Web and mobile applications can become more popular than we had anticipated. If we’re not prepared, our application can have downtime. If we are prepared, it means that our application can scale. Scaling an application comes with numerous challenges. One of those is how we reason about system performance and how that influences architecture changes we make to our system. Kay Ousterhout, Software Engineer at LightStep explains what performance means and why users struggle to reason about it in today’s systems. Kay discussed her work on Apache Spark and how jobs were decomposed to provide clarity about performance to users. We also talked about detecting bottlenecks in a system.