Image classification model selection using Amazon SageMaker JumpStart

Researchers continue to develop new model architectures for common machine learning (ML) tasks. One such task is image classification, where images are accepted as input and the model attempts to classify the image as a whole with object label outputs. With many models available today that perform this image classification task, an ML practitioner may ask questions like: “What model should I fine-tune and then deploy to achieve the best performance on my dataset?” And an ML researcher may ask questions like: “How can I generate my own fair comparison of multiple model architectures against a specified dataset while controlling training hyperparameters and computer specifications, such as GPUs, CPUs, and RAM?” The former question addresses model selection across model architectures, while the latter question concerns benchmarking trained models against a test dataset.

In this post, you will see how the TensorFlow image classification algorithm of Amazon SageMaker JumpStart can simplify the implementations required to address these questions. Together with the implementation details in a corresponding example Jupyter notebook, you will have tools available to perform model selection by exploring pareto frontiers, where improving one performance metric, such as accuracy, is not possible without worsening another metric, such as throughput.

Solution overview

The following figure illustrates the model selection trade-off for a large number of image classification models fine-tuned on the Caltech-256 dataset, which is a challenging set of 30,607 real-world images spanning 256 object categories. Each point represents a single model, point sizes are scaled with respect to the number of parameters comprising the model, and the points are color-coded based on their model architecture. For example, the light green points represent the EfficientNet architecture; each light green point is a different configuration of this architecture with unique fine-tuned model performance measurements. The figure shows the existence of a pareto frontier for model selection, where higher accuracy is exchanged for lower throughput. Ultimately, the selection of a model along the pareto frontier, or the set of pareto efficient solutions, depends on your model deployment performance requirements.

If you observe test accuracy and test throughput frontiers of interest, the set of pareto efficient solutions on the preceding figure are extracted in the following table. Rows are sorted such that test throughput is increasing and test accuracy is decreasing.

This post provides details on how to implement large-scale Amazon SageMaker benchmarking and model selection tasks. First, we introduce JumpStart and the built-in TensorFlow image classification algorithms. We then discuss high-level implementation considerations, such as JumpStart hyperparameter configurations, metric extraction from Amazon CloudWatch Logs, and launching asynchronous hyperparameter tuning jobs. Finally, we cover the implementation environment and parameterization leading to the pareto efficient solutions in the preceding table and figure.

Introduction to JumpStart TensorFlow image classification

JumpStart provides one-click fine-tuning and deployment of a wide variety of pre-trained models across popular ML tasks, as well as a selection of end-to-end solutions that solve common business problems. These features remove the heavy lifting from each step of the ML process, making it easier to develop high-quality models and reducing time to deployment. The JumpStart APIs allow you to programmatically deploy and fine-tune a vast selection of pre-trained models on your own datasets.

The JumpStart model hub provides access to a large number of TensorFlow image classification models that enable transfer learning and fine-tuning on custom datasets. As of this writing, the JumpStart model hub contains 135 TensorFlow image classification models across a variety of popular model architectures from TensorFlow Hub, to include residual networks (ResNet), MobileNet, EfficientNet, Inception, Neural Architecture Search Networks (NASNet), Big Transfer (BiT), shifted window (Swin) transformers, Class-Attention in Image Transformers (CaiT), and Data-Efficient Image Transformers (DeiT).

Vastly different internal structures comprise each model architecture. For instance, ResNet models utilize skip connections to allow for substantially deeper networks, whereas transformer-based models use self-attention mechanisms that eliminate the intrinsic locality of convolution operations in favor of more global receptive fields. In addition to the diverse feature sets these different structures provide, each model architecture has several configurations that adjust the model size, shape, and complexity within that architecture. This results in hundreds of unique image classification models available on the JumpStart model hub. Combined with built-in transfer learning and inference scripts that encompass many SageMaker features, the JumpStart API is a great launching point for ML practitioners to get started training and deploying models quickly.

Refer to Transfer learning for TensorFlow image classification models in Amazon SageMaker and the following example notebook to learn about SageMaker TensorFlow image classification in more depth, including how to run inference on a pre-trained model as well as fine-tune the pre-trained model on a custom dataset.

