Understanding Timers and Interrupts in Operating Systems

This content discusses the importance of timers and interrupts in operating systems, focusing on how they facilitate preemptive multitasking. It covers how hardware timer interrupts help in preemption of processes by the kernel, ensuring efficient task scheduling and resource management in modern operating systems.

• Operating Systems
• Timers
• Interrupts
• Preemptive Multitasking
• Process Scheduling

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1. Timers and Interrupts Marion Sudvarg, David Ferry, Chris Gill, Brian Kocoloski CSE 522S Advanced Operating Systems Washington University in St. Louis St. Louis, MO 63143 1

2. Preemptive Multitasking Last Time: CFS and CPU cgroups These mechanisms decide which, and for how long, a process can run But, how does the kernel stop a process? A system call switches execution context into the kernel but this is not sufficient What about a process that never (1) voluntarily yields the CPU or (2) makes a syscall? Must be somehow preempted to support multitasking! CSE 522S Advanced Operating Systems 2

3. CSE 422S Review: Timer Interrupts Hardware supports periodically-timed preemption of processes by the kernel System timer: fires at known periodic intervals Allows the kernel to say to the CPU: stop the currently running process and let me run after some period of time has expired Kernel programs timer interrupts to fire at known points in the future E.g., program the system timer to always interrupt the processor every 10 ms 3 CSE 522S Advanced Operating Systems

4. CSE 422S Review: Timer interrupts ./test CPU core (executing instructions) User space Kernel space Timer (hardware that supports delivery of interrupts) 4 CSE 522S Advanced Operating Systems

5. CSE 422S Review: Timer interrupts ./test ./test CPU core (executing instructions) User space Kernel space Timer (hardware that supports delivery of interrupts) 5 CSE 522S Advanced Operating Systems

6. CSE 422S Review: Timer interrupts ./test ./test Interrupt! CPU core (executing instructions) User space Kernel space Timer (hardware that supports delivery of interrupts) 6 CSE 522S Advanced Operating Systems

7. CSE 422S Review: Timer interrupts ./test ./test Interrupt! CPU core (executing instructions) User space Kernel space Timer Timer interrupt (hardware that supports delivery of interrupts) Interrupt forces the CPU to enter kernel mode where a special function called the timer interrupt will execute 7 CSE 522S Advanced Operating Systems

8. CSE 422S Review: Timer interrupts ./test ./test Interrupt! CPU core (executing instructions) User space Kernel space Timer Timer interrupt (hardware that supports delivery of interrupts) Interrupt forces the CPU to enter kernel mode where a special function called the timer interrupt will execute 8 CSE 522S Advanced Operating Systems

9. CSE 422S Review: Timer interrupts ./test User space Kernel space Timer interrupt Timer interrupt Timer interrupt 9 CSE 522S Advanced Operating Systems

10. Timer Hardware Programmable Interval Timer (PIT) Has 3 programmable counter registers Each register counts down at a constant rate When a register reaches 0, it sends an interrupt High Precision Event Timer (HPET) Similar to PIT More complex logic, allows finer-grained control Advanced Programmable Interrupt Controller (APIC) For Intel x86 hardware Can be set to TSC-Deadline mode Generates an IRQ when the TSC (timestamp counter) reaches programmed deadline BCM System Timer Raspberry Pi board system timer Has a single constant-rate counter 4 programmable registers send unique interrupts when they match the timer ARM PMU Can be programmed to generate interrupts when a counter overflows Set counter to (OVERFLOW COUNT) to receive interrupt when COUNT is reached 10 CSE 522S Advanced Operating Systems

11. Questions How can timer interrupts be used to determine an absolute, real-world measurement of time? How can timer interrupts be used to implement arbitrary user timers? (e.g., when user executes sleep(5)) 11 CSE 522S Advanced Operating Systems

12. Types of Timers Relativevs Absolute Set a timer for 2 seconds from now sleep(2) Set an alarm at 5:29:31 Periodic Set a timer to go off every 10ms (kernel timer interrupt) (poll a sensor) CSE 522S Advanced Operating Systems 12

13. Timer Implementation How are they implemented under the hood ? Timer Hardware Timeout Priority Queue Timer Insert Absolute Timeout Register Next Timeout Time Elapses Timer Hardware CPU Core Interrupt! 13 CSE 522S Advanced Operating Systems

14. Periodic Timer Implementation What s the problem? set(now + 10ms) timer(10ms,PERIODIC) Hardware 10 ms 10 ms signal timer(10ms,PERIODIC) Hardware >20 ms ?? set(now + 10ms) timer(10ms,PERIODIC) Hardware Timer Drift 10 ms 10 ms signal timer(10ms,PERIODIC) Hardware CSE 522S Advanced Operating Systems 14

15. Correct Implementation INIT: timeout = now timeout += 10ms set(timeout) timer(10ms,PERIODIC) Hardware 10 ms 10 ms Signal timer(10ms,PERIODIC) Hardware 20 ms timeout += 10ms set(timeout) timer(10ms,PERIODIC) Hardware 10 ms 10 ms Signal timer(10ms,PERIODIC) Hardware CSE 522S Advanced Operating Systems 15

16. Other Interrupts So far we ve discussed timer interrupts Other devices generate interrupts too: Keyboard/Mouse Network Card: Packets available! Hard Drive: Disk read complete! Sensors in Cyber-Physical Systems: Tolerance limit exceeded! CSE 522S Advanced Operating Systems 16

17. ARM Interrupt Overview Interrupt lines are numbered IRQ sent with line # to CPU CPU jumps to interrupt table, looks up line number (similar concept to software traps, syscalls) Lines typically edge-triggered CPU must clear line to receive new interrupts on that line Caution: Multiple devices might share a line! External Interrupt Sources Generic Interrupt Controller (GIC) nIRQ FIQ: ARM-specific Fast-path for single interrupt source Removes overhead of source lookup Has own bank of registers Eliminates context switch overhead Timers CPU CSE 522S Advanced Operating Systems 17

18. Todays Readings LKD Chapter 7: Coverage of hardware interrupts and handlers LKD Pages 216-220: Read the Hardware Clocks and Timers and The Timer Interrupt Handler sections LKD Pages 222-224: An overview of timers LSP Chapter 11: An overview of time and timer mechanisms provided to userspace (Optional) LPI Chapter 23: An in-depth reference for userspace time APIs (Optional) Linux Device Drivers Chapter 10: In-depth description of interrupt handling from a driver developer s perspective, free PDF linked from website CSE 522S Advanced Operating Systems 18