In a hybrid memory system using DRAM as the NVM cache, DRAM and NVM can be accessed in serial or parallel mode. However, we found that using either mode alone will bring access latency and bandwidth problems. In this paper, we integrate these two access modes and design a simple but accurate predictor (called SAPredictor) to help choose the appropriate access mode, thereby avoiding long access latency and bandwidth problems to improve memory performance. Our experiments show that SAPredictor achieves an accuracy rate of up to 97.1% and helps reduce access latency by up to 35.6% at fairly low costs.

The parallel hybrid memory system that combines Non-volatile Memory (NVM) and DRAM can effectively expand the memory capacity. But it puts lots of pressure on TLB due to a limited TLB capacity. The superpage technology that manages pages with a large granularity (e.g., 2MB) is usually used to improve the TLB performance. However, its coarse-grained granularity conflicts with the fine-grained page migration in the hybrid memory system, resulting in serious invalid migration and page fragmentation problems. To solve these problems, we propose to maintain the coexistence of multi-granularity pages, and design a smart TLB called GATLB to support multi-granularity page management, coalesce consecutive pages and adapt to various changes in page size. Compared with the existing TLB technologies, GATLB can not only perceive page granularity to effectively expand the TLB coverage and reduce miss rate, but also provide faster address translation with a much lower overhead. Our experimental evaluations show that GATLB can expand the TLB coverage by 7.09x, reduce the TLB miss rate by 91.1%, and shorten the address translation cycle by 49.41%.

Data deduplication, as a proven technology for effective data reduction in backup and archiving storage systems, is also showing promises in increasing the logical space capacity for storage caches by removing redundant data. However, our in-depth evaluation of the existing deduplication-aware caching algorithms reveals that they only work well when the cached block size is set to 4 KB. Unfortunately, modern storage systems often set the block size to be much larger than 4 KB, and in this scenario, the overall performance of these caching schemes drops below that of the conventional replacement algorithms without any deduplication. There are several reasons for this performance degradation. The first reason is the deduplication overhead, which is the time spent on generating the data fingerprints and their use to identify duplicate data. Such overhead offsets the benefits of deduplication. The second reason is the extremely low cache space utilization caused by read and write alignment. The third reason is that existing algorithms only exploit access locality to identify block replacement. There is a lost opportunity to effectively leverage the content usage patterns such as intensity of content redundancy and sharing in deduplication-based storage caches to further improve performance. We propose CDAC, a Content-driven Deduplication-Aware Cache, to address this problem. CDAC focuses on exploiting the content redundancy in blocks and intensity of content sharing among source addresses in cache management strategies. We have implemented CDAC based on LRU and ARC algorithms, called CDAC-LRU and CDAC-ARC respectively. Our extensive experimental results show that CDAC-LRU and CDAC-ARC outperform the state-of-the-art deduplication-aware caching algorithms, D-LRU, and D-ARC, by up to 23.83X in read cache hit ratio, with an average of 3.23X, and up to 53.3 percent in IOPS, with an average of 49.8 percent, under a real-world mixed workload when the cache size ranges from 20 to 50 percent of the workload size and the block size ranges from 4KB to 32 KB.

To improve the performance of address translation in applications with large memory footprints, techniques, such as hugepages and HW coalescing, are proposed to increase the coverage of limited hardware translation entries by exploiting the contiguous memory allocation to lower Tanslation Lookaside Buffer (TLB) miss rate. Furthermore, Page Table Caches (PTCs) are proposed to store the upper-level page table entries to reduce the TLB miss handling latency. Both increasing TLB coverage and reducing TLB miss handling latency have proved to be effective in speeding up address translation, to a certain extent. Nevertheless, our preliminary studies suggest that the structural separation between TLBs and PTCs in existing computer systems makes these two methods less effective because they are exclusively used in TLBs and PTCs respectively. In particular, the separate structures cannot dynamically adjust their sizes according to the workloads, resulting in low resource utilization and inefficient address translation. To address these issues, we propose a unified structure, called Unified - Tp,which stores PTC and TLB entries together. Besides, Our modified LRU algorithm helps identify the cold TLB and PTC entries and dynamically adjust the numbers of TLB and PTC entries to adapt to different workloads. Furthermore, we introduce a scheme of parallel search when receiving memory access requests. Our experimental results show that Unified-TP can reduce the numbers of TLB misses by an average of 35.69 % and improve the performance by an average of 11.12% compared with separately structured TLBs and PTCs.

