Brian Grant

With time-division multiplexing, many CPU threads can be interleaved. They can be blocked and queued by the OS, typically at the cost of context switches and waiting a few time slices. Thus there is no fixed limit to how many can be packed onto a machine. CPU is compressible

OTOH, swapping memory pages, even to local SSD, is painfully expensive. This is why systems hosting services that need to respond with subsecond latency disable swap. Memory is considered an incompressible resource. For simplicity, I'll ignore resources other than CPU and memory

Compressible resources like CPU can be made available quickly by the kernel with low impact to the threads that were interrupted, provided it knows which threads urgently need the resources and which ones don't. We call this latency sensitive and latency tolerant respectively

In order to reallocate incompressible resources quickly, threads need to be killed, which is obviously not low impact. (For memory, in Linux this is done by the OOM killer.) This was why Borg used priority (production priority vs not) to make memory oversubscription decisions

Borg's resource reclamation approach is described by the paper: reservations based on observed usage were computed and oversubscribed resources (latency-tolerant cpu and non-production memory) were tallied against reservations whereas guaranteed ones used limits. Complicated.

Vertical autoscaling (VA) added even more complexity. VA changed limits, but left its own padding to provide slack for reaction time and observation of demand. Ad hoc mechanisms were added to disable limit enforcement for each resource, creating a notion similar to request in K8s