1. Time Sources and Timing
David Ferry, Chris Gill
CSE 422S - Operating Systems Organization
Washington University in St. Louis
St. Louis, MO 63143

2. CSE 422S – Operating Systems Organization
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 422S – Operating Systems Organization

3. CSE 422S – Operating Systems Organization
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
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4. 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 422S –Operating Systems Organization

5. 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 422S –Operating Systems Organization

6. 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
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7. 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 (or jiffie, whichever is sooner)
Not rescheduled if no runnable tasks
jiffies is maintained as though periodic
Big power savings in idle
CSE 422S –Operating Systems Organization

8. CSE 422S –Operating Systems Organization
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 422S –Operating Systems Organization

9. Time Measurement Nuances
The timespec struct is declared in <time.h>
Stores time info (in seconds and nanoseconds)
struct timespec {
time_t tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */ };
May need to normalize when adding, subtracting, etc.
temp.tv_sec = t2.tv_sec - t1.tv_sec; // ok: monotonically non-decreasing
temp.tv_nsec = t2.tv_nsec - t1.tv_nsec; // could give negative value
Watch out for overflow (use unsigned long long etc.)
Today’s studio will give you experience measuring time (and timer resolution) in user space programs
CSE 422S –Operating Systems Organization