Date: Mon, 7 Jul 2003 15:11:23 -0700 (PDT)
From: Davide Libenzi <davidel@xmailserver.org>
To: Jamie Lokier <jamie@shareable.org>
cc: Eric Varsanyi <e0216@foo21.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: epoll vs stdin/stdout
In-Reply-To: <20030707200315.GA10939@mail.jlokier.co.uk>
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On Mon, 7 Jul 2003, Jamie Lokier wrote:

> Eric Varsanyi wrote:
> > Epoll's API/impl is great as it is IMO, not suggesting need for change, I was
> > hoping there was a good standard trick someone worked up to get around
> > this specifc end case of stdin/stdout usually being dups but sometimes
> > not. Porting my event system over to use epoll was easy/straightforward
> > except for this one minor hitch.
>
> Easy: if it's a read event, it's stdin; if it's a write event, it's stdout :)
>
> You've raised an interesting problem.  It is easy to fix this in the
> specific case of stdin/stdout, however what happens when your process
> is passed a pair of fds from some other process (or more than one
> process, using AF_UNIX), and told to read one and write the other?
> What happens when you have 10 fds from different sources, some for
> reading and some for writing (quite typical in a complex server)?
>
> With the epoll API, your process has to know whether any paids or fds
> correspond to the same file *, in order to decide whether to register
> one interested in READ+WRITE or two interests separately.
>
> Unfortunately I cannot think of a way for a process to know, in
> general, whether two fds that it is passed correspond to the same file
> *.  Well, apart from trying epoll on it and seeing what happens :/
>
> Perhaps this indicates the epoll API _is_ flawed.  Epoll maintains
> this state mapping:
>
> 	file * -> (event mask, event states)
>
> when it ought to maintain this:
>
> 	(file *, event type) -> event state
>
> In other words, perhaps epoll should be keeping registered interest in
> read events and registered interest in write events completely
> separate.

It has to keep (file*, fd) as hashing key. That will work out just fine.


> I suspect changing the API to do that wouldn't even break any of the
> existing apps.
>
> Davide, what do you think?

Not even thinking changing the API since it'll break existing apps. The
above trick will do it. Going to test it ...


- Davide

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