Hyojun Kim

Revisiting storage for smartphones

Conventional wisdom holds that storage is not a big contributor to application performance on mobile devices. Flash storage (the type most commonly used today) draws little power, and its performance is thought to exceed that of the network subsystem. In this article, we present evidence that storage performance does indeed affect the performance of several common applications such as Web browsing, maps, application install, email, and Facebook. For several Android smartphones, we find that just by varying the underlying flash storage, performance over WiFi can typically vary between 100% and 300% across applications; in one extreme scenario, the variation jumped to over 2000%. With a faster network (set up over USB), the performance variation rose even further. We identify the reasons for the strong correlation between storage and application performance to be a combination of poor flash device performance, random I/O from application databases, and heavy-handed use of synchronous writes. Based on our findings, we implement and evaluate a set of pilot solutions to address the storage performance deficiencies in smartphones.

A low-cost memory architecture with NAND XIP for mobile embedded systems

NAND flash memory has become an indispensable component in mobile embedded systems because of its versatile features such as nonvolatility, solid-state reliability, low cost and high density. Even though NAND flash memory is gaining popularity as data storage, it can be also exploited as code memory for XIP (execute-in-place). In this paper, we present a new memory architecture in which incorporates NAND flash memory into an existing memory hierarchy for code execution. The usefulness of the proposed approach is demonstrated with real embedded workloads on a real prototyping board.

Embedded NAND flash file system for mobile multimedia devices

In this work, we present a novel mobile multimedia file system, MNFS, which is specifically designed for NAND flash memory. Our design specifically addresses the needs of devices such as MP3 players, personal media players (PMPs), digital camcorders, etc. The file system has three novel features important in mobile multimedia applications: (1) predictable and uniform write latency, (2) quick file system mount, and (3) a small memory footprint. We implemented the proto-type file system on an embedded platform. In our experiments, MNFS exhibits uniform I/O latency for sequential write operation. It is mountable within 0.2 seconds and available with only 34 Kbytes of heap memory for a 128 Mbytes volume. The mounting time is 30 times faster compared to that of YAFFS, which is a well known file system for NAND flash memory, and the heap memory usage is only 5% of the YAFFS usage.

SWL: a search-while-load demand paging scheme with NAND flash memory

As mobile phones become increasingly multifunctional, the number and size of applications installed in phones are rapidly increasing. Consequently, mobile phones require more hardware resources such as NOR/NAND flash memory and DRAM, and their production cost is accordingly becoming higher. One candidate solution to reduce production cost is demand paging using MMU. However, demand paging causes unpredictably long page fault latency, and as such mobile phone manufacturers are reluctant to deploy this scheme. In this paper, we present a method that reduces the long latency of page faults by performing page fault handling in a parallelized manner, considering the characteristics of NAND-Type flash memory. We also discuss how to modify the existing page cache replacement policies so that they can exploit the benefits of the parallelized page fault handler. Experimental results show that the parallelized page fault handler improves the worst case latency of page faults significantly, by up to roughly 20%, and that the modified page cache replacement policies improve both the average and worst instruction fetch time.

Design and implementation of MLC NAND flash-based DBMS for mobile devices

Recently, Multi-Level Cell (MLC) NAND flash memory is becoming widely used as storage media for mobile devices such as mobile phones, MP3 players, PDAs and digital cameras. MLC NAND flash memory, however, has some restrictions that hard disk or Single-Level Cell (SLC) NAND flash memory do not have. Since most traditional database techniques assume hard disk, they may not provide the best attainable performance on MLC NAND flash memory. In this paper, we design and implement an MLC NAND flash-based DBMS for mobile devices, called AceDB Flashlight, which fully exploits the unique characteristics of MLC NAND flash memory. Our performance evaluations on an MLC NAND flash-based device show that the proposed DBMS significantly outperforms the existing ones.

