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Hyperparameter Evolution

📚 This guide explains hyperparameter evolution for YOLOv5 🚀. Hyperparameter evolution is a method of Hyperparameter Optimization using a Genetic Algorithm (GA) for optimization. UPDATED 25 September 2022.

Hyperparameters in ML control various aspects of training, and finding optimal values for them can be a challenge. Traditional methods like grid searches can quickly become intractable due to 1) the high dimensional search space 2) unknown correlations among the dimensions, and 3) expensive nature of evaluating the fitness at each point, making GA a suitable candidate for hyperparameter searches.

Before You Start

Clone repo and install requirements.txt in a Python>=3.7.0 environment, including PyTorch>=1.7. Models and datasets download automatically from the latest YOLOv5 release.

git clone https://github.com/ultralytics/yolov5  # clone
cd yolov5
pip install -r requirements.txt  # install

1. Initialize Hyperparameters

YOLOv5 has about 30 hyperparameters used for various training settings. These are defined in *.yaml files in the /data directory. Better initial guesses will produce better final results, so it is important to initialize these values properly before evolving. If in doubt, simply use the default values, which are optimized for YOLOv5 COCO training from scratch.

https://github.com/ultralytics/yolov5/blob/2da2466168116a9fa81f4acab744dc9fe8f90cac/data/hyps/hyp.scratch-low.yaml#L2-L34

2. Define Fitness

Fitness is the value we seek to maximize. In YOLOv5 we define a default fitness function as a weighted combination of metrics: mAP@0.5 contributes 10% of the weight and mAP@0.5:0.95 contributes the remaining 90%, with Precision P and Recall R absent. You may adjust these as you see fit or use the default fitness definition (recommended). https://github.com/ultralytics/yolov5/blob/4103ce9ad0393cc27f6c80457894ad7be0cb1f0d/utils/metrics.py#L12-L16

3. Evolve

Evolution is performed about a base scenario which we seek to improve upon. The base scenario in this example is finetuning COCO128 for 10 epochs using pretrained YOLOv5s. The base scenario training command is:

python train.py --epochs 10 --data coco128.yaml --weights yolov5s.pt --cache
To evolve hyperparameters specific to this scenario, starting from our initial values defined in Section 1., and maximizing the fitness defined in Section 2., append --evolve:
# Single-GPU
python train.py --epochs 10 --data coco128.yaml --weights yolov5s.pt --cache --evolve

# Multi-GPU
for i in 0 1 2 3 4 5 6 7; do
  sleep $(expr 30 \* $i) &&  # 30-second delay (optional)
  echo 'Starting GPU '$i'...' &&
  nohup python train.py --epochs 10 --data coco128.yaml --weights yolov5s.pt --cache --device $i --evolve > evolve_gpu_$i.log &
done

# Multi-GPU bash-while (not recommended)
for i in 0 1 2 3 4 5 6 7; do
  sleep $(expr 30 \* $i) &&  # 30-second delay (optional)
  echo 'Starting GPU '$i'...' &&
  "$(while true; do nohup python train.py... --device $i --evolve 1 > evolve_gpu_$i.log; done)" &
done

The default evolution settings will run the base scenario 300 times, i.e. for 300 generations. You can modify generations via the --evolve argument, i.e. python train.py --evolve 1000. https://github.com/ultralytics/yolov5/blob/6a3ee7cf03efb17fbffde0e68b1a854e80fe3213/train.py#L608

The main genetic operators are crossover and mutation. In this work mutation is used, with a 80% probability and a 0.04 variance to create new offspring based on a combination of the best parents from all previous generations. Results are logged to runs/evolve/exp/evolve.csv, and the highest fitness offspring is saved every generation as runs/evolve/hyp_evolved.yaml:

# YOLOv5 Hyperparameter Evolution Results
# Best generation: 287
# Last generation: 300
#    metrics/precision,       metrics/recall,      metrics/mAP_0.5, metrics/mAP_0.5:0.95,         val/box_loss,         val/obj_loss,         val/cls_loss
#              0.54634,              0.55625,              0.58201,              0.33665,             0.056451,             0.042892,             0.013441

lr0: 0.01  # initial learning rate (SGD=1E-2, Adam=1E-3)
lrf: 0.2  # final OneCycleLR learning rate (lr0 * lrf)
momentum: 0.937  # SGD momentum/Adam beta1
weight_decay: 0.0005  # optimizer weight decay 5e-4
warmup_epochs: 3.0  # warmup epochs (fractions ok)
warmup_momentum: 0.8  # warmup initial momentum
warmup_bias_lr: 0.1  # warmup initial bias lr
box: 0.05  # box loss gain
cls: 0.5  # cls loss gain
cls_pw: 1.0  # cls BCELoss positive_weight
obj: 1.0  # obj loss gain (scale with pixels)
obj_pw: 1.0  # obj BCELoss positive_weight
iou_t: 0.20  # IoU training threshold
anchor_t: 4.0  # anchor-multiple threshold
# anchors: 3  # anchors per output layer (0 to ignore)
fl_gamma: 0.0  # focal loss gamma (efficientDet default gamma=1.5)
hsv_h: 0.015  # image HSV-Hue augmentation (fraction)
hsv_s: 0.7  # image HSV-Saturation augmentation (fraction)
hsv_v: 0.4  # image HSV-Value augmentation (fraction)
degrees: 0.0  # image rotation (+/- deg)
translate: 0.1  # image translation (+/- fraction)
scale: 0.5  # image scale (+/- gain)
shear: 0.0  # image shear (+/- deg)
perspective: 0.0  # image perspective (+/- fraction), range 0-0.001
flipud: 0.0  # image flip up-down (probability)
fliplr: 0.5  # image flip left-right (probability)
mosaic: 1.0  # image mosaic (probability)
mixup: 0.0  # image mixup (probability)
copy_paste: 0.0  # segment copy-paste (probability)

We recommend a minimum of 300 generations of evolution for best results. Note that evolution is generally expensive and time consuming, as the base scenario is trained hundreds of times, possibly requiring hundreds or thousands of GPU hours.

4. Visualize

evolve.csv is plotted as evolve.png by utils.plots.plot_evolve() after evolution finishes with one subplot per hyperparameter showing fitness (y axis) vs hyperparameter values (x axis). Yellow indicates higher concentrations. Vertical distributions indicate that a parameter has been disabled and does not mutate. This is user selectable in the meta dictionary in train.py, and is useful for fixing parameters and preventing them from evolving.

evolve

W&B Sweeps

W&B sweep support was added in PR https://github.com/ultralytics/yolov5/pull/3938. To run a sweep the steps are:

  • Set the hyperparameter limits in data/hyps/sweep.yaml and define dataset path and search strategy.
  • run wandb sweep utils/logging/wandb/sweep.yaml . You can optionally pass --project name --count max_run_count
  • The above command will output an id. Start the sweep with wandb agent sweep_id. This will start W&B sweep over the defined search space in sweep.yaml. Here's a live example from this script.

Sweep Image

Environments

YOLOv5 may be run in any of the following up-to-date verified environments (with all dependencies including CUDA/CUDNN, Python and PyTorch preinstalled):

Status

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