  Modern HPC workload managers and their careful tuning contribute to the high
utilization of HPC clusters. However, due to inevitable uncertainty it is
impossible to completely avoid node idleness. Although such idle slots are
usually too short for any HPC job, they are too long to ignore them.
Function-as-a-Service (FaaS) paradigm promisingly fills this gap, and can be a
good match, as typical FaaS functions last seconds, not hours. Here we show how
to build a FaaS infrastructure on idle nodes in an HPC cluster in such a way
that it does not affect the performance of the HPC jobs significantly. We
dynamically adapt to a changing set of idle physical machines, by integrating
open-source software Slurm and OpenWhisk.
  We designed and implemented a prototype solution that allowed us to cover up
to 90\% of the idle time slots on a 50k-core cluster that runs production
workloads.
