Understanding gettimeofday() Resolution in Linux: Is it Guaranteed to be Microsecond Accurate?

When developing applications that require precise timing, such as games or performance-intensive software, the choice of timing functions can significantly impact the reliability of the application’s performance. In this blog post, we’ll look into a common question faced by developers: Is gettimeofday() guaranteed to be of microsecond resolution?

The Problem: Portability and Timing Precision

As you attempt to port a game from the Win32 API to Linux, precision in measuring time can become tricky. The original implementation uses QueryPerformanceCounter which provides high-resolution timing. In the process of porting this functionality, you turn to gettimeofday(), which can deliver the time in microseconds since the Unix epoch. However, you’d like to understand if using this function is portable and reliable across different Linux systems.

The Implementation Challenge

In your code, you’ve implemented the following method to use gettimeofday() to emulate QueryPerformanceCounter:

BOOL QueryPerformanceCounter(LARGE_INTEGER* performanceCount)
{
    gettimeofday(&currentTimeVal, NULL);
    performanceCount->QuadPart = (currentTimeVal.tv_sec - startTimeVal.tv_sec);
    performanceCount->QuadPart *= (1000 * 1000);
    performanceCount->QuadPart += (currentTimeVal.tv_usec - startTimeVal.tv_usec);

    return true;
}

While this method gives you a 64-bit variable that holds microseconds since the process startup, it’s essential to delve deeper into the limitations of gettimeofday().

The Truth About gettimeofday() Resolution

The resolution of gettimeofday() is often misunderstood. The function can indeed provide time measurements, but there are critical caveats:

• Resolution Limitations: On standard Linux systems, the resolution of gettimeofday() is typically 10 microseconds. This means that while it may provide values in microseconds, the precision may not be reliable.
• System Interference: The timing can be affected by various factors, including:
  • Running Processes: Background processes that adjust the system clock can lead to jumps in time.
  • NTP (Network Time Protocol): If configured, NTP can adjust the system time, affecting the output of gettimeofday().

These issues lead to the conclusion that the answer to the original question is no, gettimeofday() cannot be relied upon for precise and consistent microsecond measurements across all systems.

A Better Alternative: Using clock_gettime()

To ensure more reliable timing in your Linux applications, consider using clock_gettime(). This function offers several advantages over gettimeofday():

Benefits of clock_gettime()

1. More Accurate Timing:

   • You can use CLOCK_MONOTONIC to get time that is not subject to changes from system clock updates, making it ideal for measuring time intervals.

2. Reduced Issues with Multi-core Systems:

   • It handles timing better in environments with multiple processors and minimizes the disruptions that external clock settings can impose.

Next Steps: Implementing clock_gettime()

To replace gettimeofday(), you can rewrite your timing function like this:

#include <time.h>

BOOL QueryPerformanceCounter(LARGE_INTEGER* performanceCount)
{
    struct timespec ts;
    clock_gettime(CLOCK_MONOTONIC, &ts);
    performanceCount->QuadPart = ts.tv_sec * 1000000 + ts.tv_nsec / 1000; // Convert to microseconds

    return true;
}

Additional Resource

For more detailed information, consider checking out the man page for clock_gettime()—it details the various clocks available and their uses: clock_gettime() man page.

Conclusion

When developing cross-platform applications or games, knowing the limitations of timing functions is crucial. While gettimeofday() might be a common choice for obtaining the current time, its reliability can falter under specific circumstances. By adopting clock_gettime(), you can mitigate many of these issues and ensure more consistent timing behavior that is essential for performance-sensitive applications.

Now that you understand the intricacies of using gettimeofday() and the benefits of switching to clock_gettime(), you can make an informed decision for your Linux porting project.