Improving Timing Accuracy with ClockRes Techniques

Measuring and Tuning ClockRes on Linux and WindowsPrecise timekeeping is essential for many software domains: high-frequency trading, multimedia synchronization, real-time control, benchmarking, and distributed systems. One key metric is clock resolution (often called clock granularity or ClockRes)—the smallest measurable or schedulable time quantum your system’s clock and timing APIs provide. This article explains how to measure ClockRes on Linux and Windows, what limits accuracy, and practical tuning techniques to improve timing precision while balancing power and system stability.

What is ClockRes?

Clock resolution is the minimum distinguishable time interval a clock source or timer API exposes to applications. It determines the smallest sleep, wait, or timestamp increment reliably observable by software. Resolution is distinct from accuracy (how close time is to a correct reference) and precision (repeatability of measurements), but it directly affects precision in short intervals.

Common timers:

• On Linux: CLOCK_REALTIME, CLOCK_MONOTONIC, CLOCK_MONOTONIC_RAW, CLOCK_REALTIME_COARSE, CLOCK_MONOTONIC_COARSE, and high-resolution timers (hrtimers).
• On Windows: QueryPerformanceCounter (QPC), GetSystemTimePreciseAsFileTime (Windows 8+), timeGetTime, and multimedia or high-resolution timers.

Measuring ClockRes

General principles

• Use raw, high-resolution timer APIs when available (CLOCK_MONOTONIC_RAW on Linux; QPC/GetSystemTimePreciseAsFileTime on Windows).
• Measure by polling the timer in a tight loop and computing the smallest nonzero difference between consecutive readings across many samples.
• Be aware of CPU frequency scaling, power-management-induced latency, and scheduler noise; perform many iterations and control environment (isolate CPU, disable frequency scaling) for accurate results.

Measuring on Linux

1. Tools and APIs

   • clock_getres(2): returns resolution reported by the kernel for a clock (struct timespec).
   • clock_gettime(2): high-resolution timestamps for manual measurement.
   • perf, htop, and hwclock for system info.
   • cyclictest (from rt-tests) for latency testing and observing timer wakeups.
   • chrony/ntpd for synchronizing clocks (not resolution).

2. Using clock_getres

   • Example: calling clock_getres(CLOCK_MONOTONIC, &ts) returns the kernel’s idea of resolution. This is useful as a baseline but may show coarse values for *_COARSE clocks.

3. Empirical measurement (recommended)

   • Sample code pattern (C-style pseudocode):

     
     for i in 1..N:  t1 = clock_gettime(CLOCK_MONOTONIC_RAW)  do:      t2 = clock_gettime(CLOCK_MONOTONIC_RAW)  while t2 == t1  delta = t2 - t1  record delta analyze minimum and distribution 

   • Run with N large (e.g., 1e6 samples) to find the minimum observed delta and distribution tail.
   • Use CLOCK_MONOTONIC_RAW to avoid NTP/adjtime adjustments.

4. Interpreting results

   • The minimum nonzero delta approximates the timer tick or the effective granularity under current conditions.
   • Beware of reporting artifacts: CPU caching, compiler optimizations, and inlined syscalls can influence measurements.

5. Example utilities

   • clock_getres output (quick):
     • clock_getres via a tiny C program or wrapper shows values like 1ns, 1us, or 1ms depending on kernel and clock.
   • cyclictest:
     • Measures wakeup latencies for periodic timers and shows jitter which relates to real-world usable resolution for scheduling.

Measuring on Windows

1. Timer APIs

   • QueryPerformanceCounter (QPC) and QueryPerformanceFrequency: primary high-resolution timer; typically maps to HPET, TSC-Deadline, or invariant TSC depending on hardware and Windows configuration.
   • GetSystemTimePreciseAsFileTime (Windows 8+): returns system time with improved precision compared to GetSystemTimeAsFileTime; useful for timestamping.
   • timeGetDevCaps/timeBeginPeriod/timeEndPeriod and multimedia timers: affect scheduler timer resolution (legacy).
   • CreateWaitableTimerEx and SetWaitableTimer for timer-based waits.

2. Empirical measurement using QPC

   • Sample pattern:

     
     for i in 1..N:  QueryPerformanceCounter(&t1)  do:      QueryPerformanceCounter(&t2)  while t2 == t1  delta = t2 - t1  record delta_in_seconds = delta / frequency 

   • QueryPerformanceFrequency returns the tick rate; use it to convert counts to seconds.

3. Multimedia timer and system timer resolution

   • Legacy APIs: calling timeBeginPeriod(1) requests a 1ms system timer resolution. This can improve Sleep granularity and multimedia timers, but it increases power consumption and may be restricted by system policies.
   • Since Windows 10 and later many subsystems use high-resolution timers selectively; timeBeginPeriod influences global timer interrupt rate and can reduce Sleep jitter but is discouraged for broad use.

4. Interpreting Windows results

   • QPC gives very fine-grained counts; the observed minimum delta reflects the underlying hardware timestamp source (invariant TSC typically gives sub-microsecond resolution).
   • GetSystemTimePreciseAsFileTime exposes more precise system time than older APIs but is not guaranteed to be as low-latency as QPC.

