Time Sources and Timing David Ferry, Chris Gill CSE 522S - Advanced Operating Systems Washington University in St. Louis St. Louis, MO

Questions From Last Time Q: Why does the kernel create an init thread at boot instead of using the original execution context? – Robert A: The start_kernel execution context in main.c eventually becomes the main processor idle loop, a.k.a. the swapper or sched process. (Follow the start_kernel function through to cpu_idle_loop() ) CSE 522S – Advanced Operating Systems2

Questions From Last Time Q: Do we really need to issue make clean when rebuilding the kernel? – John A: Not always, but probably… CSE 522S – Advanced Operating Systems3 foo.c unistd.h #define NR_SYSCALLS 388 unistd.h #define NR_SYSCALLS 388 foo.o NR_SYSCALLS = 388 foo.o NR_SYSCALLS = 388

Questions From Last Time Q: Do we really need to issue make clean when rebuilding the kernel? – John A: Not always, but probably… CSE 522S – Advanced Operating Systems4 foo.c bar.c unistd.h #define NR_SYSCALLS 292 unistd.h #define NR_SYSCALLS 292 foo.o NR_SYSCALLS = 388 foo.o NR_SYSCALLS = 388 bar.o NR_SYSCALLS = 292 bar.o NR_SYSCALLS = 292

What is meant by Time? Absolute time since some past event, E.g.: – Seconds since Unix Epoch – Seconds since system boot Relative time, E.g.: – Seconds between two events – Ten seconds into the future – Execution time of a program segment World time, E.g.: – February 4 th, 10:37 AM An OS must approximate time to provide time-based functions for users. CSE 522S – Advanced Operating Systems5

Time Sources RTC (Real-Time Clock) – Available on most computers (not on RPI 2) – Low precision (as low as 0.5 seconds) Hardware Timers – Might be used to generate interrupts, might be queryable – Run at a variety of frequencies – Programmable Interval Timer (PIT) – High-Performance Event Timer (HPET) – Programmable Interrupt Controller (PIC) – Advanced Programmable Interrupt Controller (APIC) Processor Cycles – Timestamp Counter (TSC) on x86, 64-bit – Cycle Counter (CCNT) on ARM, 32-bit, 64-cycle divider, not accessible in user mode – Potentially very high accuracy – Can generate interrupts on x86 with TSC-deadline CSE 522S – Advanced Operating Systems6

How does the kernel schedule services? Three approaches: 1.Timer interrupts to regularly provide service e.g.: task scheduling, current time update 2.Kernel timers ( hrtimers, legacy timers) 3.Kernel threads (not time dependent) A portable OS must use whatever timer hardware is available to support actions #1 and #2. CSE 522S – Advanced Operating Systems7

Basic Timer Interrupt in Linux Historically the OS would run periodically: – Fundamental timestep is variable HZ found in /include/asm-generic/param.h – OS function tick_periodic() runs each 1/HZ seconds – jiffies variable tracks number of ticks since boot – xtime variable tracks wall-clock time – Current time since boot: jiffies * 1/HZ – Current wall time: boot_time + jiffies * 1/HZ – Timers are checked for expiry each tick As a modern approach: – Excessive power consumption (can’t really idle processors) – Inappropriate for highly virtualized systems – Might be appropriate for real-time systems CSE 522S – Advanced Operating Systems8

Modern Timer Interrupt in Linux Two fundamental operation modes: – Periodic ( CLOCK_EVT_MODE_PERIODIC ) – One-shot ( CLOCK_EVT_MODE_ONESHOT ) One-shot mode ( CONFIG_NO_HZ ): – All timer events are independent (not periodic) – Next timer event is scheduled for: Next hrtimer expiration Next 1/HZ interval, whichever is sooner Not rescheduled if no runnable tasks – jiffies is maintained as though periodic – Big power savings in idle CSE 522S – Advanced Operating Systems9

Problems with legacy time system Legacy timers could only expire each tick – Latency problems: with 10ms tick a timeout might be delivered 9.9ms late – Resolution problems: with a 10ms tick all we know was that timeout was 0-9.9ms ago Legacy timer wheel implementation problems: – Bad worst case expiration running time (O(N)) – 95-99% of timers never actually expire (e.g. http timeouts, error checking) but still incur overhead – Thus, not optimized for “millions of network timers” case CSE 522S – Advanced Operating Systems10

hrtimers subsystem The kernel now provides a high resolution timing subsystem: hrtimers – Not bound to jiffies – Can use multiple time sources – High resolution (nanosecond representation) Implementation: – Per-CPU timer list stored as a sorted red-black tree for fast expiration – Separate list for fast removal (no tree walking) – Expiration handled by dedicated interrupts CSE 522S – Advanced Operating Systems11