4x speedup of recursive rmdir in std.file

[Andrei Alexandrescu]

On 2/5/12 3:04 PM, Jay Norwood wrote:
> I can tell you that there are a couple of seconds improvement in
> the execution time running 16 threads vs 8 on the i7 on the ssd
> drive, so we aren't keeping all the cores busy with 8 threads. I
> suppose they are all blocked waiting for file system operations
> for some portion of time even with 8 threads.  I would guess that
> even on a single core it would be an advantage to have multiple
> threads available for the core to work on when it blocks waiting
> for the fs operations.
[snip]

That's why I'm saying - let's leave the decision to the user. Take a 
uint parameter for the number of threads to be used, where 0 means leave 
it to phobos, and default to 0.

Andrei