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Solid State Drives (SSDs) have many attractive features, and they are well-positioned to replace traditional HDDs in cloud and enterprise storages. However, some specific features they have such as erase-before-write and limited erase count make this transition more challenging. To tackle the issues brought by these features, Flash Translation Layer (FTL) leverages functionalities such as Garbage Collection (GC) to provide free space by erasing the blocks and cleaning the stale data. This process occupies SSD resources and causes extra waiting time for normal IO requests, and ultimately increases response time. Cloud workloads include many applications with small random write access patterns which cause frequent GC invocations. Flash occupation at the time of GC causes inconsistent performance and increases tail latency which may in turn violate SLA in these applications. In this paper, we modify the conventional GC process and propose a Cache-assisted Garbage Collection (CachedGC). CachedGC postpones writing back valid pages during the GC, erases the block at the first possible time, and hence resumes normal SSD operations sooner. CachedGC reduces request response time and tail latency by reducing the effective latency of GC. Our experimental results using a variety of workloads show that CachedGC reduces read tail latency by 19% and improves write IOPS by as much as 27%.

Power management for large last-level caches (LLCs) is important in chip multiprocessors (CMPs), as the leakage power of LLCs accounts for a significant fraction of the limited on-chip power budget. Since not all workloads running on CMPs need the entire cache, portions of a large, shared LLC can be disabled to save energy. In this article, we explore different design choices, from circuit-level cache organization to microarchitectural management policies, to propose a low-overhead runtime mechanism for energy reduction in the large, shared LLC. We first introduce a slice-based cache organization that can shut down parts of the shared LLC with minimal circuit overhead. Based on this slice-based organization, part of the shared LLC can be turned off according to the spatial and temporal cache access behavior captured by low-overhead sampling-based hardware. In order to eliminate the performance penalties caused by flushing data before powering off a cache slice, we propose data migration policies to prevent the loss of useful data in the LLC. Results show that our energy-efficient cache design (EECache) provides 14.1% energy savings at only 1.2% performance degradation and consumes negligible hardware overhead compared to prior work.

Power management for large last-level caches (LLCs) is important in chip-multiprocessors (CMPs), as the leakage power of LLCs accounts for a significant fraction of the limited on-chip power budget. Since not all workloads need the entire cache, portions of a shared LLC can be disabled to save energy. In this paper, we explore different design choices, from circuit-level cache organization to micro-architectural management policies, to propose a low-overhead run-time mechanism for energy reduction in the shared LLC. Results show that our design (EECache) provides 14.1% energy saving at only 1.2% performance degradation on average, with negligible hardware overhead.

In this work we introduce power optimizations relying on partial tag comparison (PTC) in snoop-based chip multiprocessors. Our optimizations rely on the observation that detecting tag mismatches in a snoop-based chip multiprocessor does not require aggressively processing the entire tag. In fact, a high percentage of cache mismatches could be detected by utilizing a small subset but highly informative portion of the tag bits.Based on this, we introduce a source-based snoop filtering mechanism referred to as S-PTC. In S-PTC possible remote tag mismatches are detected prior to sending the request. We reduce power as S-PTC prevents sending unnecessary snoops and avoids unessential tag lookups at the end-points. Furthermore, S-PTC improves performance as a result of early cache miss detection.S-PTC improves average performance from 2.9% to 3.5% for different configurations and for the SPLASH-2 benchmarks used in this study. Our solutions reduce snoop request bandwidth from 78.5% to 81.9% and average tag array dynamic power by about 52%.

In this work we introduce Heterogeneous Interconnect for Low Resolution Cache Access (Helia). Helia improves energy efficiency in snoop-based chip multiprocessors as it eliminates unnecessary activities in both interconnect and cache. This is achieved by using innovative snoop filtering mechanisms coupled with wire management techniques. Our optimizations rely on the observation that a high percentage of cache mismatches could be detected by utilizing a small subset but highly informative portion of the tag bits. Helia relies on the snoop controller to detect possible remote tag mismatches prior to tag array lookup. Power is reduced as a) our wire management techniques permit slow transmission of a subset of tag bits while tag mismatches are being detected and b) we avoid cache access for mismatches detected at the snoop controller. Our Evaluation shows that Helia reduces power in interconnect (dynamic: 64% to 75%, static: 45% to 50%) and cache tag array (dynamic: 57% to 58%, static: 80%) while improving average performance up to 4.4%.