Kaoutar El Maghraoui

Modeling and simulating flash based solid-state disks for operating systems

Solid-State Disks (SSDs) made out of Flash devices have gained a lot of prominence in recent years due to their increasing performance and endurance. A number of mechanisms are being proposed to improve the performance and reliability of these devices from technological and operating system perspectives, to integrate them into personal computers and enterprise systems. Most of such proposals are being implemented and evaluated directly on top of these SSDs and require sophisticated framework and infrastructure for thorough performance evaluation. On the other hand, to our knowledge, very little has been done on modeling Flash devices and building efficient Flash simulators that can be used to simulate SSDs. Such models and simulators can give insights to make design decisions, save a lot of cumbersome work for setup and implementation, save hardware costs and allow researchers to focus on the real methods that are being proposed.n This paper presents a linear model for NAND-based Flash devices based on the internal architecture of these devices. Parameters of the model are presented along with micro-benchmarks that can be used to extract these parameters. The model is validated on the STEC Zeus Flash SSD and extracted parameters are used to build a Flash simulator as a kernel extension in the AIX operating system. A key feature of the simulator is that it simulates I/O requests by maintaining minimal state information and is independent of the internal organization of a Flash SSD. The simulator is validated using commercial and raw-IO applications through experimentation on the simulator and real Flash disks.

Investigating hybrid SSD FTL schemes for Hadoop workloads

The Flash Translation Layer (FTL) is the core engine for Solid State Disks (SSD). It is responsible for managing the virtual to physical address mappings and emulating the functionality of a normal block-level device. SSD performance is highly dependent on the design of the FTL. For the last few years, several FTL schemes have been proposed. Hybrid FTL schemes have gained more popularity since they try to combine the benefits of both page-level mapping and block-level mapping schemes. Examples include BAST, FAST, LAST, etc. To provide high performance, FTL designers face several cross cutting issues: the right balance between coarse and fine grain address mapping, the asymmetric nature of reads and writes, the write amplification property of Flash memory, and the wear-out behavior of flash.n The MapReduce paradigm has become a very popular paradigm for performing parallel and distributed computations on large data. Hadoop, an open-source implementation of MapReduce, has accelerated MapReduce adoption. Flash SSD is increasingly being used as a storage solution in Hadoop deployments for faster processing and better energy utilization. Little work has been done to understand the endurance implications of SSD on Hadoop-based workloads. In this paper, using a highly flexible and reconfigurable kernel-level simulation infrastructure, we investigate the internal characteristics of various hybrid FTL schemes using a representative set of Hadoop workloads. Our investigation brings out the wear-out behavior of SSD for Hadoop-based workloads including wear-leveling details, garbage collection, translation and block/page mappings, and advocates the need for dynamic tuning of FTL parameters for these workloads.

An SMT-Selection Metric to Improve Multithreaded Applications' Performance

Simultaneous multithreading (SMT) increases CPU utilization and application performance in many circumstances, but it can be detrimental when performance is limited by application scalability or when there is significant contention for CPU resources. This paper describes an SMT-selection metric that predicts the change in application performance when the SMT level and number of application threads are varied. This metric is obtained online through hardware performance counters with little overhead, and allows the application or operating system to dynamically choose the best SMT level. We have validated the SMT-selection metric using a variety of benchmarks that capture various application characteristics on two different processor architectures. Our results show that the SMT-selection metric is capable of predicting the best SMT level for a given workload in 90% of the cases. The paper also shows that such a metric can be used with a scheduler or application optimizer to help guide its optimization decisions.

An OS-Hypervisor Infrastructure for Automated OS Crash Diagnosis and Recovery in a Virtualized Environment

Recovering from OS crashes has traditionally been done using reboot or checkpoint-restart mechanisms. Such techniques either fail to preserve the state before the crash happens or require modifications to applications. To eliminate these problems, we present a novel OS-hyper visor infrastructure for automated OS crash diagnosis and recovery in virtual servers. Our approach uses a small hidden OS-repair-image that is dynamically created from the healthy running OS instance. Upon an OS crash, the hyper visor automatically loads this repair-image to perform diagnosis and repair. The offending process is then quarantined, and the fixed OS automatically resumes running without a reboot. Our experimental evaluations demonstrated that it takes less than 3 seconds to recover from an OS crash. This approach can significantly reduce the downtime and maintenance costs in data centers. This is the first design and implementation of an OS-hyper visor combo capable of automatically resurrecting a crashed commercial server-OS.

