Ah, memories! ;-)
When AIX went multiprocessor around v.3.[45], they used a single kernel lock. Version 4 was a near complete rewrite to get rid of the single lock. I missed that fun, as I was no longer working for IBM, but I was sysadmining some AIX boxen during the 3.5-4 era.
Version 3.5 had a bug. mmap()-ing a file plus some extra space and then writing to the unbacked space would extend the file on JFS (in fact, that was how write() worked in the FS), but on NFS it would corrupt a FS-level lock so that any process that touched that FS would deadlock. Including the automounter. Which was itself single-threaded. UTCS's system programming class meant daily reboots for a while. 8-) AIX v4 fixed the problem.
AIX, like most Unixes, would happily allocate memory until you ran out of address space, completely unrelated to how much memory you had, because sparse matrices. The X server used this facility, for one thing. A process could tell the system not to kill it by handling a particular signal, but most of the system software didn't handle the signal. Including inetd. Which was usually the first thing killed when you ran out of memory, so you couldn't log into the system remotely.