In this paper, we present a runtime compilation and tuning framework for parallel programs. We extend our prior work on our auto-tuner, Active Harmony, for tunable parameters that require code generation (for example, different unroll factors). For such parameters, our auto-tuner generates and compiles new code on-the-?y. Effectively, we merge traditional feedback directed optimization and just-in-time compilation. We show that our system can leverage available parallelism in today’s HPC platforms by evaluating different code-variants on different nodes simultaneously. We evaluate our system on two parallel applications and show that our system can improve runtime execution by up to 46% compared to the original version of the program.