So I have a theory as to why we were able to get away with that in the past, but can't really any more. The approach of letting applications just allocate as m

So I have a theory as to why we were able to get away with that in the past, but can't really any more. The approach of letting applications just allocate as much memory as they wanted arose in the era of primarily 32-bit operating systems. And that meant that, at a process level, in practice 'as much memory as you want' capped out at 4GB (or 2GB if you are Win32 because... Windows reasons). And so when you're running 32 bit processes on computers with single-digit-gigs of physical RAM, and multiple-digit-gigs of disk space, and single-core processors, letting processes treat memory as 'all you can eat' and swapping out excess allocations to disk is going to work out fine. Your processor can only be running one application's code at a time, and a significant fraction of that application's entire address space can all be paged into physical memory at once, with the full amount not taking up too much disk space, so for the most part you can get away with just treating 'process address space exhausted' as being as good a signal as any of 'out of memory'. But now we're in 64-bit land, address space exhaustion is no longer a thing. If you just allow programs to keep allocating RAM until they run out of 64-bit address space, to a first approximation, 0% of your application's maximum address space will ever be able to be paged into physical RAM, and you will need several exabytes of hard drive space to hold what's paged to disk. So the problem is we now don't really have a natural limit akin to the 32-bit address ceiling to impose on applications who want to allocate memory, and it means we're instead put in the position of having to come up with policies ourselves. Stuff like setting a memory limit on a docker container. And so we wind up having to actually now think about and answer the question, 'how much memory should we let each process on this system allocate?'