The JDK Flight Recorder (JFR) was open sourced with JDK 11, and was subsequently back-ported to JDK 8u262. JFR allows for always-on production time profiling, with little overhead and with a rich set of data. This talk will discuss things to consider when using JFR to profile hundreds of thousands of JVMs in mission critical systems all over the world. We will discuss trade-offs, limits, work-arounds and insights we’ve learnt as we’ve developed the Java profiling capabilities at Datadog.