Time durations how to#
Time durations update#
Time durations code#

Time span measured using both time-of-day clock and monotonic clock I’ll leave out const = require('perf_hooks) part of the following snippets for brevity. You can also try out the experiment with () at your leisure.

Time durations update#

Let’s update our experiment to use require('perf_hooks).performance.now() instead of Date.now().

() ( process.hrtime() in older NodeJS versions).

In Node.js, the functions that returns monotonic time are:

Programming languages usually provide functions for both monotonic and time-of-day clock. Therefore, the difference between two monotonic timestamps read at two different points in time will give the accurate time duration between the two points.

But the important part is, monotonic clock is not affected by NTP corrections. Since this is just a counter, a monotonic timestamp read at a given point in time doesn’t have any use by itself. Monotonic clock is simply counter that has started from an arbitrary point in the past and moves at the same speed as the system clock.

Time durations how to#

Experiment 2 - How To Measure Time Accurately And the answer is by using a function that uses a monotonic clock rather than a time-of-day clock. time server sending an incorrect time for some reason, NTP client refreshing time after not running for a significant time, a human operator manually changing system time, & etc.)īut the important question is, can we solve this problem? YES. Though this example is a simulation, this can happen in real life systems due to various reasons (e.g. Therefore, endTimeToD is an older timestamp than the startTimeToD. However, it is fairly obvious that the reason for the ‘negative time duration’ is because the NTP client corrected the clock by moving it backwards (because it was ahead by 1 min in the beginning). The numeric value you see here may depend on various factors such as how busy your CPU is, how long did it take for the NTP client to receive time from the time server etc. This is not what a developer would expect at all! Inaccurate time span measurement with Date.now() This example only works on macOS as it relies on a few macOS shell commands/utilities, but you should be able to easily modify it to work on Linux. You can find the complete implementation of the example at the end of the article. But, we are going to simulate an NTP time correction by deliberately setting a fake time in the beginning and correcting the time using NTP while doSomething() function is being executed. In this experiment, we are going to measure the time it took to run doSomething() function. To see it and understand it clearly, let’s move on to our little experiment. Now you may probably understand why I said the snippet 1 wouldn’t work. The functions that use ‘time-of-day’ clock, such as JavaScript Date.now() returns the current system time, which could be affected by NTP corrections.This ‘correction’ of time may result in the system clock suddenly moving forward or backwards.If the NTP client finds that the local time is drifted apart from the time server’s time, it may choose to ‘correct’ it depending on how much the drift is.

NTP clients that are usually shipped with the operating system may periodically receive the time from a configured time server, and adjust the local system clock.

NTP (Network Time Protocol), is one of the network protocols that is used to fix this problem, by synchronising the clock of a computer with that of a ‘time server’ that usually has accurate time.

This ‘ clock drift’ can cause various problems in distributed systems.

Not all the clocks across all computers run at the same speed, some may run faster or slower comparatively and eventually may drift apart from each other (like some of our wall clocks sometimes do).

Time durations code#

This is a nice little code that works, until it doesn’t.īefore delving into our experiment and explaining why the above wouldn’t work, it’s important to know a few key details about time, how it is measured and maintained by the operating system.