Welcome to the 2021 ACM SIGMETRICS Conference on Measurement and Modeling of Computer Systems! ACM SIGMETRICS is the world-leading research venue in the area of computer systems performance evaluation and promotes research in performance analysis techniques and innovative applications. It was originally scheduled to be held on the campus of Tsinghua University in Beijing, China, June 14-18, 2021. However, due to the global COVID-19 pandemic situation, the SIGMETRICS 2021 Organizing Committee and the SIGMETRICS Executive Committee decided to hold the 2021 edition online. Despite being an online event, SIGMETRICS 2021 contains a set of featured components. First of all, the conference has a spectacular technical program covering exciting areas including learning, measurement and applied modeling, systems and theory. To facilitate interaction of conference attendees, we developed an online conference environment, which allows participants to interact in real-time with each other. Authors of accepted papers are encouraged to prepare a video presentation for their talks, which will be published in both ACM DL and the SIGMETRICS channel. SIGMETRICS 2021 also has three wonderful keynotes coming from academic and industry, and contains the "Highlight beyond SIGMETRICS" session to connect to other communities. There are four workshops, Mathematical Performance Modeling and Analysis (MAMA), Distributed Cloud Computing (DCC), International Workshop on Critical |