Large-scale model selection considerations

Model selection is the process of selecting the best model from a set of candidate models. This process may be applied across models of the same type with different parameter weights and across models of different types. Examples of model selection across models of the same type include fitting the same model with different hyperparameters (for example, learning rate) and early stopping to prevent the overfitting of model weights to the train dataset. Model selection across models of different types includes selecting the best model architecture (for example, Swin vs. MobileNet) and selecting the best model configurations within a single model architecture (for example, mobilenet-v1-025-128 vs. mobilenet-v3-large-100-224).

The considerations outlined in this section enable all of these model selection processes on a validation dataset.

Select hyperparameter configurations

TensorFlow image classification in JumpStart has a large number of available hyperparameters that can adjust the transfer learning script behaviors uniformly for all model architectures. These hyperparameters relate to data augmentation and preprocessing, optimizer specification, overfitting controls, and trainable layer indicators. You are encouraged to adjust the default values of these hyperparameters as necessary for your application:

model_id: str
model_version: str = "*"

hyperparameters = sagemaker.hyperparameters.retrieve_default(
    model_id=model_id, model_version=model_version
)

For this analysis and the associated notebook, all hyperparameters are set to default values except for learning rate, number of epochs, and early stopping specification. Learning rate is adjusted as a categorical parameter by the SageMaker automatic model tuning job. Because each model has unique default hyperparameter values, the discrete list of possible learning rates includes the default learning rate as well as one-fifth the default learning rate. This launches two training jobs for a single hyperparameter tuning job, and the training job with the best reported performance on the validation dataset is selected. Because the number of epochs is set to 10, which is greater than the default hyperparameter setting, the selected best training job doesn’t always correspond to the default learning rate. Finally, an early stopping criterion is utilized with a patience, or the number of epochs to continue training with no improvement, of three epochs.

One default hyperparameter setting of particular importance is train_only_on_top_layer, where, if set to True, the model’s feature extraction layers are not fine-tuned on the provided training dataset. The optimizer will only train parameters in the top fully connected classification layer with output dimensionality equal to the number of class labels in the dataset. By default, this hyperparameter is set to True, which is a setting targeted for transfer learning on small datasets. You may have a custom dataset where the feature extraction from the pre-training on the ImageNet dataset is not sufficient. In these cases, you should set train_only_on_top_layer to False. Although this setting will increase training time, you will extract more meaningful features for your problem of interest, thereby increasing accuracy.

Extract metrics from CloudWatch Logs

The JumpStart TensorFlow image classification algorithm reliably logs a variety of metrics during training that are accessible to SageMaker Estimator and HyperparameterTuner objects. The constructor of a SageMaker Estimator has a metric_definitions keyword argument, which can be used to evaluate the training job by providing a list of dictionaries with two keys: Name for the name of the metric, and Regex for the regular expression used to extract the metric from the logs. The accompanying notebook shows the implementation details. The following table lists the available metrics and associated regular expressions for all JumpStart TensorFlow image classification models.

The built-in transfer learning script provides a variety of train, validation, and test dataset metrics within these definitions, as represented by the f-string replacement values. The exact metrics available vary based on the type of classification being performed. All compiled models have a loss metric, which is represented by a cross-entropy loss for either a binary or categorical classification problem. The former is used when there is one class label; the latter is used if there are two or more class labels. If there is only a single class label, then the following metrics are computed, logged, and extractable via the f-string regular expressions in the preceding table: number of true positives (true_pos), number of false positives (false_pos), number of true negatives (true_neg), number of false negatives (false_neg), precision, recall, area under the receiver operating characteristic (ROC) curve (auc), and area under the precision-recall (PR) curve (prc). Similarly, if there are six or more class labels, a top-5 accuracy metric (top_5_accuracy) is also be computed, logged, and extractable via the preceding regular expressions.

