|Authors||K. Lund, V. Goebel and T. Plagemann|
|Title||APEX: Adaptive Disk Scheduling Framework With QoS-Support|
|Publication Type||Journal Article|
|Year of Publication||2005|
|Journal||ACM Multimedia Systems Journal|
APEX is an adaptive disk scheduling framework with QoS (Quality-of-Service) support, designed for environments with highly varying disk bandwidth usage. APEX is based on a three-layer scheduling architecture: (1) the upper layer realizes different service classes using a set of queues; (2) the mid-layer distributes available disk bandwidth among these queues; and (3) the lower layer is handled by the disk itself, which does the final ordering of disk requests. We demonstrate the use of APEX in an example scenario, a Learning-on-Demand application supported by a multimedia system, where students can search for, and play back multimedia-based learning material. In this paper, we present the scheduling concepts of APEX, which are based on an extended token bucket algorithm. The disk requests scheduled for service are assembled into batches, in order to exploit the intelligence of modern disks. Combined with a specialized work-conservation scheme, this enables APEX to apply bandwidth where it is needed, without loss of efficiency. We demonstrate, through simulations, that APEX provides both higher throughput and lower response times than other mixed-media disk schedulers, while still avoiding deadline violations for real-time requests. We also show its robustness with respect to misaligned bandwidth allocation.