Uninformed programmers, or "stop assuming virtual memory is awesome."

I decided I needed to vent a bit post-exam. So where better to find some clue-void post to answer than slashdot.

The post, and my reply:

http://news.slashdot.org/comments.pl?sid=2229598&cid=36408502

In summary, the original poster:

"In short, don't worry about fine-tuning what's "in memory". Don't change behavior based on total amount of memory in the system. Operating systems (OpenBSD aside) ALREADY DO THAT. Just let the memory manager do its job, and give it enough information (via interactivity information, memory priority, etc.) to do its job properly. Don't try to hack around problems at the wrong layers."

And my response:

"You assume that the OS will make sensible paging decisions. You assume you can hint to the OS that you're going to make sensible paging decisions. You hope the application, which is likely big, multithreaded and such, is doing the sensible thing of not wrapping large accesses to "memory things" (eg big trees of data, as an example, or image caches, or whatever takes up more than a small bit of RAM) in mutexes. You assume that your application is using memory in a sensible fashion, and not simply using a few bytes here and there in each allocated chunk."

I hate to say it, but if you're assuming that virtual memory is the be all and end all of your memory management method, you're setting yourself up for some really bad worse case behaviour. Since you have no control as to where and when the OS decides to punt pages to disk, you can't possibly begin to design methods around it. You can only hope that the OS doesn't page out the data you've got locked, or that the malloc implementation you're using doesn't handle fragmentation poorly.