Real-Time Disk Scheduling in a Mixed-Media File System

This paper presents our real-time disk scheduler called the L scheduler, which optimizes unscheduled best-effort disk requests by giving priority to best-effort disk requests while meeting real-time request deadlines. Our scheduler tries to execute real-time disk requests as much as possible in the background. Only when real-time request deadlines are endangered, our scheduler gives priority to real-time disk requests. The L disk scheduler is part of our mixed-media file system called Clockwise. An essential part of our work are extensive and detailed raw disk performance measurements. These raw performance measurements are used by the L disk scheduler for its real-time schedulability analysis and to decide whether scheduling a best-effort request before a real-time request violates real-time constraints. Further, the raw performance measurements are used by a Clockwise off-line simulator where a number of different disk schedulers are compared. We compare the L scheduler with a prioritizing Latest Start Time (LST) scheduler and non-prioritizing EDF scheduler. The L scheduler is comparable to LST in achieving low latencies for best-effort requests under light to moderate real-time loads and better in achieving low latencies for best-effort requests for extreme real-time loads. The simulator is calibrated to an actual Clockwise. Clockwise runs on a 200 MHz Pentium-Pro based PC with PCI bus, multiple SCSI controllers and disks on Linux 2.2.x and the Nemesis kernel. Clockwise's performance is dictated by the hardware: all available bandwidth can be committed to real-time streams, provided hardware overloads do not occur.