== Quote from Andrei Alexandrescu (seewebsiteforem...@erdani.org)'s article > Brad Roberts wrote: > > bearophile wrote: > > >> What the heck is going on? When does memory mapping actually help?< > >> You are scanning the file linearly, and the memory window you use is > >> probably very small. In such situation a memory mapping is probably > >> not the best thing. A memory mapping is useful when you for example > >> operate with random access on a wider sliding window on the file. > > > > You can drop the 'sliding' part. mmap tends to help when doing random > > access (or sequential but non-contiguous maybe) over a file. Pure > > streaming is handled pretty well by both patterns. One nicity with mmap > > is that you can hint to the os how you'll be using it via madvise. You > > can't do that with [f]read. > This all would make perfect sense if the performance was about the same > in the two cases. But in fact memory mapping introduced a large > *pessimization*. Why? I am supposedly copying less data and doing less > work. This is very odd.
If I had to guess, I'd say that the OS assumes every file will be read in a linear manner from front to back, and optimizes accordingly. There's no way of knowing how a memory-mapped file will be accessed however, so no such optimization occurs. Sean
