EvLag – A Tool for Monitoring and Lagging Linux Input Devices

Shengmei Liu

and

Mark Claypool

Introduction

• Latency degrades interactivity
• Particularly for games
• Highly interactive
• Player performance matters
• Techniques to mitigate (latency compensation)
• But need more studies of latency on games and game actions

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Introduction

• Typically, via user study
• Add latency
• Play game
• Measure perf, QoE
• Repeat

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• Must add lag to game
• Limits games (open source)
• Re-implement code (bugs)

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Introduction

EvLag

• Sits outside game, so can use with close source (commercial)
• Latency to any Linux input device
• Also logging of input events

This paper 🡪 features, implementation, evaluation, example use

EvLag

Outline

• Introduction (done)
• Related Work (next)
• Implementation
• Evaluation
• Use
• Conclusion

Related Work

• Network latency tools
• Networking: dummynet [4], Clumsy [31], MahiMahi [18], netem [32]
• Can be used for network latency, but not local latency
• Local latency studies
• [7, 8, 13, 16, 17, 20, 21, 29]
• cloud-based game systems [26, 30]
• All use same technique – modify small game/app with latency
• Measuring local latency
• Camera, sensors [13, 24, 27, 33]
• We use to verify EvLag works

EvLag – Overview

Features

• Fixed latency to any input device
• Lag multiple devices at once (e.g., keyboard and mouse)
• Log input events, one file per device

Implementation

• Linux evdev
• Generalize raw input as character devices
• /dev/input
• Three threads
• Reader, Writer – handle events
• Main – setup, handles timers

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EvLag – Implementation

Reader

EvLag – Implementation

Main

EvLag – Usage

EvLag – Utilities

EvParse – parse EvLag log files

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EvDetect – automatically find device given string

Outline

• Introduction (done)
• Related Work (done)
• Implementation (done)
• Evaluation (next)
• Use
• Conclusion

Evaluation – Measuring �Local Latency

Evaluation – Results

EvLag – Example Study

• User study,

43 participants

Shengmei Liu, Atsuo Kuwahara, James Scovell, Jamie Sherman, and Mark Claypool. “Lower is Better? The Effects of Local Latencies on Competitive First-Person Shooter Game Players”, In Proceedings of the ACM Conference on Human Factors in Computing Systems (CHI), Virtual Conference, May 8-13, 2021. Online at: http://www.cs.wpi.edu/~claypool/papers/csgo-lag-21/

EvLag – Example Study

EvLag – Demo

https://web.cs.wpi.edu/~claypool/papers/evlag/

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Summary

• Understanding latency helpful for designs and techniques to compensate
• User studies must modify code, limiting use of commercial games
• EvLag – adds latency to input devices in Linux
• Can be used for any application
• Also logs input

Download:

apt package:

https://web.cs.wpi.edu/~claypool/papers/evlag/install.html

git repo:

https://claypool@bitbucket.org/claypool/evlag.git

Future Work

• Additional features
• Enhanced parsing of logs (game controllers)
• Different lag for each device
• Additional studies of latency
• Games, game actions, input devices
• Similar tools for other platforms
• Windows, Mac, Android, iOS

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EvLag – A Tool for Monitoring and Lagging Linux Input Devices

Shengmei Liu

and

Mark Claypool

Thank-you for your attention!