램의 높은 백분위 응답 시간을 최대 30 %까지 감소시킬 수 있다. 또한 최근의 컴퓨터 시스템은 보다 많은 메모리 대역폭과 용량을 필요로하는 더 많은 CPU 코어를 단일 소켓으로 통합하는 방향으로 진화하고 있다. 이러한 소켓 당 채널 수를 늘리는 것은 대역폭 요구에 대한 일반적인 해결책이며, 최신의 DRAM 인터페이스의 발전 양상은 증가한 채널을 보다 잘 활용하기 위해 데이터 버스 폭이 감소되고 버스트 길이가 증가한다. 그러나 길어진 버스트 길이는 DRAM 액세스 대기 시간을 증가시킨다. 추가적으로 최신의 응용프로그램은 더 많은 메모리 용량을 요구하며, 미세 공정으로 메모리 용량을 증가시키는 방법론은 수십 년 동안 사용되었지만, 20 nm 이하의 미세공정에서는 더 이상 공정 미세화를 통해 메모리 밀도를 증가시키기가 어려운 상황이며, 적층형 메모리를 사용하여 용량을 증가시키는 방법을 사용한다. 이러한 상황에서, 실제 최신의 멀티코어 머신에서 SPEC CPU 2006 응용프로그램을 멀티코어에서 실행하였을 때, 항상 시스템의 모든 메모리 컨트롤러가 완전히 활용되지 않는다는 사실을 관찰했다. 이러한 유휴 채널을 사용하기 위해 하나의 메모리 채널의 피크 대역폭을 높이고 3D 스택 메모리의 버스트 대기 시간을 줄이기 위해 본 학위 논문에서는 메모리 채널 공유 아키텍처를 제안하였으며, 하드웨어 블록을 제안하였다. 이러한 채널 공유를 통해 멀티 프로그램 된 응용프로그램 및 다중 스레드 응용프로그램 성능이 각각 4.3 % 및 3.6 %로 향상되었으며 평균 읽기 대기 시간은 8.22 % 및 10.18 %로 감소하였다.Contents Abstract i Contents iv List of Figures vi List of Tables viii Introduction 1 1.1 3D-XPath: High-Density Managed DRAM Architecture with Cost-effective Alternative Paths for Memory Transactions 5 1.2 Boosting Bandwidth – Dynamic Channel Sharing on 3D Stacked Memory 9 1.3 Research contribution 13 1.4 Outline 14 3D-stacked Heterogeneous Memory Architecture with Cost-effective Extra Block Transfer Paths 17 2.1 Background 17 2.1.1 Heterogeneous Main Memory Systems 17 2.1.2 Specialized DRAM 19 2.1.3 3D-stacked Memory 22 2.2 HIGH-DENSITY DRAM ARCHITECTURE 27 2.2.1 Key Design Challenges 29 2.2.2 Plausible High-density DRAM Designs 33 2.3 3D-STACKED DRAM WITH ALTERNATIVE PATHS FOR MEMORY TRANSACTIONS 37 2.3.1 3D-XPath Architecture 41 2.3.2 3D-XPath Management 46 2.4 EXPERIMENTAL METHODOLOGY 52 2.5 EVALUATION 56 2.5.1 OLDI Workloads 56 2.5.2 Non-OLDI Workloads 61 2.5.3 Sensitivity Analysis 66 2.6 RELATED WORK 70 Boosting bandwidth –Dynamic Channel Sharing on 3D Stacked Memory 72 3.1 Background: Memory Operations 72 3.1.1. Memory Controller 72 3.1.2 DRAM column access sequence 73 3.2 Related Work 74 3.3. CHANNEL SHARING ENABLED MEMORY SYSTEM 76 3.3.1 Hardware Requirements 78 3.3.2 Operation Sequence 81 3.4 Analysis 87 3.4.1 Experiment Environment 87 3.4.2 Performance 88 3.4.3 Overhead 90 CONCLUSION 92 REFERENCES 94 국문초록 107Docto

ネットワーク仮想化におけるパケット処理のための並列メモリシステムアーキテクチャ

京都大学新制・課程博士博士(情報学)甲第23326号情博第762号新制||情||130(附属図書館)京都大学大学院情報学研究科通信情報システム専攻(主査)教授 大木 英司, 教授 守倉 正博, 教授 岡部 寿男学位規則第4条第1項該当Doctor of InformaticsKyoto UniversityDFA

Design Guidelines for High-Performance SCM Hierarchies

With emerging storage-class memory (SCM) nearing commercialization, there is evidence that it will deliver the much-anticipated high density and access latencies within only a few factors of DRAM. Nevertheless, the latency-sensitive nature of memory-resident services makes seamless integration of SCM in servers questionable. In this paper, we ask the question of how best to introduce SCM for such servers to improve overall performance/cost over existing DRAM-only architectures. We first show that even with the most optimistic latency projections for SCM, the higher memory access latency results in prohibitive performance degradation. However, we find that deployment of a modestly sized high-bandwidth 3D stacked DRAM cache makes the performance of an SCM-mostly memory system competitive. The high degree of spatial locality that memory-resident services exhibit not only simplifies the DRAM cache's design as page-based, but also enables the amortization of increased SCM access latencies and the mitigation of SCM's read/write latency disparity. We identify the set of memory hierarchy design parameters that plays a key role in the performance and cost of a memory system combining an SCM technology and a 3D stacked DRAM cache. We then introduce a methodology to drive provisioning for each of these design parameters under a target performance/cost goal. Finally, we use our methodology to derive concrete results for specific SCM technologies. With PCM as a case study, we show that a two bits/cell technology hits the performance/cost sweet spot, reducing the memory subsystem cost by 40% while keeping performance within 3% of the best performing DRAM-only system, whereas single-level and triple-level cell organizations are impractical for use as memory replacements.Comment: Published at MEMSYS'1

Reducing Memory Traffic with Approximate Compression

Memory bandwidth is a critical resource in modern systems and has an increasing demand. The large number of on-chip cores and specialized accelerators improves the potential processing throughput but also calls for higher data rates. In addition, new emerging data-intensive applications further increase memory traffic. On the other hand, memory bandwidth is pin limited and power constrained and is therefore more difficult to scale. This thesis proposes lossy memory compression as a means to reduce data volumes by exploiting the ability of applications to tolerate approximations in parts of their datasets. A reduction of off-chip memory traffic leads to reduced memory latency, which in turn improves the performance and energy efficiency of the system. The first part of this thesis introduces Approximate Value Reconstruction (AVR), which combines a low-complexity downsampling compressor with an LLC design able to co-locate compressed and uncompressed data. Two separate thresholds are employed to limit the error introduced by approximation. For applications that tolerate aggressive approximation in large fractions of their data, AVR reduces memory traffic by up to 70%, execution time by up to 55%, and energy costs by up to 20% introducing up to 1.2% error to the application output. The second part of this thesis proposes Memory Squeeze (MemSZ), introducing a parallelized implementation of the more advanced Squeeze (SZ) compression method. Furthermore, MemSZ improves on the error limiting capability of AVR by keeping track of life-time accumulated error. An alternate memory compression architecture is also proposed, which utilizes 3D-stacked DRAM as a last-level cache. MemSZ improves execution time, energy and memory traffic over AVR by up to 15%, 9%, and 64%, respectively