Prompt-Ladder: Memory-efficient prompt tuning for vision-language models on edge devices

The pre-trained vision-language models (VLMs) have been the foundation for diverse intelligent services in human life. Common VLMs hold large parameter scales and require heavy memory overhead for model pre-training, which poses challenges in adapting them to edge devices. To enable memory-efficient VLMs, previous works mainly focus on the prompt engineering technique that utilizes trainable soft prompts instead of manually designing hard prompts. However, to update fewer than 3% of prompt parameters, these studies still require the back-propagation chain to traverse pre-trained models with extensive parameters. Consequently, the intermediate activation variables and gradients occupy a significant amount of memory resources, greatly hindering their adaptation on resource-constrained edge devices. In view of the above, we propose a memory-efficient prompt-tuning method, named Prompt-Ladder. Our main idea is to adopt a lightweight ladder network as an agent to bypass VLMs during back-propagation for the parameter optimization of the designed multi-model prompt module. The ladder network fuses the intermediate output of VLMs as a guide and selects important parameters of VLMs to initialize for the maintenance of model performance. We also share parameters of the ladder network between text and image data to obtain a more semantically aligned representation across modalities for the optimization of the prompt module. The experiments across seven datasets demonstrate that Prompt-Ladder can significantly reduce memory resource usage by at least 27% compared to baselines while maintaining relatively good performance.