Bulk bitwise operations, i. e., bitwise operations on large bit vectors, are prevalent in a wide range of important application domains, including databases, graph processing, genome analysis, cryptography, and hyper-dimensional computing. In conventional systems, the performance and energy efficiency of bulk bitwise operations are bottlenecked by data movement between the compute units (e.g., CPUs and GPUs) and the memory hierarchy. In-flash processing (i. e., processing data inside NAND flash chips) has a high potential to accelerate bulk bitwise operations by fundamentally reducing data movement through the entire memory hierarchy, especially when the processed data does not fit into main memory. We identify two key limitations of the state-of-the-art in-flash processing technique for bulk bitwise operations; (i) it falls short of maximally exploiting the bit-level parallelism of bulk bitwise operations that could be enabled by leveraging the unique cell-array architecture and operating principles of NAND flash memory; (ii) it is unreliable because it is not designed to take into account the highly error-prone nature of NAND flash memory. We propose Flash-Cosmos (Flash C omputation with-O ne-S hot M ulti-O perand S ensing), a new in-flash processing technique that significantly increases the performance and energy efficiency of bulk bitwise operations while providing high reliability. Flash-Cosmos introduces two key mechanisms that can be easily supported in modern NAND flash chips: (i) M ulti-W ordline S ensing (MWS), which enables bulk bitwise operations on a large number of operands (tens of operands) with a single sensing operation, and (ii) E nhanced S LC-mode P rogramming (ESP), which enables reliable computation inside NAND flash memory. We demonstrate the feasibility of performing bulk bitwise operations with high reliability in Flash-Cosmos by testing 160 real 3D NAND flash chips. Our evaluation shows that Flash-Cosmos improves average performance and energy efficiency by $3.5 \times /32 \times$ and $3.3 \times /95 \times$, respectively, over the state-of-the-art in-flash/outside-storage processing techniques across three real-world applications.

Aggressive memory density scaling causes modern DRAM devices to suffer from RowHammer, a phenomenon where rapidly activating (i.e., hammering) a DRAM row can cause bit-flips in physically-nearby rows. Recent studies demonstrate that modern DDR4/LPDDR4 DRAM chips, including chips previously marketed as RowHammer-safe, are even more vulnerable to RowHammer than older DDR3 DRAM chips. Many works show that attackers can exploit RowHammer bit-flips to reliably mount system-level attacks to escalate privilege and leak private data. Therefore, it is critical to ensure RowHammersafe operation on all DRAM-based systems as they become increasingly more vulnerable to RowHammer. Unfortunately, state-of-the-art RowHammer mitigation mechanisms face two major challenges. First, they incur increasingly higher performance and/or area overheads when applied to more vulnerable DRAM chips. Second, they require either closely-guarded proprietary information about the DRAM chips' physical circuit layouts or modifications to the DRAM chip design.In this paper, we show that it is possible to efficiently and scalably prevent RowHammer bit-flips without knowledge of or modification to DRAM internals. To this end, we introduce BlockHammer, a low-cost, effective, and easy-to-adopt RowHammer mitigation mechanism that prevents all RowHammer bit-flips while overcoming the two key challenges. BlockHammer selectively throttles memory accesses that could otherwise potentially cause RowHammer bit-flips. The key idea of BlockHammer is to (1) track row activation rates using area-efficient Bloom filters, and (2) use the tracking data to ensure that no row is ever activated rapidly enough to induce RowHammer bit-flips. By guaranteeing that no DRAM row ever experiences a RowHammer-unsafe activation rate, BlockHammer (1) makes it impossible for a RowHammer bit-flip to occur and (2) greatly reduces a RowHammer attack's impact on the performance of co-running benign applications. Our evaluations across a comprehensive range of 280 workloads show that, compared to the best of six state-of-the-art RowHammer mitigation mechanisms (all of which require knowledge of or modification to DRAM internals), BlockHammer provides (1) competitive performance and energy when the system is not under a RowHammer attack and (2) significantly better performance and energy when the system is under a RowHammer attack.

RowHammer is a circuit-level DRAM vulnerability, first rigorously analyzed and introduced in 2014, where repeatedly accessing data in a DRAM row can cause bit flips in nearby rows. The RowHammer vulnerability has since garnered significant interest in both computer architecture and computer security research communities because it stems from physical circuit-level interference effects that worsen with continued DRAM density scaling. As DRAM manufacturers primarily depend on density scaling to increase DRAM capacity, future DRAM chips will likely be more vulnerable toRowHammer than those of the past. Many RowHammer mitigation mechanisms have been proposed by both industry and academia, but it is unclear whether these mechanisms will remain viable solutions forfuture devices, as their overheads increase with DRAM's vulnerability to RowHammer. In order to shed more light on how RowHammer affects modern and future devices at the circuit-level, wefirst present an experimental characterization of RowHammer on 1580 DRAM chips (408X DDR3, 652X DDR4, and 520X LPDDR4) from 300 DRAM modules (60X DDR3, 110X DDR4, and 130X LPDDR4) with RowHammer protection mechanisms disabled, spanning multiple different technology nodes from across each of the three major DRAM manufacturers. Our studies definitively show that newer DRAM chips are more vulnerable to RowHammer: as device feature size reduces, the number of activations needed to induce a RowHammer bit flip also reduces, to as few as 9.6k (4.8k to two rows each) in the most vulnerable chip we tested. We evaluate five state-of-the-art RowHammer mitigation mechanisms using cycle-accurate simulation in the context of real data taken from our chips to study how the mitigation mechanisms scale with chip vulnerability. Wefind that existing mechanisms either are not scalable or suffer from prohibitively large performance overheads in projected future devices given our observed trends of RowHammer vulnerability. Thus, it is critical to research more effective solutions to RowHammer.

Important workloads, such as machine learning and graph analytics applications, heavily involve sparse linear algebra operations. These operations use sparse matrix compression as an effective means to avoid storing zeros and performing unnecessary computation on zero elements. However, compression techniques like Compressed Sparse Row (CSR) that are widely used today introduce significant instruction overhead and expensive pointer-chasing operations to discover the positions of the non-zero elements. In this paper, we identify the discovery of the positions (i.e., indexing) of non-zero elements as a key bottleneck in sparse matrix-based workloads, which greatly reduces the benefits of compression.

The effectiveness of deep neural networks (DNN) in vision, speech, and language processing has prompted a tremendous demand for energy-efficient high-performance DNN inference systems. Due to the increasing memory intensity of most DNN workloads, main memory can dominate the system's energy consumption and stall time. One effective way to reduce the energy consumption and increase the performance of DNN inference systems is by using approximate memory, which operates with reduced supply voltage and reduced access latency parameters that violate standard specifications. Using approximate memory reduces reliability, leading to higher bit error rates. Fortunately, neural networks have an intrinsic capacity to tolerate increased bit errors. This can enable energy-efficient and high-performance neural network inference using approximate DRAM devices.