Real-time Disk Scheduling Algorithm Research Articles

A New Optimized Real-Time Disk Scheduling Algorithm

this paper, a new approach of disk scheduling has been proposed to improve the throughput of modern storage device. After the invention of disk with movable head, researchers are making efforts continuously to improve the I/O performance by implementing many intelligent scheduling algorithms of disk accesses. Speeds of processor and memory capacity have been increased several times than the speed of the disk. Due to this difference in the speed of processor and the disk, I/O performance of disk has become an important bottleneck. Therefore, it is needed to develop some advanced methods for using disk more efficiently. Management of disk performance is an important aspect of operating system research and development. In this paper, a new disk scheduling algorithm has been developed and implemented to reduce the number of head movements. Experiment has been carried out to compare the performance of proposed algorithm with the performance of all six classical disk scheduling algorithms. Experiment shows that the numbers of head movements in existing classical disk scheduling algorithms such as FCFS, SSTF, SCAN, C-SCAN, LOOK and C-LOOK are high. But in proposed new optimized real-time disk scheduling algorithm, head movements are reduced and hence it maximizes the throughput for the modern storage devices. KeywordsSSTF, SCAN, C-SCAN, LOOK, C-LOOK, Scheduling

A new real time disk-scheduling method based on GSR algorithm

Disk scheduling has an important role in QOS guarantee of soft real-time environments such as video-on-demand and multimedia servers. Since now, some disk-scheduling algorithms have been proposed to schedule real-time disk requests. One of the most recent algorithms is global seek-optimizing real-time (GSR) that schedules the disk requests with different ready times by a global regrouping scheme. In the present paper, we propose a real-time disk-scheduling algorithm based on GSR that is called IGSR (improved GSR). IGSR creates the scan-groups of the requests and tries to find a good feasible schedule by optimized grouping with considering another chance for tasks that miss their deadlines at initial grouping. With regard to the admission policy of tasks, two different version of proposed method are presented: the first one has been designed for the case that all the disk requests available simultaneously and second one has been designed for the case that requests are admitted dynamically (GSR does not support the second one). It means that in the second case, the request queue may change when a task is running but in the first one it does not change. Simulation results showed IGSR outperformed GSR and some other related works in terms of maximum supportable streams, number of missed deadlines, and disk throughput.

An Efficient Real-Time Disk-Scheduling Framework with Adaptive Quality Guarantee

A multimedia server requires a real-time disk-scheduling algorithm to deliver isochronous data for real-time streams. Traditional disk-scheduling algorithms focus on providing good quality in a best effort manner. In this paper, we propose a novel real-time disk-scheduling algorithm called WRR-SCAN (weighted-round-robin-SCAN) to provide quality guarantees for all in-service streams encoded at variable bit rates and bounded response times for aperiodic jobs. WRR-SCAN divides a real-time stream into guaranteed jobs and optional jobs. The admission control admits a stream as long as its guaranteed jobs are satisfied. Such a decision is made in 0(1) time as WRR-SCAN reserves a fixed weight for each stream. WRR-SCAN incorporates an aggressive policy to dynamically reclaim unused bandwidth during runtime. The reclaimed bandwidth is used to serve optional jobs or more aperiodic jobs. We conducted a set of simulations to compare WRR-SCAN with a set of referred disk-scheduling algorithms. The evaluations are conducted on a commonly used disk simulator with traces from a real multimedia server. The experimental results show that WRR-SCAN provides significantly better quality for real-time streams and yields considerably shorter response times for aperiodic jobs.

GSR: A global seek-optimizing real-time disk-scheduling algorithm

Earliest-deadline-first (EDF) is good for scheduling real-time tasks in order to meet timing constraint. However, it is not good enough for scheduling real-time disk tasks to achieve high disk throughput. In contrast, although SCAN can maximize disk throughput, its schedule results may violate real-time requirements. Thus, during the past few years, various approaches were proposed to combine EDF and SCAN (e.g., SCAN-EDF and RG-SCAN) to resolve the real-time disk-scheduling problem. However, in previous schemes, real-time tasks can only be rescheduled by SCAN within a local group. Such restriction limited the obtained data throughput. In this paper, we proposed a new globally rescheduling scheme for real-time disk scheduling. First, we formulate the relations between the EDF schedule and the SCAN schedule of input tasks as EDF-to-SCAN mapping (ESM). Then, on the basis of ESM, we propose a new real-time disk-scheduling algorithm: globally seek-optimizing rescheduling (GSR) scheme. Different from previous approaches, a task in GSR may be rescheduled to anywhere in the input schedule to optimize data throughput. Owing to such a globally rescheduling characteristic, GSR obtains a higher disk throughput than previous approaches. Furthermore, we also extend the GSR to serve fairly non-real-time tasks. Experiments show that given 15 real-time tasks, our data throughput is 1.1 times that of RG-SCAN. In addition, in a mixed workload, compared with RG-SCAN, our GSR achieves over 7% improvement in data throughput and 33% improvement in average response time.

Multi-dimensional storage virtualization

Most state-of-the-art commercial storage virtualization systems focus only on one particular storage attribute, capacity. This paper describes the design, implementation and evaluation of a multi-dimensional storage virtualization system called Stonehenge, which is able to virtualize a cluster-based physical storage system along multiple dimensions, including bandwidth, capacity, and latency. As a result, Stonehenge is able to multiplex multiple virtual disks, each with a distinct bandwidth, capacity, and latency attribute, on a single physical storage system as if they are separate physical disks. A key enabling technology for Stonehenge is an efficiency-aware real-time disk scheduling algorithm called dual-queue disk scheduling, which maximizes disk utilization efficiency while providing Quality of Service (QoS) guarantees. To optimize disk utilization efficiency, Stonehenge exploits run-time measurements extensively, for admission control, computing latency-derived bandwidth requirement, and predicting disk service time.

