>> Incidentally, some context for the AIX approach to the OOM problem: a >> process may exclude itself from OOM vulnerability altogether. It places >> itself in "early allocation" mode, which means at the time it creates >> virtual memory, it reserves enough backing store for the worst case. The >> memory manager does not send such a process the SIGDANGER signal or >> terminate it when it runs out of paging space. Before c. 2000, this was >> the only mode. Now the default is late allocation mode, which is similar >> to Linux. > >This is an interesting approach. It feels like some programs might be >interested in choosing this mode instead of risking OOM. It's the way virtual memory always worked when it was first invented. The system not only reserved space to back every page of virtual memory; it assigned the particular blocks for it. Late allocation was a later innovation, and I believe its main goal was to make it possible to use the cheaper disk drives for paging instead of drums. Late allocation gives you better locality on disk, so the seeking doesn't eat you alive (drums don't seek). Even then, I assume (but am not sure) that the system at least reserved the space in an account somewhere so at pageout time there was guaranteed to be a place to which to page out. Overcommitting page space to save on disk space was a later idea. I was surprised to see AIX do late allocation by default, because IBM's traditional style is bulletproof systems. A system where a process can be killed at unpredictable times because of resource demands of unrelated processes doesn't really fit that style. It's really a fairly unusual application that benefits from late allocation: one that creates a lot more virtual memory than it ever touches. For example, a sparse array. Or am I missing something? -- Bryan Henderson IBM Almaden Research Center San Jose CA Filesystems - To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
