Major Description
Currently FairScheduler, CapacityScheduler and FifoScheduler each have a method private void validateConf(Configuration conf).
All three methods validate the minimum and maximum scheduler allocations for cpu and memory (with minor difference). FairScheduler supports 0 as minimum allocation for cpu and memory, while CapacityScheduler and FifoScheduler do not. We can move this code to AbstractYarnScheduler (avoids code duplication) and make it protected for individual schedulers to override.
Why do we care about a minimum allocation of 0 for cpu and memory?
We contribute to a project called Apache Myriad that run yarn on mesos. Myriad supports a feature call fine grained scaling (fgs). In fgs, a NM is launched with zero capacity (0 cpu and 0 mem). When a yarn container is to be run on the NM, a mesos offer for that node is accepted and the NM capacity is dynamically scaled up to match the accepted mesos offer. On completion of the yarn container, resources are returned back to Mesos and the NM capacity is scaled down back to zero (cpu & mem).
In ResourceTrackerService.registerNodeManager, yarn checks if the NM capacity is at-least as much as yarn.scheduler.minimum-allocation-mb and yarn.scheduler.minimum-allocation-vcores. These values can be set to 0 in yarn-site.xml (so a zero capacity NM is possible). However, the validateConf methods in CapacityScheduler and FifoScheduler do not allow for 0 values for these properties (The FairScheduler one does allow for 0). This behaviour should be consistent or at-least be override able.