Emerging byte-addressable, non-volatile memory technologies offer performance within an order of magnitude of DRAM, prompting their inclusion in the processor memory subsystem. However, such load/store accessible Persistent Memory (PM) has implications on system design, both hardware and software. In this paper, we explore system software support to enable low-overhead PM access by new and legacy applications. To this end, we implement PMFS, a light-weight POSIX file system that exploits PM's byteaddressability to avoid overheads of block-oriented storage and enable direct PM access by applications (with memorymapped I/O). PMFS exploits the processor's paging and memory ordering features for optimizations such as finegrained logging (for consistency) and transparent large page support (for faster memory-mapped I/O). To provide strong consistency guarantees, PMFS requires only a simple hardware primitive that provides software enforceable guarantees of durability and ordering of stores to PM. Finally, PMFS uses the processor's existing features to protect PM from stray writes, thereby improving reliability.Using a hardware emulator, we evaluate PMFS's performance with several workloads over a range of PM performance characteristics. PMFS shows significant (up to an order of magnitude) gains over traditional file systems (such as ext4) on a RAMDISK-like PM block device, demonstrating the benefits of optimizing system software for PM.

The advent of non-volatile memory (NVM) will fundamentally change the dichotomy between memory and durable storage in database management systems (DBMSs). These new NVM devices are almost as fast as DRAM, but all writes to it are potentially persistent even after power loss. Existing DBMSs are unable to take full advantage of this technology because their internal architectures are predicated on the assumption that memory is volatile. With NVM, many of the components of legacy DBMSs are unnecessary and will degrade the performance of data intensive applications.To better understand these issues, we implemented three engines in a modular DBMS testbed that are based on different storage management architectures: (1) in-place updates, (2) copy-on-write updates, and (3) log-structured updates. We then present NVMaware variants of these architectures that leverage the persistence and byte-addressability properties of NVM in their storage and recovery methods. Our experimental evaluation on an NVM hardware emulator shows that these engines achieve up to 5.5× higher throughput than their traditional counterparts while reducing the amount of wear due to write operations by up to 2×. We also demonstrate that our NVM-aware recovery protocols allow these engines to recover almost instantaneously after the DBMS restarts.

After nearly a decade of anticipation, scalable nonvolatile memory DIMMs are finally commercially available with the release of the Intel® Optane™ DC Persistent Memory Module (or just "Optane DC PMM"). This new nonvolatile DIMM supports byte-granularity accesses with access times on the order of DRAM, while also providing data storage that survives power outages.This work comprises the first in-depth, scholarly, performance review of Intel's Optane DC PMM, exploring its capabilities as a main memory device, and as persistent, byte-addressable memory exposed to user-space applications. For the past several months, our group has had access to machines with Optane DC memory and has investigated the Optane DC PMM's performance characteristics. This report details the chip's performance under a number of modes and scenarios, and across a wide variety of both micro-and macro-scale benchmarks. In total, this report represents approximately 330 hours of machine time.Optane DC memory occupies a tier in-between SSDs and DRAM. It has higher latency (346 ns) than DRAM but lower latency than an SSD. Unlike DRAM, its bandwidth is asymmetric with respect to access type: for a single Optane DC PMM, its max read bandwidth is 6.6 GB/s, whereas its max write bandwidth is 2.3 GB/s. However, the expected price point of Optane DC memory means that machines with large quantities of Optane DC memory are feasible -our test machine has 3 TB of Optane DC memory across two sockets.Optane DC PMMs can be used as large memory devices with a DRAM cache to hide their lower bandwidth and higher latency. When used in this Memory (or cached) mode, Optane DC memory has little impact on applications with small memory footprints. Applications with larger memory footprints may experience some slow-down relative to DRAM, but are now able to keep much more data in memory.In contrast, in App Direct (or uncached) mode, Optane DC PMMs can be used as a persistent storage device. When used under a file system, this configuration can result in significant performance gains, especially when the file system is optimized to use the load/store interface of the Optane DC PMM and the application uses many small, persistent writes. For instance, using the NOVA-relaxed NVMM file system, we can improve the performance of Kyoto Cabinet by almost 2×.In App Direct mode, Optane DC PMMs can also be used to enable user-space persistence where the application explicitly controls its writes into persistent Optane DC media. By modifying the actual application, application programmers can gain additional performance benefits since persistent updates bypass both the kernel and file system. In our experiments, modified applications that used user-space Optane DC persistence generally outperformed their file system counterparts; for instance, the user-space persistent version of RocksDB performed almost 2× faster than the equivalent program utilizing an NVMM-aware file system. This early report is only the beginning in an effort to understand these new memory devices. We hope that ...

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