Ultralytics YOLO Hyperparameter Tuning Guide

Introduction

Hyperparameter tuning is not just a one-time set-up but an iterative process aimed at optimizing the machine learning model's performance metrics, such as accuracy, precision, and recall. In the context of Ultralytics YOLO, these hyperparameters could range from learning rate to architectural details, such as the number of layers or types of activation functions used.

What are Hyperparameters?

Hyperparameters are high-level, structural settings for the algorithm. They are set prior to the training phase and remain constant during it. Here are some commonly tuned hyperparameters in Ultralytics YOLO:

Learning Rate lr0: Determines the step size at each iteration while moving towards a minimum in the loss function.
Batch Size batch: Number of images processed simultaneously in a forward pass.
Number of Epochs epochs: An epoch is one complete forward and backward pass of all the training examples.
Architecture Specifics: Such as channel counts, number of layers, types of activation functions, etc.

Hyperparameter Tuning Visual

For a full list of augmentation hyperparameters used in YOLOv8 please refer to the configurations page.

Genetic Evolution and Mutation

Ultralytics YOLO uses genetic algorithms to optimize hyperparameters. Genetic algorithms are inspired by the mechanism of natural selection and genetics.

Mutation: In the context of Ultralytics YOLO, mutation helps in locally searching the hyperparameter space by applying small, random changes to existing hyperparameters, producing new candidates for evaluation.
Crossover: Although crossover is a popular genetic algorithm technique, it is not currently used in Ultralytics YOLO for hyperparameter tuning. The focus is mainly on mutation for generating new hyperparameter sets.

Preparing for Hyperparameter Tuning

Before you begin the tuning process, it's important to:

Identify the Metrics: Determine the metrics you will use to evaluate the model's performance. This could be AP50, F1-score, or others.
Set the Tuning Budget: Define how much computational resources you're willing to allocate. Hyperparameter tuning can be computationally intensive.

Steps Involved

Initialize Hyperparameters

Start with a reasonable set of initial hyperparameters. This could either be the default hyperparameters set by Ultralytics YOLO or something based on your domain knowledge or previous experiments.

Mutate Hyperparameters

Use the _mutate method to produce a new set of hyperparameters based on the existing set.

Train Model

Training is performed using the mutated set of hyperparameters. The training performance is then assessed.

Evaluate Model

Use metrics like AP50, F1-score, or custom metrics to evaluate the model's performance.

Log Results

It's crucial to log both the performance metrics and the corresponding hyperparameters for future reference.

Repeat

The process is repeated until either the set number of iterations is reached or the performance metric is satisfactory.

Usage Example

Here's how to use the model.tune() method to utilize the Tuner class for hyperparameter tuning of YOLOv8n on COCO8 for 30 epochs with an AdamW optimizer and skipping plotting, checkpointing and validation other than on final epoch for faster Tuning.

Example

Python

from ultralytics import YOLO

# Initialize the YOLO model
model = YOLO('yolov8n.pt')

# Tune hyperparameters on COCO8 for 30 epochs
model.tune(data='coco8.yaml', epochs=30, iterations=300, optimizer='AdamW', plots=False, save=False, val=False)

Results

After you've successfully completed the hyperparameter tuning process, you will obtain several files and directories that encapsulate the results of the tuning. The following describes each:

File Structure

Here's what the directory structure of the results will look like. Training directories like train1/ contain individual tuning iterations, i.e. one model trained with one set of hyperparameters. The tune/ directory contains tuning results from all the individual model trainings:

runs/
└── detect/
    ├── train1/
    ├── train2/
    ├── ...
    └── tune/
        ├── best_hyperparameters.yaml
        ├── best_fitness.png
        ├── tune_results.csv
        ├── tune_scatter_plots.png
        └── weights/
            ├── last.pt
            └── best.pt

File Descriptions

best_hyperparameters.yaml

This YAML file contains the best-performing hyperparameters found during the tuning process. You can use this file to initialize future trainings with these optimized settings.

