The performance testing and optimization of cloud applications is challenging, because manual tuning of cloud computing stacks is tedious and automated tuning tools are rare used for cloud services. To address this issue, we introduce KeenTune, an automated tuning tool designed to optimize application performance and facilitate performance testing. KeenTune is a lightweight and flexible tool that can be deployed with to-be-tuned applications with negligible impact on their performance. Specifically, KeenTune uses a surrogate model that can be implemented with machine learning models to filter out less relevant parameters for efficient tuning. Our empirical evaluation shows that KeenTune significantly enhances the throughput performance of Nginx web servers, resulting in performance improvements of up to 90.43% and 117.23% in certain cases. This study highlights the benefits of using KeenTune for achieving efficient and effective performance testing of cloud applications.The video for KeenTune is presented at https://youtu.be/7l_w2EfXAuo and the source code can be found at https://github.com/alibaba/keentune