1. Scheduling 6.894 Lecture 6

2. What is Scheduling? An O/S often has many pending tasks. –Threads, async callbacks, device input. The order may matter. –Policy, correctness, or efficiency. Providing sufficient control is not easy. –Mechanisms must allow policy to be expressed.

3. Scheduling Policy Examples Allocate cycles in proportion to money. Maintain high throughput under high load. Never delay high pri thread by > 1ms. Maintain good interactive response. Can we enforce policy with the thread scheduler?

4. Pitfall: Priority Inversion Low-priority thread X holds a lock. High-priority thread Y waits for the lock. Medium-priority thread Z pre-empts X. Y is indefinitely delayed despite high priority.

5. Pitfall: Long Code Paths Large-granularity locks are convenient. –Non-pre-emptable threads are an extreme case. May delay high-priority processing.

6. Pitfall: Efficiency Efficient disk use requires unfairness. –Shortest-seek-first vs FIFO. –Read-ahead vs data needed now. Efficient paging policy creates delays. –O/S may swap out my idle Emacs to free memory. –What happens when I type a key? Thread scheduler doesn’t control these.

7. Pitfall: Multiple Schedulers Every resource with multiple waiting threads has a scheduler. Locks, disk driver, memory allocator. The schedulers may not cooperate or even be explicit.

8. Pitfall: Server Processes User-level servers schedule requests. –X11, DNS, NFS. They usually don’t know about kernel’s scheduling policy. Network packet scheduling also interferes.

9. Pitfall: Hardware Schedulers Memory system scheduled among CPUs. I/O bus scheduled among devices. Interrupt controller chooses next interrupt. Hardware doesn’t know about O/S policy. O/S often doesn’t understand hardware.

10. Scheduling is a System Problem Thread/process scheduler can’t enforce policies by itself. Needs cooperation from: –All resource schedulers. –Software structure. Conflicting goals may limit effectiveness.

11. Example: UNIX Goals: –Simple kernel concurrency model. Limited pre-emption. –Quick response to device interrupts. Many kinds of execution environments. Some transitions are not possible. Some transitions can’t be controlled.

12. UNIX Environments User Kernel Half Process User Half Kernel Half Process User Half Timer Soft Interrupt Network Soft Interrupt Device Interrupt Device Interrupt Timer Interrupt

13. UNIX: Process User Half Interruptable. Pre-emptable via timer interrupt. –We don’t trust user processes. Enters kernel half via system calls, faults. –Save user state on stack. –Raise privilege level. –Jump to known point in the kernel. Each process has a stack and saved registers.

14. UNIX: Process Kernel Half Executes system calls for its user process. –May involve many steps separated by sleep(). Interruptable. –May postpone interrupts in critical sections. Not pre-emptable. –Simplifies concurrent programming. –No context switch until voluntary sleep(). –No user process runs if a kernel half is runnable. Each kernel half has a stack and saved registers. Many processes may be sleep()ing in the kernel.

15. UNIX: Device Interrupts Device hardware asks CPU for an interrupt. –To signal new input or completion of output. –Cheaper than polling, lower latency. Interrupts take priority over u/k half. –Save current state on stack. –Mask other interrupts. –Run interrupt handler function. –Return and restore state. The real-time clock is a device.

16. UNIX: Soft Interrupts Device interrupt handlers must be short. Expensive processing deferred to soft intr. –Can’t do it in kernel-half: process not known. –Example: TCP protocol input processing. –Example: periodic process scheduling. Devices can interrupt soft intr. Soft intr has priority over user & kernel processes. –But only entered on return from device intr. –Similar to async callback. –Can’t be high-pri thread, since no pre-emption.

17. UNIX Environments User Kernel Half Process User Half Kernel Half Process User Half Soft Interrupt Device Interrupt Transfer w/ choice Transfer, limited choice Transfer, no choice