Byte-addressable, non-volatile memory (NVRAM) combines the benefits of DRAM and flash memory. However, due to its slower speed than DRAM, it is best to deploy it in combination with typical DRAM. In such Hybrid NVRAM systems, frequently accessed, hotpages can be stored in DRAM while other cold pages can reside in NVRAM, providing the benefits of both high performance (from DRAM) and lower power consumption and cost/performance (from NVRAM). While the idea seems beneficial, realizing an efficient hybrid NVRAM system requires careful page migration and accurate data temperature measurement. Existing solutions, however, often cause invalid migrations due to inaccurate data temperature accounting, because hot and cold pages are separately identified in DRAM and NVRAM regions. Moreover, since a new NVRAM frame is always allocated for each page swapped back NVRAM, a large amount of unnecessary NVRAM writes are generated during each page migration. Based on these observations, we propose APMigrate, an adaptive data migration approach for hybrid NVRAM systems. APMigrate consist of two parts, UIMigrate and LazyWriteback. UIMigrate focuses on eliminating invalid page migrations by considering data temperature in the entire DRAM-NVRAM space, while LazyWriteback focus on rewriting only dirty data back when the page is swapped back to NVRAM. Our experiments using SPEC 2006 show that APMigrate can reduce the number of migrations and improves performance by up to 90 percent compared to existing state-of-the-art approaches. For some workloads, LazyWriteback can reduce unnecessary NVRAM writes for existing page migrations by up to 75 percent.

Byte-addressable, non-volatile memory (NVRAM) combines the benefits of DRAM and flash memory. Its slower speed compared to DRAM, however, makes it hard to entirely replace DRAM with NVRAM. Hybrid NVRAM systems that equip both DRAM and NVRAM on the memory bus become a better solution: frequently accessed, hot pages can be stored in DRAM while other cold pages can reside in NVRAM. This way, the system gets the benefits of both high performance (from DRAM) and lower power consumption and cost/performance (from NVRAM). Realizing an efficient hybrid NVRAM system requires careful page migration and accurate data temperature measurement. Existing solutions, however, often cause invalid migrations due to inaccurate data temperature accounting, because hot and cold pages are separately identified in DRAM and NVRAM regions.Based on this observation, we propose UIMigrate, an adaptive data migration approach for hybrid NVRAM systems. The key idea is to consider data temperature across the whole DRAM-NVRAM space when determining whether a page should be migrated between DRAM and NVRAM. In addition, UIMigrate adapts workload changes by dynamically adjusting migration decisions as workload changes. Our experiments using SPEC 2006 show that UIMigrate can reduce the number of migrations and improves performance by up to 90.4% compared to existing state-of-the-art approaches.

Lossless data reduction techniques, particularly compression and deduplication, have emerged as effective approaches to tackling the combined challenge of explosive growth in data volumes but lagging growth in network bandwidth, to improve space and bandwidth efficiency in the cloud storage environment. However, our observations reveal that traditional deduplication solutions are rendered essentially useless in detecting and removing redundant data from the compressed packages in the cloud, which are poised to greatly increase in their presence and popularity. This is because even uncompressed, compressed and differently compressed packages of the exact same contents tend to have completely different byte stream patterns, whose redundancy cannot be identified by comparing their fingerprints. This, combined with different compressed packets mixed with different data but containing significant duplicate data, will further exacerbate the problem in the cloud storage environment. To address this fundamental problem, we propose Z-Dedup, a novel deduplication system that is able to detect and remove redundant data in compressed packages, by exploiting some key invariant information embedded in the metadata of compressed packages such as file-based checksum and original file length information. Our evaluations show that Z-Dedup can significantly improve both space and bandwidth efficiency over traditional approaches by eliminating 1.61% to 98.75% redundant data of a compressed package based on our collected datasets, and even more storage space and bandwidth are expected to be saved after the storage servers have accumulated more compressed contents.

