[sterlind]

to me the line between machine and cluster is mostly about real-time and fate-sharing. multiple cores on a single machine can expect memory accesses to succeed, caches to be coherent, interrupts to trigger within a deadline, clocks not to skew, cores in a CPU not to drop out, etc.

in a cluster, communication isn't real-time. packets drop, fetches fail, clocks skew, machines reboot.

IPC is a gray area. the remote process might die, its threads might be preempted, etc. RTOSes make IPC work more like a single machine, while regular OSes make IPC more like a network call.

so to me, the datacenter-as-mainframe idea falls apart because you need massive amounts of software infrastructure to treat a cluster like a mainframe. you have to use Paxos or Raft for serializing operations, you have to shard data and handle failures, etc. etc.

but it's definitely getting closer, thanks to lots of distributed systems engineering.