As Joel mentioned earlier, from an Oracle perspective, one of the key
things we are looking for is a nice clean *common* wait point. We don't
really care whether this common wait point is the old libaio:async-poll,
epoll, or "wait_for_async". And if "wait_for_async" has the added
benefit of scaling, all the better.
However, it is desirable for that common wait-routine to have the
ability to return explicit completions, instead of requiring a follow-on
call to some other query/wait for events/completions for each of the
different type of async submissions done (poll, pid, i/o, ...).
Obviously not a "must-have", but desirable.
It is also desirable (if possible) to have immediate completions (either
immediate errs or async submissions that complete synchronously)
communicated at submission time, instead of via the common wait-routine.
Finally, it is agreed that neg-errno is a much better approach for the
return code. The threading/concurrency issues associated w/ the current
unix errno has always been buggy area for Oracle Networking code.
Regards, -Scot
Linus Torvalds wrote:
On Mon, 5 Feb 2007, bert hubert wrote:
From my end as an application developer, yes please. Either make it
perfectly ok to have thousands of outstanding asynchronous system calls (I
work with thousands of separate sockets), or allow me to select/poll/epoll
on the "async fd".
No can do.
Allocating an fd is actually too expensive, exactly because a lot of these
operations are supposed to be a few hundred ns, and taking locks is simply
a bad idea.
But if you want to, we could have a *separate* "convert async cookie to
fd" so that you can poll for it, or something.
I doubt very many people want to do that. It would tend to simply be nicer
to do
async(poll);
async(waitpid);
async(.. wait foranything else ..)
followed by a
wait_for_async();
That's just a much NICER approach, I would argue. And it automatically
and very naturally solves the "wait for different kinds of events"
question, in a way that "poll()" never did (except by turning all events
into file descriptors or signals).
Alternatively, something like SIGIO ('SIGASYS'?) might be considered, but,
well, the fd might be easier.
Again. NO WAY. Signals are just damn expensive. At most, it would be an
option again, but if you want high performance, signals simply aren't very
good. They are also a nice way to make your user-space code very racy.
In fact, perhaps the communication channel might simply *be* an fd. Queueing
up syscalls sounds remarkably like sending datagrams.
I'm the first to say that file descriptors is the UNIX way, but so are
processes, and I think this is MUCH better done as a "process" interface.
In other words, instead of doing it as a filedescriptor, do it as a
"micro-fork/exec", and have the "wait()" equivalent. It's just that we
don't fork a "real process", and we don't exec a "real program", we just
exec a single system call.
If you think of it in those terms, it all makes sense *without* any file
descriptors what-so-ever, and the "wait_for_async()" interface also makes
a ton of sense (it really *is* "waitpid()" for the system call).
Linus
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