This paper documents the findings of our analysis of the implementation of our six-year NSF S-STEM scholarship program. One major finding was that, for underrepresented students to major in computer science, knowing the major existed and understanding the nature of the program were the most important factors. Also, the academic support system and hands-on nature of the major had a significant impact on scholarship recipients' persistence in the major. Evidence demonstrated that scholarship recipients had a 10%+ higher year-to-year persistence rate from their freshmen to sophomore year than that of all computer science students of the same entering classes. For all computer science students, college computer science major GPAs were not strongly correlated with their high school GPAs, financial need, or ACT math scores. This paper also presents lessons learned and resulting recommendations for future new scholarship administrators, as our lessons can likely be applied to other grants that recruit and deal with underrepresented groups.

The design and implementation of traditional file systems typically use the one-to-one mapping of logical files to their physical metadata representations. File system optimizations generally follow this rigid mapping and miss opportunities for an entire class of optimizations. We designed, implemented, and evaluated a composite-file file system, which allows many-to-one mappings of files to metadata. Through exploring different mapping strategies, our empirical evaluation shows up to a 27% performance improvement under web server and software development workloads, for both disks and SSDs. This result demonstrates that our approach of relaxing file-to-metadata mapping is promising.

One important aspect of privacy is the ability to securely delete sensitive data from electronic storage in such a way that it cannot be recovered; we call this action secure deletion . Short of physically destroying the entire storage medium, existing software secure-deletion solutions tend to be piecemeal at best -- they may only work for one type of storage or file system, may force the user to delete all files instead of selected ones, may require the added complexities of encryption and key storage, may require extensive changes and additions to the computer's operating system or storage firmware, and may not handle system crashes gracefully. We present TrueErase, a holistic secure-deletion framework for individual systems that contain sensitive data. Through design, implementation, verification, and evaluation on both a hard drive and NAND flash, TrueErase shows that it is possible to construct a per-file, secure-deletion framework that can accommodate different storage media and legacy file systems, require limited changes to legacy systems, and handle common crash scenarios. TrueErase can serve as a building block by cryptographic systems that securely delete information by erasing encryption keys. The overhead is dependent on spatial locality, number of sensitive files, and workload (computational- or I/O-bound).

Computer science Ph.D. training provides numerous opportunities to prepare doctoral graduates to write research grant proposals. However, writing scholarship grant proposals is a very different process, and a newcomer might go through many attempts before obtaining their first awarded grant. This paper documents our four proposal submissions prior to acquiring our first NSF S-STEM grant for the Department of Computer Science at Florida State University. This paper also highlights major issues to consider when writing such proposals. We hope that future newcomers will be able to avoid some of the pitfalls we encountered in obtaining scholarship grants of a similar nature.

The ability to securely delete sensitive data from electronic storage is becoming important. However, current per-file deletion solutions tend to be limited to a segment of the operating system's storage data path or specific to particular file systems or storage media.

The specification of the sporadic server real-time scheduling policy in the IEEE POSIX standard is defective, and needs to be corrected. Via experiments using a POSIX sporadic server implementation under Linux, as well as simulations, we have shown and confirmed previously unreported defects. We propose and demonstrate a corrected sporadic server formulation that eliminates these defects without changes to the syntax of the API or any significant increase in implementation complexity.

Many contemporary disk drives have built-in queues and schedulers. These features can improve I/O performance, by offloading work from the system's main processor, avoiding disk idle time, and taking advantage of vendor-specific disk characteristics. At the same time, they pose challenges for scheduling requests that have real-time requirements, since the operating system has less visibility and control over service times. While it may be possible for an operating system to obtain more predictable real-time performance by bypassing the on-disk queue and scheduler, the diversity and continuing evolution of disk drives make it difficult to extract the necessary detailed timing characteristics of a specific disk, and to generalize that approach to all hard drives. This paper demonstrates three techniques we developed in the Linux operating system to bound real-time request response times for disks with internal queues and schedulers. The first technique is to use the disk's built-in starvation prevention scheme. The second is to prevent requests from being sent to the disk when real-time requests are waiting to be served. The third is to limit the length of the on-disk queue in addition to the second technique. Our results show the ability to guarantee a wide range of desired response times while still allowing the disk to perform scheduling optimizations. These techniques can be generalized to disks from different vendors.

While wireless vehicular ad hoc networks (VANETs) are attracting greater commercial interest, current research has not adequately captured the real-world urban constraints in VANET deployment. In this work, we evaluated the feasibility and benefits of deploying a VANET in urban settings with a wireless mesh backbone infrastructure. We modeled urban street layouts, traffic rules, RF attenuation due to physical obstacles, and the use of multiple radio channels Our results indicate that the performance improves in dense networks when routing decisions are limited to mesh nodes, whereas it improves in sparse networks when mobile nodes also participate in routing. We also show that the effect of signal attenuation due to physical obstacles can potentially be parametrized in simulations using empirical real-world measurements.

