Modern server and data center applications are characterized not only by big datasets, but also by large instruction footprints that incur frequent cache and Translation Lookaside Buffer (TLB) misses due to instruction accesses. Instruction TLB misses are particularly problematic since they cause pipeline stalls that significantly harm performance.

Despite processor vendors reporting that cache prefetchers operating with virtual addresses are permitted to cross page boundaries, academia is focused on optimizing cache prefetching for patterns within page boundaries. This work reveals that page-cross prefetching at the first-level data cache (L1D) is seldom beneficial across different execution phases and workloads while showing that state-of-the-art L1D prefetchers are not very accurate at prefetching across page boundaries. In response, we propose $M O K A$, a holistic framework for designing Page-Cross Filters, i.e., microarchitectural schemes that ensure effective and accurate prefetching across page boundaries. MOKA combines (i) hashed perceptron predictors that use prefetcher-independent program features, (ii) predictors that adapt decisions based on the system state (e.g., TLB pressure), and (iii) a scheme to dynamically optimize predictions across different execution phases and workload types. We use the MOKA framework to prototype a Page-Cross Filter, named DRIPPER, for three relevant L1D prefetchers (Berti [60], IPCP [61], BOP [57]). We show that DRIPPER accurately enables pagecross prefetching only when it is beneficial for performance. For instance, Berti [60] (state-of-the-art prefetcher) combined with DRIPPER improves single-core geomean performance over Berti that always permits page-cross prefetches and Berti that always discards page-cross prefetches by $\mathbf{1 . 7 \%}(\mathbf{1 . 2 \%})$ and $\mathbf{2 . 5 \%}(\mathbf{2 . 1 \%})$ across 218 seen (178 unseen) workloads, respectively. Across 300 8 -core mixes, the corresponding geomean speedups are $2.0 \%$ and $3.3 \%$. Finally, we show that DRIPPER provides consistent benefits when both 4KB pages and 2MB large pages are used.

To alleviate the performance and energy overheads of contemporary applications with large data footprints, we propose the Two Level Perceptron (TLP) predictor, a neural mechanism that effectively combines predicting whether an access will be off-chip with adaptive prefetch filtering at the first-level data cache (L1D). TLP is composed of two connected microarchitectural perceptron predictors, named First Level Predictor (FLP) and Second Level Predictor (SLP). FLP performs accurate off-chip prediction by using several program features based on virtual addresses and a novel selective delay component. The novelty of SLP relies on leveraging off-chip prediction to drive L1D prefetch filtering by using physical addresses and the FLP prediction as features. TLP constitutes the first hardware proposal targeting both off-chip prediction and prefetch filtering using a multilevel perceptron hardware approach. TLP only requires 7KB of storage. To demonstrate the benefits of TLP we compare its performance with state-of-the-art approaches using off-chip prediction and prefetch filtering on a wide range of single-core and multi-core workloads. Our experiments show that TLP reduces the average DRAM transactions by 30.7% and 17.7%, as compared to a baseline using state-of-the-art cache prefetchers but no off-chip prediction mechanism, across the single-core and multi-core workloads, respectively, while recent work significantly increases DRAM transactions. As a result, TLP achieves geometric mean performance speedups of 6.2% and 11.8% across single-core and multi-core workloads, respectively. In addition, our evaluation demonstrates that TLP is effective independently of the L1D prefetching logic.

Graph-processing workloads have become widespread due to their relevance on a wide range of application domains such as network analysis, path-planning, bioinformatics, and machine learning. Graph-processing workloads have massive data footprints that exceed cache storage capacity and exhibit highly irregular memory access patterns due to data-dependent graph traversals. This irregular behaviour causes graph-processing workloads to exhibit poor data locality, undermining their performance.This paper makes two fundamental observations on the memory access patterns of graph-processing workloads: First, conventional cache hierarchies become mostly useless when dealing with graph-processing workloads, since 78.6% of the accesses that miss in the L1 Data Cache (L1D) result in misses in the L2 Cache (L2C) and in the Last Level Cache (LLC), requiring a DRAM access. Second, it is possible to predict whether a memory access will be served by DRAM or not in the context of graph-processing workloads by observing strides between accesses triggered by instructions with the same Program Counter (PC). Our key insight is that bypassing the L2C and the LLC for highly irregular accesses significantly reduces latency cost while also reducing pressure on the lower levels of the cache hierarchy.Based on these observations, this paper proposes the Large Predictor (LP), a low-cost micro-architectural predictor capable of distinguishing between regular and irregular memory accesses. We propose to serve accesses tagged as regular by LP via the standard memory hierarchy, while irregular access are served via the Side Data Cache (SDC). The SDC is a private per-core set-associative cache placed alongside the L1D specifically aimed at reducing the latency cost of highly irregular accesses while avoiding polluting the rest of the cache hierarchy with data that exhibits poor locality. SDC coupled with LP yields geometric mean speed-ups of 20.3% and 20.2% on single- and multi-core scenarios, respectively, over an architecture featuring a conventional cache hierarchy across a set of contemporary graph-processing workloads. In addition, SDC combined with LP outperforms the Transpose-based Cache Replacement (T-OPT), the state-of-the-art cache replacement policy for graph-processing applications, by 10.9% and 13.8% on single-core and multi-core contexts, respectively. Regarding the hardware budget, SDC coupled with LP requires 10KB of storage per core.

