The JIT compiler was active, but most of the computationally heavy parts were ahead of time compiled. The compiler had a number of tweaks to increase the quality of the generated code (though at the expense of a good amount of compile time). Generating the ahead of time image probably took a good 5-10 minutes (much more if we had tried to do it on the KNL nodes that we actually ran on), so it was infeasible and would have been a waste of time to try to do so in a JITted fashion. Some JIT compilation probably still happened (for code paths that weren't in the pre-compilation trace, e.g. logging code or exception code paths hit by a few processes).