Processing data where it makes sense: Enabling in-memory computation

This retrospective paper describes the RowHammer problem in dynamic random access memory (DRAM), which was initially introduced by Kim et al. at the ISCA 2014 Conference. RowHammer is a prime (and perhaps the first) example of how a circuit-level failure mechanism can cause a practical and widespread system security vulnerability. It is the phenomenon that repeatedly accessing a row in a modern DRAM chip causes bit flips in physically adjacent rows at consistently predictable bit locations. RowHammer is caused by a hardware failure mechanism called DRAM disturbance errors , which is a manifestation of circuit-level cell-to-cell interference in a scaled memory technology. Researchers from Google Project Zero demonstrated in 2015 that this hardware failure mechanism can be effectively exploited by user-level programs to gain kernel privileges on real systems. Many other follow-up works demonstrated other practical attacks exploiting RowHammer. In this paper, we comprehensively survey the scientific literature on RowHammer-based attacks as well as mitigation techniques to prevent RowHammer. We also discuss what other related vulnerabilities may be lurking in DRAM and other types of memories, e.g., NAND flash memory or phase change memory, that can potentially threaten the foundations of secure systems, as the memory technologies scale to higher densities. We conclude by describing and advocating a principled approach to memory reliability and security research that can enable us to better anticipate and prevent such vulnerabilities.

RowHammer: A Retrospective

We propose a new DRAM-based true random number generator (TRNG) that leverages DRAM cells as an entropy source. The key idea is to intentionally violate the DRAM access timing parameters and use the resulting errors as the source of randomness. Our technique specifically decreases the DRAM row activation latency (timing parameter tRCD) below manufacturer-recommended specifications, to induce read errors, or activation failures, that exhibit true random behavior. We then aggregate the resulting data from multiple cells to obtain a TRNG capable of providing a high throughput of random numbers at low latency. 
To demonstrate that our TRNG design is viable using commodity DRAM chips, we rigorously characterize the behavior of activation failures in 282 state-of-the-art LPDDR4 devices from three major DRAM manufacturers. We verify our observations using four additional DDR3 DRAM devices from the same manufacturers. Our results show that many cells in each device produce random data that remains robust over both time and temperature variation. We use our observations to develop D-RanGe, a methodology for extracting true random numbers from commodity DRAM devices with high throughput and low latency by deliberately violating the read access timing parameters. We evaluate the quality of our TRNG using the commonly-used NIST statistical test suite for randomness and find that D-RaNGe: 1) successfully passes each test, and 2) generates true random numbers with over two orders of magnitude higher throughput than the previous highest-throughput DRAM-based TRNG.

D-RaNGe: Using Commodity DRAM Devices to Generate True Random Numbers with Low Latency and High Throughput

Specialized on-chip accelerators are widely used to improve the energy efficiency of computing systems. Recent advances in memory technology have enabled near-data accelerators (NDAs), which reside off-chip close to main memory and can yield further benefits than on-chip accelerators. However, enforcing coherence with the rest of the system, which is already a major challenge for accelerators, becomes more difficult for NDAs. This is because (1) the cost of communication between NDAs and CPUs is high, and (2) NDA applications generate a lot of off-chip data movement. As a result, as we show in this work, existing coherence mechanisms eliminate most of the benefits of NDAs. We extensively analyze these mechanisms, and observe that (1) the majority of off-chip coherence traffic is unnecessary, and (2) much of the off-chip traffic can be eliminated if a coherence mechanism has insight into the memory accesses performed by the NDA. Based on our observations, we propose CoNDA, a coherence mechanism that lets an NDA optimistically execute an NDA kernel, under the assumption that the NDA has all necessary coherence permissions. This optimistic execution allows CoNDA to gather information on the memory accesses performed by the NDA and by the rest of the system. CoNDA exploits this information to avoid performing unnecessary coherence requests, and thus, significantly reduces data movement for coherence. We evaluate CoNDA using state-of-the-art graph processing and hybrid in-memory database workloads. Averaged across all of our workloads operating on modest data set sizes, CoNDA improves performance by 19.6% over the highest-performance prior coherence mechanism (66.0%/51.7% over a CPU-only/NDA-only system) and reduces memory system energy consumption by 18.0% over the most energy-efficient prior coherence mechanism (43.7% over CPU- only). CoNDA comes within 10.4% and 4.4% of the performance and energy of an ideal mechanism with no cost for coherence. The benefits of CoNDA increase with large data sets, as CoNDA improves performance over the highest-performance prior coherence mechanism by 38.3% (8.4x/7.7x over CPU-only/NDA-only), and comes within 10.2% of an ideal no-cost coherence mechanism.

