Case in point, a sort-of general purpose IO library was using mmap as the OS interface, and presenting a read/write style API. We had various bugs related to th

Case in point, a sort-of general purpose IO library was using mmap as the OS interface, and presenting a read/write style API. We had various bugs related to this (e.g. extending or truncating a mmap'ed file is slightly tricky to do correctly etc.). I ripped out the mmap codepath, fixing the mmap-related bugs, and for some benchmarks performance improved by a factor of 20. Now, there are certainly places where mmap is awesome. E.g. if you can push the mmap semantics up to the application level, or you need the sharing semantics etc.