Data deduplication, as a proven technology for effective data reduction in backup and archive storage systems, also demonstrates the promise in increasing the logical space capacity of storage caches by removing redundant data. However, our in-depth evaluation of the existing deduplication-aware caching algorithms reveals that they do improve the hit ratios compared to the caching algorithms without deduplication, especially when the cache block size is set to 4KB. But when the block size is larger than 4KB, a clear trend for modern storage systems, their hit ratios are significantly reduced. A slight increase in hit ratios due to deduplicationmay not be able to improve the overall storage performance because of the high overhead created by deduplication. To address this problem, in this paper we propose CDAC, a Content-driven Deduplication-Aware Cache, which focuses on exploiting the blocks' content redundancy and their intensity of content sharing among source addresses in cache management strategies. We have implemented CDAC based on LRU and ARC algorithms, called CDAC-LRU and CDAC-ARC respectively. Our extensive experimental results show that CDACLRU and CDAC-ARC outperform the state-of-the-art deduplication-aware caching algorithms, D-LRU and DARC, by up to 19.49X in read cache hit ratio, with an average of 1.95X under real-world traces when the cache size ranges from 20% to 80% of the working set size and the block size ranges from 4KB to 64 KB.

Integrating the data deduplication function into Solid State Drives (SSDs) helps avoid writing duplicate contents to NAND flash chips, which will not only effectively reduce the number of Program/Erase (P/E) operations to extend the device's lifespan but also proportionally enlarge the effective capacity of SSD to improve the performance of its behind-the-scenes maintenance tasks such as wear-leveling (WL) and garbage-collection (GC). However, these benefits of deduplication come at a non-trivial computational cost incurred by the embedded SSD controller to compute cryptographic hashes. To address this overhead problem, some researchers have suggested replacing cryptographic hashes with error correction codes (ECCs) already embedded in the SSD chips to detect the duplicate contents. However, all existing attempts have ignored the impact of the data randomization (scrambler) module that is widely used in modern SSDs, thus making it impractical to directly integrate ECC-based deduplication into commercial SSDs. In this work, we revisit SSD's internal structure and propose the first deduplicatable SSD that can bypass the data scrambler module to enable the low-cost ECC-based data deduplication. Specifically, we propose two design solutions, one on the host side and the other on the device side, to enable ECC-based deduplication. Based on our approach, we can effectively exploit SSD's built-in ECC module to calculate the hash values of stored data for data deduplication. We have evaluated our SES-Dedup approach by replaying data traces in an SSD simulator and found that it can remove up to 30.8% redundant data with up to 17.0% write performance improvement over the baseline SSD.

Soft error is a type of transient errors that occur due in part to reductions in capacitance and operating voltages in modern electronic components. Recently, the problem of soft errors has become more prevalent due to several design factors, including aggressive device scaling and newer energy-efficient designs, thus significantly threatening the reliability of computer systems. Since the occurrence of soft errors is non-deterministic, detecting them and recovering from them can be quite challenging. A common way to detect soft errors is to execute two identical program instances and then compare their results. Although this approach is effective, it is not efficient as both non-trivial computation and memory resources must be invested to support such redundant executions.