During training, metrics specified to a SageMaker Estimator are emitted to CloudWatch Logs. When the training is complete, you can invoke the SageMaker DescribeTrainingJob API and inspect the FinalMetricDataList key in the JSON response:

tuner: sagemaker.tuner.HyperparameterTuner
session: sagemaker.Session

training_job_name = tuner.best_training_job()
description = session.describe_training_job(training_job_name)
metrics = description["FinalMetricDataList"]

This API requires only the job name to be provided to the query, so, once completed, metrics can be obtained in future analyses so long as the training job name is appropriately logged and recoverable. For this model selection task, hyperparameter tuning job names are stored and subsequent analyses reattach a HyperparameterTuner object given the tuning job name, extract the best training job name from the attached hyperparameter tuner, and then invoke the DescribeTrainingJob API as described earlier to obtain metrics associated with the best training job.

Launch asynchronous hyperparameter tuning jobs

Refer to the corresponding notebook for implementation details on asynchronously launching hyperparameter tuning jobs, which uses the Python standard library’s concurrent futures module, a high-level interface for asynchronously running callables. Several SageMaker-related considerations are implemented in this solution:

• Each AWS account is affiliated with SageMaker service quotas. You should view your current limits to fully utilize your resources and potentially request resource limit increases as needed.
• Frequent API calls to create many simultaneous hyperparameter tuning jobs may exceed the Python SDK rate and throw throttling exceptions. A resolution to this is to create a SageMaker Boto3 client with a custom retry configuration.
• What happens if your script encounters an error or the script is stopped before completion? For such a large model selection or benchmarking study, you can log tuning job names and provide convenience functions to reattach hyperparameter tuning jobs that already exist:

tuning_job_name: str
session: sagemaker.Session

tuner = sagemaker.tuner.HyperparameterTuner.attach(tuning_job_name, session)

Analysis details and discussion

The analysis in this post performs transfer learning for model IDs in the JumpStart TensorFlow image classification algorithm on the Caltech-256 dataset. All training jobs were performed on the SageMaker training instance ml.g4dn.xlarge, which contains a single NVIDIA T4 GPU.

The test dataset is evaluated on the training instance at the end of training. Model selection is performed prior to the test dataset evaluation to set model weights to the epoch with the best validation set performance. Test throughput is not optimized: the dataset batch size is set to the default training hyperparameter batch size, which isn’t adjusted to maximize GPU memory usage; reported test throughput includes data loading time because the dataset isn’t pre-cached; and distributed inference across multiple GPUs isn’t utilized. For these reasons, this throughput is a good relative measurement, but actual throughput would depend heavily on your inference endpoint deployment configurations for the trained model.

Although the JumpStart model hub contains many image classification architecture types, this pareto frontier is dominated by select Swin, EfficientNet, and MobileNet models. Swin models are larger and relatively more accurate, whereas MobileNet models are smaller, relatively less accurate, and suitable for resource constraints of mobile devices. It’s important to note that this frontier is conditioned on a variety of factors, including the exact dataset used and the fine-tuning hyperparameters selected. You may find that your custom dataset produces a different set of pareto efficient solutions, and you may desire longer training times with different hyperparameters, such as more data augmentation or fine-tuning more than just the top classification layer of the model.

Conclusion

In this post, we showed how to run large-scale model selection or benchmarking tasks using the JumpStart model hub. This solution can help you choose the best model for your needs. We encourage you to try out and explore this solution on your own dataset.

References

More information is available at the following resources:

• Image Classification – TensorFlow
• Run image classification with Amazon SageMaker JumpStart
• Build high performing image classification models using Amazon SageMaker JumpStart

About the authors

Dr. Kyle Ulrich is an Applied Scientist with the Amazon SageMaker built-in algorithms team. His research interests include scalable machine learning algorithms, computer vision, time series, Bayesian non-parametrics, and Gaussian processes. His PhD is from Duke University and he has published papers in NeurIPS, Cell, and Neuron.

Dr. Ashish Khetan is a Senior Applied Scientist with Amazon SageMaker built-in algorithms and helps develop machine learning algorithms. He got his PhD from University of Illinois Urbana Champaign. He is an active researcher in machine learning and statistical inference and has published many papers in NeurIPS, ICML, ICLR, JMLR, ACL, and EMNLP conferences.

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