Brian Grant

QoS is something that wouldn't matter if just one process were running per host system or if all the processes steadily used a constant amount of cpu, memory, and other resources. Because they vary, reserving max capacity needed for each would leave systems poorly utilized

Oversubscription mitigates that by packing more applications onto a system than could all fit at their peak requirements. It's kind of like a bank: not everyone can withdraw at the same time. The question is then: what happens when apps demand more resources than they can get?

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.