In deduplication-based backup systems, the removal of redundant data transforms the otherwise logically adjacent data chunks into physically scattered chunks on the disks. This, in effect, changes the retrieval operations from sequential to random and significantly degrades the performance of restoring data. These scattered chunks are called fragmented data and many techniques have been proposed to identify and sequentially rewrite such fragmented data to new address areas, trading off the increased storage space for reduced number of random reads (disk seeks) to improve the restore performance. However, existing solutions for backup workloads share a common assumption that every read operation involves a large fixed-size window of contiguous chunks, which restricts the fragment identification to a fixed-size read window. This can lead to inaccurate identifications due to false positives since the data fragments can vary in size and appear in any different and unpredictable address locations. Based on these observations, we propose FGdefrag , a Fine-Grained defragmentation approach that uses variable-sized and adaptively located data groups, instead of using fixed-size read windows, to accurately identify and effectively remove fragmented data. When we compare its performance to those of existing solutions, FGdefrag not only reduces the amount of rewritten data but also significantly improves the restore performance. Our experimental results show that FGdefrag can improve the restore performance by 14 to 329 percent, while simultaneously reducing the rewritten data by 25 to 87 percent.

Mobile databases and key-value stores provide consistency and durability through write-ahead logging. The traditional logging scheme appends the log records to the end of the log file and flushes the records to durable storage using fsync(). Due to the large block size of the underlying file system and the Journaling of Journal anomaly, the logging latency becomes the main bottleneck of the mobile databases. Our experimental results indicate that the logging latency accounts for more than 90% of the overall insert latency on a Samsung Galaxy S4 smartphone. Moreover, we observe a significant write amplification (up to 122×) induced by the traditional logging scheme. In this paper we present xLog, a fast transaction logging service leveraging qNVRAM, a nearly non-volatile memory for mobile devices. From our experimental results, xLog logs up to 77× faster than the traditional logging scheme, and speeds up the LevelDB Put operation by up to 10.7×. Moreover, xLog drastically reduces the write amplification of the traditional logging scheme, from 122× to less than 1.6×.

Our experimental study and analysis reveal that the bottlenecks of existing hardware transactional memory systems are largely rooted in the extra data movements in version management and in the inefficient scheduling of conflicting transactions in conflict management, particularly in the presence of high-contention and coarse-grained applications. In order to address this problem, we propose an integrated Pseudo-Associativity and Relaxed-Order approach to hardware Transactional Memory, called PARO-TM. It exploits the extra pseudo-associative space in the data cache to hold the new value of each transactional modification, and maintains the mappings between the old and new versions via an implicit pseudo-associative hash algorithm (i.e., by inverting the specific bit of the SET index). PARO-TM can branch out the speculative version from the old version upon each transactional modification on demand without a dedicated hardware component to hold the uncommitted data. This means that it is able to automatically access the proper version upon the transaction's commit or abort. Moreover, PARO-TM augments multi-version support in a chained directory to schedule conflicting transactions in a relaxed-order manner to further reduce their overheads. We compare PARO-TM with the state-of-the-art LogTM-SE, TCC, DynTM, and SUV-TM systems and find that PARO-TM consistently outperforms these four representative HTMs. This performance advantage of PARO-TM is far more pronounced under the high-contention and coarse-grained applications in the STAMP benchmark suite, for which PARO-TM is motivated and designed.

In order to maintain the transactional semantics, Transactional Memory (TM) must guarantee isolated read and write operations in each transaction, meaning that it must spend a non-negligible and potentially significant amount of time on keeping track of the transactional modifications in its undo or redo log and switching to the proper version at the end of each transaction. Existing TMs failed to minimize the overheads incurred by these operations that are poised to impose more significant TM overheads in current and future many-core CMPs. A direct consequence of this is that extra and different data movements are needed to manage these modifications depending on commit or abort. To address this problem, we propose a novel Single-Update Version-management (SUV) scheme to redirect each transactional store operation to another memory address, track the mapping information between the original and redirected addresses, and switch to the proper version of data upon the transaction's commit or abort. There is only one data update (movement) in our SUV regardless of commit or abort, thus significantly reducing the TM overheads while allowing it to exploit more thread parallelism. We use SUV to replace version-management schemes in some existing hardware TMs to assess SUV's performance advantages. Our extensive execution-driven experiments show that SUV-TM consistently outperforms the state-of-the-art HTM schemes Log TM-SE, FasTM and DynTM under the STAMP benchmark suite. Moreover, we use CACTI to estimate the hardware overheads of SUV and find it is feasible in hardware implementation.

