Debugging high throughput, low-latency C/C++ systems in production is hard. At Google we developed XRay, a function call tracing system that allows Google engineers to get accurate function call traces with negligible overhead when off and moderate overhead when on, suitable for services deployed in production. XRay enables efficient function call entry/exit logging with high accuracy timestamps, and can be dynamically enabled and disabled. This white paper describes the XRay tracing system and its implementation. It also describes future plans with open sourcing XRay and engaging open source communities.