The growing interest in wireless Vehicular Ad Hoc Networks (VANETs) has prompted greater research into simulation models that better reflect urban VANET deployments. Still, we lack a systematic understanding of the required level of simulation details in modeling various real-world urban constraints. In this work, we developed a series of simulation models that account for street layout, traffic rules, multilane roads, acceleration-deceleration, and RF attenuation due to obstacles. Using real and controlled synthetic maps, we evaluated the sensitivity of the simulation results toward these details. Our results indicate that the delivery ratio and packet delays in VANETs are more sensitive to the clustering effect of vehicles at intersections and their acceleration/deceleration. The VANET performance appears to be only marginally affected by the simulation of multiple lanes and careful synchronization at traffic signals. We also found that the performance in dense VANETs improves significantly when routing decisions are limited to a wireless backbone of mesh nodes, whereas in sparse VANETs, performance improves when vehicles also participate in ad hoc routing. Finally, through measurement and analysis of signal strengths around urban city blocks, we show that the effect of signal attenuation due to physical obstacles can potentially be parameterized in simulations. Our work provides a starting point for further understanding and development of more accurate VANET simulation model.

Device drivers are integral components of operating systems. The computational workloads imposed by device drivers tend to be aperiodic and unpredictable because they are triggered in response to events that occur in the device, and may arbitrarily block or preempt other time-critical tasks. This characteristic poses significant challenges in real-time systems, where schedulability analysis is essential to guarantee system-wide timing constraints. At the same time, device driver workloads cannot be ignored. Demand-based schedulability analysis is a technique that has been successful in validating the timing constraints in both single and multiprocessor systems. In this paper we present two approaches to demand-based schedulability analysis of systems that include device drivers. First, we derive load-bound functions using empirical measurement techniques. Second, we modify the scheduling of network device driver tasks in Linux to implement an algorithm for which a load-bound function can be derived analytically. We demonstrate the practicality of our approach through detailed experiments with a network device under Linux. Our results show that, even though the network device driver does not conform to conventional periodic or sporadic task models, it can be successfully modeled using hyperbolic load-bound functions that are fitted to empirical performance measurements

Reducing power consumption for server-class computers is important, since increased energy usage causes more heat dissipation, greater cooling requirements, reduced computational density, and higher operating costs. For a typical data center, storage accounts for 27% of energy consumption. Conventional server-class RAIDs cannot easily reduce power because loads are balanced to use all disks, even for light loads. We have built the power-aware RAID (PARAID), which reduces energy use of commodity server-class disks without specialized hardware. PARAID uses a skewed striping pattern to adapt to the system load by varying the number of powered disks. By spinning disks down during light loads, PARAID can reduce power consumption, while still meeting performance demands, by matching the number of powered disks to the system load. Reliability is achieved by limiting disk power cycles and using different RAID encoding schemes. Based on our five-disk prototype, PARAID uses up to 34% less power than conventional RAIDs while achieving similar performance and reliability.

Modern file systems assume the use of disk, a system-wide performance bottleneck for over a decade. Current disk caching and RAM file systems either impose high overhead to access memory content or fail to provide mechanisms to achieve data persistence across reboots.The Conquest file system is based on the observation that memory is becoming inexpensive, which enables all file system services to be delivered from memory, except for providing large storage capacity. Unlike caching, Conquest uses memory with battery backup as persistent storage, and provides specialized and separate data paths to memory and disk. Therefore, the memory data path contains no disk-related complexity. The disk data path consists of optimizations only for the specialized disk usage pattern.Compared to a memory-based file system, Conquest incurs little performance overhead. Compared to several disk-based file systems, Conquest achieves 1.3x to 19x faster memory performance, and 1.4x to 2.0x faster performance when exercising both memory and disk. Conquest realizes most of the benefits of persistent RAM at a fraction of the cost of a RAM-only solution. It also demonstrates that disk-related optimizations impose high overheads for accessing memory content in a memory-rich environment.

No abstract available.

Summary form only given. The Conquest file system is designed to provide a transition from disk- to persistent-RAM-based storage. Initially, we assume 2 to 4 Gbytes of persistent RAM and the popular single-user desktop environment. Unlike other memory file systems, Conquest can incrementally assume more responsibility for in-core storage as memory prices decline. The Conquest approach realizes most of the benefits of persistent-RAM-based file systems before persistent RAM becomes cheaply abundant. Conquest also benefits from the removal of disks as the primary storage by identifying disk-related complexities and isolating them from the critical path where possible. The Conquest prototype is operational under Linux 2.4.2. It is POSIX compliant and supports both in-core and on-disk storage.

Optimistic replication of data is becoming increasingly popular in mobile environments, but its performance and scaling characteristics are not well understood. This paper presents a simulation evaluation of optimistic replicated filing in a mobile environment. We first compare full and selective optimistic replication systems to capture the properties required for scaling. We then show that the presence of portable computers in optimistically replicated filing systems achieves a 60-percent cost reduction (e.g., computing resources) with only a 10-percent degradation of service quality (e.g., consistency of data perceived by users). This finding reveals certain similarities between the network disconnection interval and frequency of data synchronization. The research suggests new guidelines for design of optimistic replication systems.