memory-mapped files

[Andrei Alexandrescu]

Indeed, time and again, "testing is believing".

I tried a simple line splitting program in D with and without memory 
mapping against a 140MB file. The program just reads the entire file and 
does some simple string processing on it.

The loop pattern looks like this:

     foreach (line; byLineDirect(stdin))
     {
         auto r = splitter(line, "|||");
         write(r.head, ":");
         r.next;
         writeln(r.head);
     }

The byLineDirect returns a range that uses memory mapped files when 
possible, or simple fread calls otherwise.

The memory-mapped version takes 2.15 seconds on average. I was fighting 
against Perl's equivalent 2.45. At some point I decided to try without 
memory mapping and I consistently got 1.75 seconds. What the heck is 
going on? When does memory mapping actually help?


Andrei