k8s's scheduling model is that scheduler is able to see the entire state of the cluster, thus can optimistically make optimal decisions on scheduling, especially for those long running jobs that are very picky in practice. Although k8s by default only runs the default scheduler, you could in theory run multiple schedulers in parallel (the omega model).

Mesos's pessimistic two level offer model makes it hard for second level scheduler to make optimal decisions because it might not get the right offer it needs. At the same time, first level scheduler lacks application specific information to make the right decision to send the right offer to the second level scheduler, thus the problem. We evaluated many first level scheduling algorithms, and ironically found that "random" first level scheduler sometimes works better than DRF for long running services scheduling.

> Could you say what was unique about the Mesos threading model?

Mesos uses a component called libprocess (think of it as C++ version of erlang). Each actor in the system (mesos master, mesos agent) is single threaded. Thus, all i/o operations need to be non-blocking to not block the actor. This makes it hard to integrate 3rdparty C++ libraries, especially those that involves I/O as they might have a different threading model.

Golang solved this problem using go-routing and bake that into the language. So the golang libraries, especially those involve I/O, are much more composable than C++ IMO.