Format: YAML
Usage: Hyperparameter results

Example:

  # 558/900 iterations complete ✅ (45536.81s)
  # Results saved to /usr/src/ultralytics/runs/detect/tune
  # Best fitness=0.64297 observed at iteration 498
  # Best fitness metrics are {'metrics/precision(B)': 0.87247, 'metrics/recall(B)': 0.71387, 'metrics/mAP50(B)': 0.79106, 'metrics/mAP50-95(B)': 0.62651, 'val/box_loss': 2.79884, 'val/cls_loss': 2.72386, 'val/dfl_loss': 0.68503, 'fitness': 0.64297}
  # Best fitness model is /usr/src/ultralytics/runs/detect/train498
  # Best fitness hyperparameters are printed below.

  lr0: 0.00269
  lrf: 0.00288
  momentum: 0.73375
  weight_decay: 0.00015
  warmup_epochs: 1.22935
  warmup_momentum: 0.1525
  box: 18.27875
  cls: 1.32899
  dfl: 0.56016
  hsv_h: 0.01148
  hsv_s: 0.53554
  hsv_v: 0.13636
  degrees: 0.0
  translate: 0.12431
  scale: 0.07643
  shear: 0.0
  perspective: 0.0
  flipud: 0.0
  fliplr: 0.08631
  mosaic: 0.42551
  mixup: 0.0
  copy_paste: 0.0

best_fitness.png

This is a plot displaying fitness (typically a performance metric like AP50) against the number of iterations. It helps you visualize how well the genetic algorithm performed over time.

Format: PNG
Usage: Performance visualization

Hyperparameter Tuning Fitness vs Iteration

tune_results.csv

A CSV file containing detailed results of each iteration during the tuning. Each row in the file represents one iteration, and it includes metrics like fitness score, precision, recall, as well as the hyperparameters used.

Format: CSV
Usage: Per-iteration results tracking.

Example:

  fitness,lr0,lrf,momentum,weight_decay,warmup_epochs,warmup_momentum,box,cls,dfl,hsv_h,hsv_s,hsv_v,degrees,translate,scale,shear,perspective,flipud,fliplr,mosaic,mixup,copy_paste
  0.05021,0.01,0.01,0.937,0.0005,3.0,0.8,7.5,0.5,1.5,0.015,0.7,0.4,0.0,0.1,0.5,0.0,0.0,0.0,0.5,1.0,0.0,0.0
  0.07217,0.01003,0.00967,0.93897,0.00049,2.79757,0.81075,7.5,0.50746,1.44826,0.01503,0.72948,0.40658,0.0,0.0987,0.4922,0.0,0.0,0.0,0.49729,1.0,0.0,0.0
  0.06584,0.01003,0.00855,0.91009,0.00073,3.42176,0.95,8.64301,0.54594,1.72261,0.01503,0.59179,0.40658,0.0,0.0987,0.46955,0.0,0.0,0.0,0.49729,0.80187,0.0,0.0

tune_scatter_plots.png

This file contains scatter plots generated from tune_results.csv, helping you visualize relationships between different hyperparameters and performance metrics. Note that hyperparameters initialized to 0 will not be tuned, such as degrees and shear below.

Format: PNG
Usage: Exploratory data analysis

Hyperparameter Tuning Scatter Plots

weights/

This directory contains the saved PyTorch models for the last and the best iterations during the hyperparameter tuning process.

last.pt: The last.pt are the weights from the last epoch of training.
best.pt: The best.pt weights for the iteration that achieved the best fitness score.

Using these results, you can make more informed decisions for your future model trainings and analyses. Feel free to consult these artifacts to understand how well your model performed and how you might improve it further.

Conclusion

The hyperparameter tuning process in Ultralytics YOLO is simplified yet powerful, thanks to its genetic algorithm-based approach focused on mutation. Following the steps outlined in this guide will assist you in systematically tuning your model to achieve better performance.