At the moment driver and executor pods are created using the following requests and limits:
Unlimited (but can be specified using spark.[driver,executor].cores)
[driver,executor].memory + [driver,executor].
Specifying the requests like this leads to problems if the pods only get the requested amount of resources and nothing of the optional (limit) resources, as it can happen in a fully utilized cluster.
Let’s say we have a node with 100GiB memory and 5 pods with 20 GiB memory and 5 GiB memoryOverhead.
At the beginning all 5 pods use 20 GiB of memory and all is well. If a pod then starts using its overhead memory it will get killed as there is no more memory available, even though we told spark
that it can use 25 GiB of memory.
Instead of requesting `[driver,executor].memory`, we should just request `[driver,executor].memory + [driver,executor].
As it turns out, there can be performance problems if we only have `executor.cores` available (which means we have one core per task). This was raised here and is the reason that the cpu limit was set to unlimited.
This issue stems from the fact that in general there will be more than one thread per task, resulting in performance impacts if there is only one core available.
However, I am not sure that just setting the limit to unlimited is the best solution because it means that even if the Kubernetes cluster can perfectly satisfy the resource requests, performance might be very bad.
I think we should guarantee that an executor is able to do its work well (without performance issues or getting killed - as could happen in the memory case) with the resources it gets guaranteed from Kubernetes.
One way to solve this could be to request more than 1 core from Kubernetes per task. The exact amount we should request is unclear to me (it largely depends on how many threads actually get spawned for a task).
We would need to find a way to determine this somehow automatically or at least come up with a better default value than 1 core per task.
Does somebody have ideas or thoughts on how to solve this best?