1. Time Sources and Timing
David Ferry, Chris Gill, Brian Kocoloski
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?
… to a computer 
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3. CSE 422S –Operating Systems Organization
What is meant by Time?
… to a computer 
Q: is time a resource?
Q: or, is time a means with which to share other resources?
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4. CSE 422S –Operating Systems Organization
What is meant by Time?
… to a computer 
Q: is time a resource?
Q: or, is time a means with which to share other resources?
What time of day is it? ($ date)
Wake me up in 5 seconds ($ sleep 5)
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5. 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.:
January 30th, 9:00 AM
An OS must approximate time to provide time-based functions for users.
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6. How does the kernel keep track of time?
Some hardware support
Real-time clock: track system time even when machine is off
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
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
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7. Timer interrupts ./test CPU core (executing instructions) User space
Kernel space
Timer
(hardware that supports delivery of interrupts)
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8. Timer interrupts ./test ./test CPU core (executing instructions)
User space
Kernel space
Timer
(hardware that supports delivery of interrupts)
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9. Timer interrupts ./test ./test Interrupt! CPU core
(executing instructions)
User space
Kernel space
Timer
(hardware that supports delivery of interrupts)
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10. Timer interrupts ./test ./test Interrupt! CPU core
(executing instructions)
User space
Kernel space
Timer
(hardware that supports delivery of interrupts)
Timer interrupt
Interrupt forces the CPU to enter kernel mode where a special function called the timer interrupt will execute
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11. Timer interrupts ./test ./test Interrupt! CPU core
(executing instructions)
User space
Kernel space
Timer
(hardware that supports delivery of interrupts)
Timer interrupt
Interrupt forces the CPU to enter kernel mode where a special function called the timer interrupt will execute
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12. CSE 422S –Operating Systems Organization
Timer interrupts
./test
User space
Kernel space
Timer interrupt
Timer interrupt
Timer interrupt
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13. 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)
Comparatively low precision (as low as 0.5 seconds)
Higher Precision 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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14. CSE 422S –Operating Systems Organization
Questions
How can timer interrupts used to determine an absolute, real-world measurement of time? How can time interrupts be used to implement arbitrary user timers? (e.g., when user executes sleep(5))
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15. CSE 422S –Operating Systems Organization
Timer Tick Rate: HZ
The kernel operates on the granularity of timer interrupts
Every 1/HZ seconds, stop the currently running process and figure out what to do next (run another process, run kernel-level services, etc.)
What should HZ be set to?
High HZ = frequent timer interrupts
Low HZ = infrequent timer interrupts
Tradeoff between high throughput and low latency
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16. CSE 422S –Operating Systems Organization
Latency vs Throughput
Target: low latency, high throughput
Throughput
Latency
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17. CSE 422S –Operating Systems Organization
Latency vs Throughput
Target: low latency, high throughput .
Throughput . . .
10 HZ
Latency
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18. CSE 422S –Operating Systems Organization
Latency vs Throughput
Target: low latency, high throughput .
Throughput .
Where should 100 HZ go? . .
10 HZ
Latency
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19. CSE 422S –Operating Systems Organization
Latency vs Throughput
Target: low latency, high throughput .
1 HZ
Throughput . . .
10 HZ
100 HZ
1000 HZ
Latency
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20. Basic Timer Interrupt in Linux
Historically the OS would run periodically:
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:
Unnecessary interrupts when only few processes running
Excessive power consumption (can’t really idle processors)
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21. 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 interrupt is programmed for the next timer expiration
Not rescheduled if no runnable tasks
jiffies is maintained as though periodic
Big power savings in idle
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22. Measuring Time in Userspace
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
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