1. Time Sources and Timing
David Ferry, Chris Gill
CSE 522S - Advanced Operating Systems
Washington University in St. Louis
St. Louis, MO 63143

2. Questions About the Previous Studio
Q: Why does the kernel create an init thread at boot instead of using the original execution context? 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 Systems

3. Questions About the Previous Studio
Q: Do we really need to issue make clean when rebuilding the kernel? A: Not always, but probably a good idea to do so anyway
unistd.h
#define NR_SYSCALLS 292
baz.c
foo.c
foo.o
NR_SYSCALLS = 388
baz.o
NR_SYSCALLS = 292
CSE 522S – Advanced Operating Systems

4. Today: What is meant by Time?
Absolute time since some fixed past event, e.g.:
Seconds since start of the Unix Epoch
(00:00:00 UTC on 1 January 1970)
Seconds since system boot
Relative time, E.g.:
Seconds between two events
Ten seconds into the future (from now)
Execution time of a program segment
World time, E.g.:
February 4th, 10:37 AM
An OS must approximate time to provide time-based functions for users.
CSE 522S – Advanced Operating Systems

5. CSE 522S – Advanced Operating Systems
Time Sources
RTC (Real-Time Clock)
Available on most computers (not on RPi 2 or 3 unless you add it)
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 Systems

6. How does the kernel schedule services?
Three approaches:
Timer interrupts to regularly provide service e.g.: task scheduling, current time update
Kernel timers (hrtimers, legacy timers)
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 Systems

7. 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
Concerns with this 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 Systems

8. Modern Timer Interrupt in Linux
Two fundamental operating 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 Systems

9. 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 Systems

10. CSE 522S – Advanced Operating Systems
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 Systems