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Date: Fri, 14 Feb 2003 17:50:04 -0800 (PST)
From: Davide Libenzi <davidel@xmailserver.org>
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To: Jamie Lokier <jamie@shareable.org>
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Subject: Re: Synchronous signal delivery..
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On Sat, 15 Feb 2003, Jamie Lokier wrote:

> Davide Libenzi wrote:
> > > And when that's done you have some nice bonuses:
> > >
> > > 	- All event types are reported equally fast, and in a single
> > > 	  system call (read()).
> > >
> > > 	- The order in which events occurred is preserved.
> > > 	  (This is lost when you have to scan multiple queues).
> > >
> > > 	- Hierarchies of event sets of any kind are possible.
> > > 	  (epoll has solved the logical problems of this already).
> > >
> > > 	- Less code duplicated.
> > >
> > > 	- Adding new kinds of kernel events becomes _very_ simple.
> >
> > Hmm ... using read() you'll lose the timeout capability, that IMHO is
> > pretty nice.
>
> Very good point.
>
> Timeouts could be events too - probably a good idea as they can then
> be absolute, relative, attached to different system clocks (monotonic
> vs. timeofday).  I think the POSIX timer work is like that.

POSIX timers might work, and events might be captured using the sigfd()
descriptor.



> It seems like a good idea to be able to attach one timeout event in
> the same system call as the event_read call itself - because it is
> _so_ common to vary the expiry time every time.
>
> Then again, it is also extremely common to write this:
>
> 	gettimeofday(...)
> 	// calculate time until next application timer expires.
> 	// Note also race condition here, if we're preempted.
> 	read_events(..., next_app_time - timeofday)
> 	// we need to know the current time.
> 	gettimeofday(...)
>
> So perhaps the current select/poll/epoll timeout method is not
> particularly optimal as it is?

What's bad in epoll_wait() to get events from all pollable descriptors ?




- Davide

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