On Jul 07, 1999 at 01:10:38AM -0400, Zach Brown wrote:
> the sigio/siginfo model is a few orders of magnitude cheaper than
> poll/select as you scale the number of fds you're watching. The reasons
> for this being that select()/poll() have that large chunk of state to
> throw around every syscall, and the real world behaviour of very rarely
> ever returning more than than a few active pollfds
Yes; that's effectively what the "delta" method I'm using now gets
rid of; it only passes around state changes instead of the entire state.
I agree that passing around the entire state is quite un-efficient; that's
what I would like to get rid of. We just need to agree on a new method.
> with the sigio/siginfo model you register your interest in the fd at fd
> creation. from then on, when a POLL_ event happens on the fd we notice
> that it has an rt signal queue registered and a siginfo struct is tacked
> onto the end. these code paths can be nice and light. the siginfo
> enqueueing can be pointed at multiple queues by registering a process
> group with F_SETOWN, etc.
Yes, but I also need support for temporarily "de-registering" interest
in an fd, as well selectively choosing read/write/close events.
> its important to notice that we don't actually use signal delivery for
> this sigio/siginfo stuff, we mask the signal and use signwaitinfo() to
> block or pop the next siginfo struct off the queue. dealing with async
> signals jumping in would be annoying, and to do it right one would
> probably want to simply enqueue the siginfo delivered to the signal
> handler into a nice fifo that the real thread of execution would deal
> with.. instead of doing all this grossness, we just let the kernel
> maintain the siginfo queue.
In this case, it doesn't seem all that different than a poll() type
call, or an event queue that Peter was talking about. If the signal
is blocked, then sigwaitinfo() effectively becomes a poll(), but with
no timeout parameter.
> its quite like the 'delta poll' system proposed, but with differently
> inelegant semantics. I'd say if one were to design an event
> queueing/notification system and add a new api for it, we'd want to do it
> correctly from the get-go and lose the similarity to existing interfaces
> entirely unless they really makes sense to behave like them (which it
> doesn't in the poll() case, imho)
I agree. One aspect of the design that should be explored is whether
the new call should report "events", or "state". My current implementation
reports state; (think of it as a level-triggered design), while the
siginfo approach appears to be more of an "edge-triggered" design. I
just looked at Banga's USENIX paper and they have a nice discussion of
this issue.
> setup sigio and such on new fd (dorky, we have to do this in
> linux rather than inheriting it from the listening fd.
> but it has yet to show up on the profile radar, so,
> whatever :))
Hah. It showed up on my profiling; the kernel I'm running has routines
so the child fd inherits certain settings from the parent.
> read() in the header (usually done in one read, but rarely
> will block and require falling back to a POLL_IN on
> the new fd)
Well, we _NEVER_ want to block. Ever. And in my particular case,
it is quite common for the header/data to be spread out over several
reads.
> of course, this could change if you had a situation where you could burn
> through events like nothing else and simply couldn't deal with the
> lock-step..
Correct. I need this for a web caching proxy. The above loop won't work
in my particular case.
> > Also, I would guess that you would start getting into locking problems,
> > and how to cancel a signal which has already been posted.
>
> locking problems?
For asynchronous signal delivery, you alluded to this problem earlier
as well. Since you're blocking signals, this isn't a problem.
> yes, the possibility of getting stale events in the queue is _annoying_.
> This is going to be a problem in any system that passes state deltas to
> the process in a queued manner. hacks could be put in, and perhaps
> should, to remove events in the queue for a fd when it is closed, etc.
>
> take the web server case again. it is quite possible to close() an fd
> while there is an event queued for it, and then accept() a new fd that now
> has a bogus event coming down the pipe for it. I get around this garbage
> in the cheesy web server by doing deferred close()s on fds based on the
> length of the queue when I stopped being interested in the fd (and as such
> turned off sigio delivery). Its gross.
Exactly. Sometimes, we just want to close() an fd, even if there are
pending events, and then immediately re-open with a new fd. Deferred
closes are not an option. What I do at the moment is remove queued
events/state when the fd is closed. (actually, my implementation sucks
a bit, as I re-scan the state for this particular case).
> but even with these problems, the rt signal queue is quite powerful. to
> do better would require a fair bit of engineering, and one might quickly
> be bogged down in featuritis.
Well, I'm sure that we have a lot of engineering talent around here. :-)
--
Jonathan
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