Examining storage performance on mobile devices

Conventional wisdom holds that storage is not a big contributor to application performance or energy consumption on mobile devices. Flash storage (the type most commonly used today) draws little power, and its performance is thought to exceed that of the network subsystem. In this paper we present initial evidence to the contrary even for common applications such as web browsing or application install. We find that just by varying the underlying flash storage, performance of web browsing over WiFi can vary roughly by 500%, and of application install by 300%. With a faster network (setup over USB), storage is taxed even more and the performance variation rose to roughly 700% for web browsing! The performance variation can be attributed to the characteristics of the storage device, the workload pattern (random or sequential), and the operating system itself. We also find that lower storage performance leads to increased CPU consumption, thus having an indirect impact on energy.

MNFS: mobile multimedia file system for NAND flash based storage device

In this work, we present a novel mobile multimedia file system, MNFS, which is specifically designed for NAND flash memory. It is designed for mobile multimedia devices such as MP3 player, Personal Media Player (PMP), digital camcorder, etc. Our file system has three novel features important in mobile multimedia applications: (1) predictable and uniform write latency, (2) quick file system mount, and (3) small memory footprint. We implement the proto-type file system on ARM9 embedded platform. In experiments, MNFS exhibits uniform I/O latency for sequential write operation. It is mountable within 0.2 seconds, and available with only 34Kbytes heap memory for 128Mbytes volume. Compared to YAFFS which is the de facto standard file system for NAND flash memory, the mounting time is 30 times faster and the heap memory usage is only 5% of YAFFS usage.

A page padding method for fragmented flash storage

Today, flash memory is widely used for various kinds of products. Unlike a hard disk, it has neither mechanical parts nor seek-delay. Therefore, a user may expect steady performance under disk fragmentation in flash storage. However, most commercial products do not satisfy this expectation. For example, a SDMMC card can be written in 18.7Mbytes/sec speed sequentially, but its write speed is slowed down to 3.2Mbytes/sec when it is seriously fragmented. It is only 18% of the original performance. In this paper, we analyze the reason for performance degradation in a flash disk, and propose an FTL level optimization technique, named the page padding method, to lessen the fragmentation effect. We applied the technique to the Log-block FTL algorithm and showed that it can enhance write performance by 150% in a severely fragmented flash disk.

Impact of flash memory on video-on-demand storage: analysis of tradeoffs

There is no doubt that video-on-demand (VoD) services are very popular these days. However, disk storage is a serious bottleneck limiting the scalability of a VoD server. Disk throughput degrades dramatically due to seek time overhead when the server is called upon to serve a large number of simultaneous video streams. To address the performance problem of disk, buffer cache algorithms that utilize RAM have been proposed. Interval caching is a state-of-the-art caching algorithm for a VoD server. Flash Memory Solid-State Drive (SSD) is a relatively new storage technology. Its excellent random read performance, low power consumption, and sharply dropping cost per gigabyte are opening new opportunities to efficiently use the device for enterprise systems. On the other hand, it has deficiencies such as poor small random write performance and limited number of erase operations. In this paper, we analyze tradeoffs and potential impact that flash memory SSD can have for a VoD server. Performance of various commercially available flash memory SSD models is studied. We find that low-end flash memory SSD provides better performance than the high-end one while costing less than the high-end one when the I/O request size is large, which is typical for a VoD server. Because of the wear problem and asymmetric read/write performance of flash memory SSD, we claim that interval caching cannot be used with it. Instead, we propose using file-level Least Frequently Used (LFU) due to the highly skewed video access pattern of the VoD workload. We compare the performance of interval caching with RAM and file-level LFU with flash memory by simulation experiments. In addition, from the cost-effectiveness analysis of three different storage configurations, we find that flash memory with hard disk drive is the most cost-effective solution compared to DRAM with hard disk drive or hard disk drive only.

A New Transactional Flash Translation Layer for Embedded Database Systems Based on MLC NAND Flash Memory

This paper presents a novel transactional flash translation layer that is specially designed for embedded database systems based on multi-level cell NAND flash memory. It effectively overcomes the insufficient reliability and slow write speed of MLC NAND flash memory. We also propose a desirable architecture for an embedded database system running on flash memory.