UNIX does *not* fully support asynchronous I/O

[Jim Giles]

>From article <11576:Aug2503:18:3790 at kramden.acf.nyu.edu>, by brnstnd at kramden.acf.nyu.edu (Dan Bernstein):
> [...]
> I'm afraid Jim is right, though he drastically overestimates the effect
> of this failure on small machines. Let me explain.

True.  But many "small" machines aren't very small any more.  This will
increasingly be true.

> [...]
> Under UNIX, there's at least one big extra step. write(fd,buf,n) first
> *copies* the data to a buffer inside the kernel's space. This takes CPU
> time. Do you see now what Jim is complaining about?

Takes more than CPU time.  If the program is consuming and producing
data at (or exceeding) the maximum I/O rate, the buffers will always
be busy when I make a request.  I will, therefore, be forced to wait.
With real asynchronous I/O, I only wait when I need the next I/O to
have _completed_, not when I make the request.  This means that _none_
of the I/O will overlap user calculation in the UNIX style, while
_all_ of the I/O will be overlapped with genuine asynchronous I/O.

J. Giles

P.S. Dan:  I have been on vacation and will get to your email when I
catch up with other things.