The garbage collector (GC) is a crucial component of language runtimes, offering correctness guarantees and high productivity in exchange for a run-time overhead. Concurrent collectors run alongside application threads (mutators) and share CPU resources. A likely point of contention between mutators and GC threads and, consequently, a potential overhead source is the shared last-level cache (LLC).

The increase in working set sizes of contemporary applications outpaces the growth in cache sizes, resulting in frequent main memory accesses that deteriorate system performance due to the disparity between processor and memory speeds. Prefetching data blocks into the cache hierarchy ahead of demand accesses has proven successful at attenuating this bottleneck. However, spatial cache prefetchers operating in the physical address space leave significant performance on the table by limiting their pattern detection within 4KB physical page boundaries when modern systems use page sizes larger than 4KB to mitigate the address translation overheads. This paper exploits the high usage of large pages in modern systems to increase the effectiveness of spatial cache prefetching. We design and propose the Page-size Propagation Module (PPM), a $\mu$architectural scheme that propagates the page size information to the lower-level cache prefetchers, enabling safe prefetching beyond 4KB physical page boundaries when the accessed blocks reside in large pages, at the cost of augmenting the first-level caches’ Miss Status Holding Register (MSHR) entries with one additional bit. PPM is compatible with any cache prefetcher without implying design modifications. We capitalize on PPM’s benefits by designing a module that consists of two page size aware prefetchers that inherently use different page sizes to drive prefetching. The composite module uses adaptive logic to dynamically enable the most appropriate page size aware prefetcher. Finally, we show that the proposed designs are transparent to which cache prefetcher is used. We apply the proposed page size exploitation techniques to four state-of-the-art spatial cache prefetchers. Our evaluation shows that our proposals improve single-core geomean performance by up to 8.1% (2.1% at minimum) over the original implementation of the considered prefetchers, across 80 memory-intensive workloads. In multi-core contexts, we report geomean speedups up to 7.7% across different cache prefetchers and core configurations.

Frequent Translation Lookaside Buffer (TLB) misses incur high performance and energy costs due to page walks required for fetching the corresponding address translations. Prefetching page table entries (PTEs) ahead of demand TLB accesses can mitigate the address translation performance bottleneck, but each prefetch requires traversing the page table, triggering additional accesses to the memory hierarchy. Therefore, TLB prefetching is a costly technique that may undermine performance when the prefetches are not accurate.In this paper we exploit the locality in the last level of the page table to reduce the cost and enhance the effectiveness of TLB prefetching by fetching cache-line adjacent PTEs "for free". We propose Sampling-Based Free TLB Prefetching (SBFP), a dynamic scheme that predicts the usefulness of these "free" PTEs and prefetches only the ones most likely to prevent TLB misses. We demonstrate that combining SBFP with novel and state-of-the-art TLB prefetchers significantly improves miss coverage and reduces most memory accesses due to page walks.Moreover, we propose Agile TLB Prefetcher (ATP), a novel composite TLB prefetcher particularly designed to maximize the benefits of SBFP. ATP efficiently combines three low-cost TLB prefetchers and disables TLB prefetching for those execution phases that do not benefit from it. Unlike state-of-the-art TLB prefetchers that correlate patterns with only one feature (e.g., strides, PC, distances), ATP correlates patterns with multiple features and dynamically enables the most appropriate TLB prefetcher per TLB miss.To alleviate the address translation performance bottleneck, we propose a unified solution that combines ATP and SBFP. Across an extensive set of industrial workloads provided by Qualcomm, ATP coupled with SBFP improves geometric speedup by 16.2%, and eliminates on average 37% of the memory references due to page walks. Considering the SPEC CPU 2006 and SPEC CPU 2017 benchmark suites, ATP with SBFP increases geometric speedup by 11.1%, and eliminates page walk memory references by 26%. Applied to big data workloads (GAP suite, XSBench), ATP with SBFP yields a geometric speedup of 11.8% while reducing page walk memory references by 5%. Over the best state-of-the-art TLB prefetcher for each benchmark suite, ATP with SBFP achieves speedups of 8.7%, 3.4%, and 4.2% for the Qualcomm, SPEC, and GAP+XSBench workloads, respectively.

The effort to reduce address translation overheads has typically targeted data accesses since they constitute the overwhelming portion of the second-level TLB (STLB) misses in desktop and HPC applications. The address translation cost of instruction accesses has been relatively neglected due to historically small instruction footprints. However, state-of-the-art datacenter and server applications feature massive instruction footprints owing to deep software stacks, resulting in high STLB miss rates for instruction accesses.