Vassal: Loadable Scheduler Support for Multi-Policy Scheduling

1. Vassal: Loadable Scheduler Support for Multi-Policy Scheduling George M. Candea, Oracle Corporation Michael B. Jones, Microsoft Research

2. Loadable Scheduler Overview Loadable schedulers special Windows NT 4.0 drivers Coexist with standard Windows NT scheduler Applications can use scheduling algorithms tailored to their needs System can provide behaviors not otherwise available Only one loadable scheduler loaded at once in this prototype

3. Windows NT Scheduling Threads schedulable unit Threads in one of several states: Running, Standby, Ready, Waiting, Terminated Dispatcher schedules threads based on: Priority Processor Affinity

4. Windows NT Priorities 32 priorities in three classes: Real-Time (16-31) Variable (1-15) Idle (0) Real-Time threads Use strict priority scheduling Round-robin within priority Variable threads Subject to priority boosts, decays

5. Unscheduled CPU Time Interrupts Deferred Procedure Calls (DPCs) Interrupts needing substantial work queue DPCs

6. Deferred Procedure Calls DPCs: Are preempted by interrupts Preempt normal threads May not block Are run in FIFO order Typical Uses: I/O Completion Processing Background Driver Housekeeping

7. Scheduling Events Reschedules triggered by: Quantum end Priority or affinity changes Blocking Wakeups Timers queue DPCs Which may wake up threads

8. Timer Management Hardware Abstraction Layer (HAL) provides kernel periodic timer Resolution selectable from 1 to 15ms Default typically 10ms or 15ms Not all HALs implement all values MP HAL provides 1, 2, 4, 8, 16ms Some HALs just implement 10ms!

9. Windows NT Drivers General kernel extension mechanism Kernel loaded dynamic link libraries Drivers implement facilities such as: Filesystems Network protocols Hardware device drivers Vassal schedulers implemented as Windows NT drivers

10. Loadable Scheduler Infrastructure Four tasks (T1-T4) with two schedulers (A & B)

11. Primitives Added by Vassal RegisterScheduler(scheduler, decision_maker, message_dispatcher) Used by schedulers to register with system MessageToScheduler(scheduler, buffer, buflen) Used by threads to communicate with loaded schedulers SetSchedulerEvent(scheduler, performance_counter_reading) Used by schedulers to request that call to decision function be made at specific time

12. Integration of Vassal into Windows NT 4.0

13. Use of Real-Time Scheduler /* Tell system to use the real-time scheduler */ status = MessageToScheduler (rt_sched, {JOIN}); if (status != SUCCESS) { error (“Could not join R/T scheduling class.”); } /* Calculate how long our loop iterations take */ estimate = Calibrate(); /* Start the loop 1 ms from now */ status = MessageToScheduler (rt_sched, {SET, 1000}); if (status != SUCCESS) { error (“Could not set deadline.”); } /* We want one iteration every 300 µs */ while (1) { status = MessageToScheduler (rt_sched, {SET, 300 – estimate}); … }

14. Execution of the Sample Code

15. Context Switch Times Measured context switch times on Pentium-133 running original and modified systems (in µs)

16. Periodic Wakeup Times Periodic wakeup times on Pentium-133 using multimedia timers on original system and sample scheduler’s events on modified system (in µs). Desired value is 1000µs.