Towards More Effective Solution Retrieval in IT Support Services Using Systems Log

Technical support agents working in the IT support services field resolve IT problems. They are often faced with the daunting task of identifying the correct solution document through a search system from large corpora of IT support documents. Based on the observation that system logs may contain critical information for identifying the root cause of IT problems, we explore the idea of automatic query expansion by using system logs as a bridge to link queries with the most relevant documents. Given the original query from a user such as a technical support agent, an intermediate query is first formed by adding key terms extracted from system logs using domain-specific rules. Based on topic models, further key terms are selected from corpora of IT support documents, which are combined with the intermediate query to form the final query. Our experimental results show that expanding queries using system logs together with topic models yields better performance in retrieving relevant IT support documents than using topic models only.

A flexible OS-based approach for characterizing solid-state disk endurance

The performance and power benefits of Flash memory have paved its adoption in mass storage devices in the form of Solid-State Disks (SSDs). Despite these benefits, Flash memorys limited write endurance remains a big impediment to its wide adoption in the enterprise server market. Existing research efforts have mostly focused on proposing various mechanisms and algorithms to improve SSDs performance and reliability. However, there is still a lack of flexible tools that allow characterizing SSD endurance (i.e., wear-out behavior) and investigating its impact on applications without affecting the lifetime of the real SSD device. To address this issue, SolidSim, a kernel-level simulator has been enhanced with capabilities to simulate state-of-the-art wear-leveling, garbage-collection and other advanced internal management techniques of an SSD. These extensions have further increased SolidSims flexibility to study both SSD performance and endurance characteristics. Our approach allows investigating these characteristics without requiring any changes to applications or gathering any workload traces. The paper presents insights into wear-out behavior including logical, physical and translation characteristics, and correlates them with application behavior and SSD life-times using a set of representative workloads.

A Hybrid Approach for Large Cache Performance Studies

Recent technology trends are leading to the possibility of computer systems having last level caches significantly larger than those that exist today. Traditionally, cache effectiveness has been modeled through trace-driven simulation tools, however, these tools are not up to the task of modeling very large caches. Because of the limited length of available traces, the tools cannot capture behavior across long enough periods of time to adequately simulate a very large cache. We present mprofiler, a tool that characterizes the memory access pattern of workloads, and present a novel hybrid modeling technique that models cache behavior across a much larger time scale than previously possible. Our methodology combines memory access patterns captured by different tools (e.g., mprofiler) at different time scales and develops analytical techniques that allow spanning the required time frame and predicting the performance of very large caches.

Towards an immortal operating system in virtual environments

We show how a commercial OS can be successfully recovered from a crash.Support from the virtualization layer (Hypervisor) can significantly help in diagnosis and recovery of the OS.We evaluate the time taken to automatically recover from an OS crash for different workloads.This technology can significantly reduce the downtime and maintenance costs in data centers.This technology can be easily integrated into the support operations of OS vendors. Many OS crashes are caused by bugs in kernel extensions or device drivers while the OS itself may have been tested rigorously. To make an OS immortal we must resurrect the OS from these crashes. We present a novel OS-hypervisor infrastructure that allows automated and transparent OS crash diagnosis and recovery in a virtual environment. This infrastructure eliminates the need for reboots or checkpoint-restart mechanisms, which require preserving the states of critical applications before the crash happens and also require extensive modifications to those applications. At the core of our approach is a small hidden OS-repair-image that is dynamically created from the healthy running OS instance. When an OS crashes, the hypervisor dynamically loads this repair-image to perform diagnosis and repair. One way of repair we have experimented with, is to quarantine the offending process and resume the running of the fixed OS automatically without a reboot. Experimental evaluations demonstrated that it takes less than 3s to recover from an OS crash. This approach can significantly reduce the downtime and maintenance costs in data centers, and is the first design and implementation of an OS-hypervisor combo capable of automatically resurrecting a crashed commercial server-OS. In addition to online diagnosis and recovery, this infrastructure can also be used for offline diagnosis and can be incorporated into the technical support tools of the OS vendor. Additionally, we have used parts of this infrastructure to speed-up the diagnosis of AIX OS-crashes for the IBM technical support teams.