Factors limiting ClockRes

• Hardware: TSC, HPET, APIC timer, platform-specific timer implementations set lower bounds.
• BIOS/firmware: BIOS or firmware settings can select timers or disable certain features.
• Kernel and drivers: kernel timer tick configuration (tickless kernels vs periodic tick), hrtimers, and governor settings shape effective resolution.
• Power management: CPU P-states/C-states and frequency scaling introduce jitter and delay, especially when cores wake from deep sleep.
• Scheduler noise: other processes, interrupts, and kernel activity cause jitter and affect the ability to observe or use the theoretical resolution.
• Virtualization: hypervisors may virtualize timers or add latency; nested virtualization can further degrade resolution.

Tuning techniques

Linux tuning

1. Choose the right clock

   • Use CLOCK_MONOTONIC_RAW for measuring elapsed time without adjustments.
   • For scheduling, use high-resolution timers (hrtimers) where available.

2. Kernel parameters and configuration

   • Enable CONFIG_HZ_1000 or use tickless configuration (NOHZ_FULL/NOHZ_IDLE) depending on workload. A 1000Hz tick historically improved timer granularity for legacy interfaces, but modern systems prefer tickless with hrtimers for low-latency workloads.
   • Enable PREEMPT_RT (real-time patches) for lowest latency in real-time systems.

3. CPU isolation and affinity

   • Isolate a CPU core with isolcpus or use cset to pin timing-sensitive threads to a dedicated core to reduce scheduler interference.
   • Use sched_setaffinity and SCHED_FIFO for real-time threads.

4. Disable power-saving features during measurement/tuning

   • Set CPU governor to performance and minimize C-state depth to reduce wake latency: echo performance > /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor; adjust /dev/cpu_dma_latency to prevent deep C-states.
   • Use cpupower or tuned profiles.

5. Use high-resolution timers and APIs

   • Use timerfd_create with TFD_TIMER_REALTIME or TFD_TIMER_ABSTIME for precise wakeups (with CLOCK_MONOTONIC).
   • Use posix timers and clock_nanosleep with CLOCK_MONOTONIC.

6. Real-time patches and priorities

   • PREEMPT_RT provides lower worst-case latencies; combine with SCHED_FIFO and appropriate priorities for deterministic scheduling.

7. Avoid noisy devices and interrupts

   • Move interrupt affinities away from the timing CPU(s) using /proc/irq/*/smp_affinity.
   • Disable unnecessary device drivers during critical measurements.

Windows tuning

1. Prefer QPC and GetSystemTimePreciseAsFileTime

   • Use QueryPerformanceCounter for high-resolution timestamps; fallback to GetSystemTimePreciseAsFileTime when wall-clock synchronized time is required.

2. Control system timer resolution carefully

   • timeBeginPeriod(1) can reduce Sleep granularity to ~1ms system-wide; use sparingly because it raises power use and affects the entire system.
   • On modern Windows, timer coalescing and power features may limit benefits from timeBeginPeriod; test on target systems.

3. Use multimedia/high-resolution timers only when needed

   • CreateWaitableTimerEx with high resolution to request precise wakeups. Use proper privileges and handle power implications.

4. Affinity and priority

   • Set thread affinity and use real-time thread priority classes when deterministic scheduling is required (Use carefully—can starve system threads).
   • Use SetThreadPriority with THREAD_PRIORITY_TIME_CRITICAL for critical tasks; ensure watchdogs and safety controls to avoid system hangs.

5. Power settings

   • Choose a high-performance power plan, disable deep C-states (in firmware or via OS/driver tools), and disable CPU throttling to reduce latency.

6. Driver and firmware updates

   • Ensure chipset drivers, BIOS/UEFI, and platform firmware are up to date—some timer improvements arrive in microcode or firmware.

Practical examples & measurement snippets

• Linux C (conceptual):

  // Use clock_gettime(CLOCK_MONOTONIC_RAW) in a tight loop, record deltas, analyze histogram. 

• Windows C/C++ (conceptual):

  // Use QueryPerformanceCounter and QueryPerformanceFrequency, loop to capture minimum delta and distribution. 

Run tests both idle and under load to see how resolution and jitter behave in real-world scenarios.

Balancing precision, power, and stability

Raising timer resolution or forcing frequent wakeups improves timing precision but increases power use and thermal output and may hurt battery life on mobile platforms. Real-time tuning (high priorities, PREEMPT_RT, timeBeginPeriod(1)) should be applied only to the machines and workloads that require them. For general-purpose applications, prefer using APIs that provide the best available resolution without globally changing system timer behavior.

Summary

• ClockRes is the smallest observable time increment; measure it empirically with high-resolution timers (CLOCK_MONOTONIC_RAW on Linux; QueryPerformanceCounter on Windows).
• System-reported resolution (clock_getres) is a starting point; real-world resolution depends on hardware, kernel/OS behavior, power management, and scheduler noise.
• Tuning involves selecting proper clocks, adjusting kernel/OS settings, isolating CPUs, disabling deep power states, using real-time priorities, and updating firmware/drivers.
• Always weigh improved timing precision against increased power consumption and system-wide effects.