re: disk-based fds in select/poll

[Dan Kegel]

Pierre Phaneuf <[e-mail]> wrote:
> It's fairly widely-known that select/poll returns immediately when
> testing a filesystem-based file descriptor for writability or
> readability.
> 
> On top of this, even when in non-blocking mode, read() could block if
> the pages needed aren't in core. sendfile() behaves in a similar way.
> 
> What would be needed to alleviate this?
> ...
> I remember seeing a suggestion by Linus for an event-based I/O
> interface, similar to kqueue on FreeBSD but much simpler. I'd just say
> "I want it too!", ok? :-)
>
> SGI's AIO might be a solution here, does it use threads? I'm trying to
> avoid context switching as much as possible, to keep the CPU cache as
> warm as possible.

IMHO, you want AIO.  SGI's is fine for now.  I hear rumors that there will be
something even better coming in 2.5, though I have no details.

Or you could use explicit userspace threads... say, divide up your
network connections among 8 or so threads.  Then if one thread blocks,
the others are there to usefully soak up the CPU time.

Readiness events for readahead completion on disk files used to 
seem like a neat idea to me, but now AIO seems more appealing
in the long run, since they handle random access properly.

- Dan