Cache-Aware Real-Time Disk Scheduling

Previous real-time disk scheduling algorithms assume that each disk request incurs a physical disk mechanical operation and only consider how to move the disk head under real-time constraints. However, with the increased capacity of on-disk cache, modern disk drives read-ahead data aggressively. Thus, the on-disk cache may service many disk requests without incurring physical disk access. By exploring the design methodology of on-disk cache, in this paper, we propose cacheaware real-time disk scheduling algorithms that take the on-disk cache into consideration during scheduling. Therefore, the scheduling algorithm can help the cache replacement scheme to minimize the cache miss ratio. Besides, the service timing estimation is more accurate in schedulability analysis since the cache effect is considered during scheduling. A simulation-based evaluation shows the proposed scheduling algorithms to be highly successful as compared with the classical real-time disk scheduling algorithms. For example, under sequential workload with 10 sequential streams, the data throughput of our scheme is 1.1 times that of DM-SCAN.

Reschedulable-Group-SCAN scheme for mixed real-time/non-real-time disk scheduling in a multimedia system

Real-time disk scheduling is important for time-critical multimedia applications. Previous approaches, such as SCAN-earliest deadline first (EDF) or DM-SCAN, applied the SCAN scheme to reschedule service sequence of input tasks to reduce tasks' service time. However, they required the input to be in EDF order. In DM-SCAN, a deadline modification scheme was employed to obtain a pseudo-EDF sequence from non-EDF ordered input tasks. Because the modified deadlines were smaller than the original ones, number of tasks that could be rescheduled is decreased and thus data throughput is reduced. In this paper, we propose Reschedulable-Group-SCAN (RG-SCAN), a new real-time disk scheduling algorithm using the concept of Reschedulable-Group (R-Group). Differing from previous approaches, RG-SCAN has no limitation on the input task's sequence. In addition, by exploiting the service time's reduction after rescheduling in each R-Group, RG-SCAN has been extended to serve mixed real-time and non-real-time workloads. As shown in experimental results, our approach can support more tasks than DM-SCAN, both real-time and non-real-time. Additionally, our approach can provide larger data throughput and offer better response time to non-real-time tasks. For example, given 30 random-generated real-time tasks, the number of non-real-time tasks that can be supported by RG-SCAN is 1.3 times that supported by DM-SCAN. In addition, our data throughput is 1.1 times DM-SCAN's.

Disk scheduling in video editing systems

Modern video servers support both video-on-demand and nonlinear editing applications. Video-on-demand servers enable the user to view video clips or movies from a video database, while nonlinear editing systems enable the user to manipulate the content of the video database. Applications such as video and news editing systems require that the underlying storage server be able to concurrently record live broadcast information, modify prerecorded data, and broadcast an authored presentation. A multimedia storage server that efficiently supports such a diverse group of activities constitutes the focus of this study. A novel real-time disk scheduling algorithm is presented that treats both read and write requests in a homogeneous manner in order to ensure that their deadlines are met. Due to real-time demands of movie viewing, read requests have to be fulfilled within certain deadlines; otherwise, they are considered lost. Since the data to be written into disk is stored in main memory buffers, write requests can be postponed until critical read requests are processed. However, write requests still have to be processed within reasonable delays and without the possibility of indefinite postponement. This is due to the physical constraint of the limited size of the main memory write buffers. The new algorithm schedules both read and write requests appropriately, to minimize the amount of disk reads that do not meet their presentation deadlines, and to avoid indefinite postponement and large buffer sizes in the case of disk writes. Simulation results demonstrate that the proposed algorithm offers low violations of read deadlines, reduces waiting time for lower priority disk requests, and improves the throughput of the storage server by enhancing the utilization of available disk bandwidth.

Real-Time Disk Scheduling for Mirrored Disk Systems

This paper presents a real-time model of the mirrored disk system. Earliest Deadline First(EDF) and Feasible Earliest Deadline First(F-EDF) are real-time disk scheduling algorithms for mirrored disk systems studied in this paper. Comparing real-time and non-real-time scheduling algorithms, it is clear that F-EDF is the best one for real-time mirrored disk systems.

Performance evaluation of two new disk scheduling algorithms for real-time systems

In this paper, we present two new disk scheduling algorithms for real-time systems. The two algorithms, called SSEDO (for Shortest Seek and Earliest Deadline by Ordering) and SSEDV (for Shortest Seek and Earliest Deadline by Value), combine deadline information and disk service time information in different ways. The basic idea behind these new algorithms is to give the disk I/O request with the earliest deadline a high priority, but if a request with a larger deadline is "very" close to the current disk arm position, then it may be assigned the highest priority. The performance of SSEDO and SSEDV algorithms is compared with three other proposed real-time disk scheduling algorithms ED, P-SCAN, and FD-SCAN, as well as four conventional algorithms SSTF, SCAN, C-SCAN, and FCFS. An important aspect of the performance study is that the evaluation is not done in isolation with respect to the disk, but as part of an integrated collection of protocols necessary to support a real-time transaction system. The transaction system model is validated on an actual real-time transaction system testbed, called RT-CARAT. The performance results show that SSEDV outperforms SSEDO; that both of these new algorithms can improve performance of up to 38% over previously-known real-time disk scheduling algorithms; and that all of these real-time scheduling algorithms are significantly better than non-real-time algorithms in the sense of minimizing the transaction loss ratio.