https://dl.acm.org/doi/pdf/10.1145/3307650.3322266

CoNDA: efficient cache coherence support for near-data accelerators

Processing-using-DRAM has been proposed for a limited set of basic operations (i.e., logic operations, addition). However, in order to enable full adoption of processing-using-DRAM, it is necessary to provide support for more complex operations. In this paper, we propose SIMDRAM, a flexible general-purpose processing-using-DRAM framework that (1) enables the efficient implementation of complex operations, and (2) provides a flexible mechanism tosupport the implementation of arbitrary user-defined operations. The SIMDRAM framework comprises three key steps. The first step builds an efficient MAJ/NOT representation of a given desired operation. The second step allocates DRAM rows that are reserved for computation to the operation’s input and output operands, and generates the required sequence of DRAM commands to perform the MAJ/NOT implementation of the desired operation in DRAM. The third step uses the SIMDRAM control unit located inside the memory controller to manage the computation of the operation from start to end, by executing the DRAM commands generated in the second step of the framework. We design the hardware and ISA support for SIMDRAM framework to (1) address key system integration challenges, and (2) allow programmers to employ new SIMDRAM operations without hardware changes. We evaluate SIMDRAM for reliability, area overhead, throughput, and energy efficiency using a wide range of operations and seven real-world applications to demonstrate SIMDRAM’s generality. Our evaluations using a single DRAM bank show that (1) over 16 operations, SIMDRAM provides 2.0X the throughput and 2.6X the energy efficiency of Ambit, a state-of-the-art processing-using-DRAM mechanism; (2) over seven real-world applications, SIMDRAM provides 2.5X the performance of Ambit. Using 16 DRAM banks, SIMDRAM provides (1) 88X and 5.8X the throughput, and 257X and 31X the energy efficiency, of a CPU and a high-end GPU, respectively, over 16 operations; (2) 21X and 2.1X the performance of the CPU and GPU, over seven real-world applications. SIMDRAM incurs an area overhead of only 0.2% in a high-end CPU.

SIMDRAM: a framework for bit-serial SIMD processing using DRAM

We are experiencing an explosive growth in the number of consumer devices, including smartphones, tablets, web-based computers such as Chromebooks, and wearable devices. For this class of devices, energy efficiency is a first-class concern due to the limited battery capacity and thermal power budget. We find that data movement is a major contributor to the total system energy and execution time in consumer devices. The energy and performance costs of moving data between the memory system and the compute units are significantly higher than the costs of computation. As a result, addressing data movement is crucial for consumer devices. In this work, we comprehensively analyze the energy and performance impact of data movement for several widely-used Google consumer workloads: (1) the Chrome web browser; (2) TensorFlow Mobile, Google's machine learning framework; (3) video playback, and (4) video capture, both of which are used in many video services such as YouTube and Google Hangouts. We find that processing-in-memory (PIM) can significantly reduce data movement for all of these workloads, by performing part of the computation close to memory. Each workload contains simple primitives and functions that contribute to a significant amount of the overall data movement. We investigate whether these primitives and functions are feasible to implement using PIM, given the limited area and power constraints of consumer devices. Our analysis shows that offloading these primitives to PIM logic, consisting of either simple cores or specialized accelerators, eliminates a large amount of data movement, and significantly reduces total system energy (by an average of 55.4% across the workloads) and execution time (by an average of 54.2%).

Google Workloads for Consumer Devices: Mitigating Data Movement Bottlenecks

Mobile computing has grown drastically over the past decade. Despite the rapid pace of advancements, mobile device understanding, benchmarking, and evaluation are still in their infancies, both in industry and academia. This article presents an industry perspective on the challenges facing mobile computer architecture, specifically involving mobile workloads, benchmarking, and experimental methodology, with the hope of fostering new research within the community to address pending problems. These challenges pose a threat to the systematic development of future mobile systems, which, if addressed, can elevate the entire mobile ecosystem to the next level.

Two Billion Devices and Counting

Provided are methods, systems, and apparatus for managing and controlling memory caches, in particular, system level caches outside of those closest to the CPU. The processes and representative hardware structures that implement the processes are designed to allow for detailed control over the behavior of such system level caches. Caching policies are developed based on policy identifiers, where a policy identifier corresponds to a collection of parameters that control the behavior of a set of cache management structures. For a given cache, one policy identifier is stored in each line of the cache.

Allan Knies

Papers

Google Workloads for Consumer Devices: Mitigating Data Movement Bottlenecks

Two Billion Devices and Counting

Memory system architecture

System-level cache

Way partitioning for a system-level cache