kinX: latency measurement

(2018-04-17)

This post is part of a series of posts about the kinX project.

Latency measurement

End-to-end latency consists of 3 parts:

  1. input latency (keyboard)
  2. processing latency (computer)
  3. output latency (monitor)

During the development of the kinX keyboard controller, I realized that measuring processing latency was quite simple with my hardware: I could start a timer when sending a key press HID report to the computer and measure the elapsed time when I would receive a reply from the computer.

The key to send is the Caps Lock key, because unlike other keys it results in a reply: a HID report telling the keyboard to turn the Caps Lock LED on.

Measurement device

To make this measurement technique accessible to as many people as possible, I decided to pull it out of my kinX keyboard controller and instead build it using the FRDM-K66F evaluation board, which uses the same microcontroller.

The FRDM-K66F can be bought for about 60 USD at big electronics shops, e.g. Digi-Key.

Find the firmware at https://github.com/kinx-project/measure-fw

Baseline

To determine the lowest processing latency one can possibly get for userspace applications on a Linux system, I wrote a small program which uses Linux’s evdev API to receive key presses and react to a Caps Lock keypress as quickly as it can by turning the Caps Lock LED on.

Find the program at https://github.com/kinx-project/measure-evdev

The following layers are exercised when measuring this program:

  • USB host controller
  • Linux kernel (USB and input subsystems)
  • input event API (evdev)

Notably, graphical interfaces such as X11 or Wayland are excluded.

The measurements can be verified using Wireshark’s usbmon capturing, which provides a view of the USB bus from the computer’s perspective, excluding USB poll latency and USB transaction time.

Using Ubuntu 17.10, I measured a processing latency of 152 μs on average.

Emacs

Now let’s see whether my current editor of choice adds significant latency.

Using a few lines of Emacs Lisp, I instructed Emacs to turn on the Caps Lock LED whenever a key is inserted into the current buffer. In combination with remapping the Caps Lock key to any other key (e.g. “a”), this allows us to measure Emacs’s processing latency.

On the same Ubuntu 17.10 installation used above, Emacs 25.2.2 adds on average 278 μs to the baseline processing latency.

Find the code at https://github.com/kinx-project/measure-emacs

End-to-end latency

With the kinX keyboard controller, we can achieve the following end-to-end latency:

contributor latency
Matrix scan ≈ 100 μs
USB poll ≈ 125 μs
Linux ≈ 152 μs
Emacs ≈ 278 μs


This sums up to ≈ 655 μs on average. On top of that, we have output latency within [0, 16ms] due to the 60 Hz refresh rate of our monitors.

Note that using a compositor adds one frame of output latency.

Input latency perception

A natural question to ask is how well humans can perceive input latency. After all, keyboards have been manufactured for many years, and if input latency was really that important, surely manufacturers would have picked up on this fact by now?

I ran a little unscientific experiment in the hope to further my understanding of this question at the most recent Chaos Communication Congress in Leipzig.

In the experiment, I let 17 people play a game on a specially prepared keyboard. In each round, the game reconfigures the keyboard to either have additional input latency or not, decided at random. The player can then type a few keys and make a decision. If the player can correctly indicate whether additional input latency was present in more than 50% of the cases, the player is said to be able to distinguish latency at that level. On each level, the game decreases the additional input latency: it starts with 100ms, then 75ms, then 50ms, etc.

The most sensitive player reliably recognized an additional 15ms of input latency.

Some players could not distinguish 75ms of additional input latency.

Every player could distinguish 100ms of additional input latency.

My take-away is that many people cannot perceive slight differences in input latency at all, explaining why keyboard manufacturers don’t optimize for low latency.

Reducing input latency still seems worthwhile to me: even if the reduction happens under the threshold at which you can perceive differences in input latency, it buys you more leeway in the entire stack. In other words, you might now be able to turn on an expensive editor feature which previously slowed down typing too much.

Conclusion

When I started looking into input latency, my keyboard had dozens of milliseconds of latency. I found an easy win in the old firmware, then hit a wall, started the kinX project and eventually ended up with a keyboard with just 0.225ms input latency.

Even if I had not reduced the input latency of my keyboard at all, I feel that this project was a valuable learning experience: I now know a lot more about PCB design, ARM microcontrollers, USB, HID, etc.

Typing on a self-built keyboard feels good: be it because of the warm fuzzy feeling of enjoying the fruits of your labor, or whether the input latency indeed is lower, I’m happy with the result either way.

Lastly, I can now decidedly claim that the processing latency of modern computers is perfectly fine (remember our 152 μs + 278 μs measurement for Linux + Emacs), and as long as you pick decent peripherals, your end-to-end latency will be fine, too.

What’s next?

By far the biggest factor in the end-to-end latency is the monitor’s refresh rate, so getting a monitor with a high refresh rate and no additional processing latency would be the next step in reducing the end-to-end latency.

As far as the keyboard goes, the matrix scan could be eliminated by wiring up each individual key to a microcontroller with enough GPIO pins. The USB poll delay could be eliminated by switching to USB 3, but I don’t know of any microcontrollers which have USB 3 built-in yet. Both of these improvements are likely not worth the effort.