Due to the relatively low bandwidth of WAN (Wide Area Network) that supports cloud backup services, both the backup time and restore time in the cloud backup environment are in desperate need for reduction to make cloud backup a practical and affordable service for small businesses and telecommuters alike. Existing solutions that employ the deduplication technology for cloud backup services only focus on removing redundant data from transmission during backup operations to reduce the backup time, while paying little attention to the restore time that we argue is an important aspect and affects the overall quality of service of the cloud backup services. In this paper, we propose a CAusality Based deduplication performance booster for both cloud backup and restore operations, called CABdedupe, which captures the causal relationship among chronological versions of datasets that are processed in multiple backups/restores, to remove the unmodified data from transmission during not only backup operations but also restore operations, thus to improve both the backup and restore performances. CABdedupe is a middleware that is orthogonal to and can be integrated into any existing backup system. Our extensive experiments, where we integrate CABdedupe into two existing backup systems and feed real world datasets, show that both the backup time and restore time are significantly reduced, with a reduction ratio of up to 103 : 1.

As more threads added to execute the multi-threaded applications in the many-core era, memory contentions among different threads impose a severe challenge to both the programmability and performance. Existing studies show that Transactional Memory (TM) is able to solve the programmability problem and scale well on the fine-grained applications in the SPLASH-2 benchmark suite. As more investigations on the coarse-grained applications in the STAMP benchmark suite, the long-running transactions block the parallelism among the concurrent transactions and failed to obtain the performance returns when the number of threads is beyond 4. In order to address this problem, we propose TMTLS, which combines TM with Thread-Level Speculation (TLS) to limit the number of concurrent executing transactions due to the memory contention in the runtime, divides the coarse-grained transactions into several epochs and assigns them to the available threads to speculatively exploit the parallelism in the coarse-grained transactions. This proposal not only alleviates the memory contention among the threads but also shortens the execution period of the coarse-grained transactions. Moreover, it further reduces the serializing overheads due to the transactional conflicts among the transactions. Our evaluation show this method achieves an average speedup of 2.27 over the baseline TM system under the 4 high-contention and coarse-grained applications selected from the STAMP benchmark suite on a 16-core CMP.

Existing de-duplication solutions in cloud backup environment either obtain high compression ratios at the cost of heavy de-duplication overheads in terms of increased latency and reduced throughput, or maintain small de-duplication overheads at the cost of low compression ratios causing high data transmission costs, which results in a large backup window. In this paper, we present SAM, a Semantic-Aware Multitiered source de-duplication framework that first combines the global file-level de-duplication and local chunk-level deduplication, and further exploits file semantics in each stage in the framework, to obtain an optimal tradeoff between the deduplication efficiency and de-duplication overhead and finally achieve a shorter backup window than existing approaches. Our experimental results with real world datasets show that SAM not only has a higher de-duplication efficiency/overhead ratio than existing solutions, but also shortens the backup window by an average of 38.7%.

Beyond the storage savings brought by chunk-level de-duplication in backup and archiving systems, a prominent challenge facing this technology is how to efficiently and effectively identify the duplicate chunks. Most of the chunk fingerprints used to identify individual chunks are stored on disks due to the limited main memory capacity. Checking for chunk fingerprint match on disk for every input chunk is known to be a severe performance bottleneck for the backup process. On the other hand, our intuitions and analyses of real backup data both indicate that duplicate chunks tend to strongly concentrate according to the data ownership. Motivated by this observation and to avoid or alleviate the aforementioned backup performance bottleneck, we propose DAM, a dataownership-aware multi-layered de-duplication scheme that exploits the data chunks' ownership and uses a tri-layered de-duplication approach to narrow the search space for duplicate chunks to reduce the total disk accesses. Our experimental results with real world datasets on DAM show it reduces the disk accesses by an average of 60.8% and shortens the de-duplication time by an average of 46.3%.