Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait

From: Pavel Machek (
Date: Wed Feb 07 2001 - 18:15:13 EST


> > > Reading write(2):
> > >
> > > EAGAIN Non-blocking I/O has been selected using O_NONBLOCK and there was
> > > no room in the pipe or socket connected to fd to write the data
> > > immediately.
> > >
> > > I see no reason why "aio function have to block waiting for requests".
> >
> > That was my reasoning too with READA etc, but Linus seems to want that we
> > can block while submitting the I/O (as throttling, Linus?) just not
> > until completion.
> Note the "in the pipe or socket" part.
> ^^^^ ^^^^^^
> EAGAIN is _not_ a valid return value for block devices or for regular
> files. And in fact it _cannot_ be, because select() is defined to always
> return 1 on them - so if a write() were to return EAGAIN, user space would
> have nothing to wait on. Busy waiting is evil.

So you consider inability to select() on regular files _feature_?

It can be a pretty serious problem with slow block devices
(floppy). It also hurts when you are trying to do high-performance
reads/writes. [I know it hurt in userspace sherlock search engine --
kind of small altavista.]

How do you write high-performance ftp server without threads if select
on regular file always returns "ready"?

> Remember: in the end you HAVE to wait somewhere. You're always going to be
> able to generate data faster than the disk can take it. SOMETHING

Userspace wants to _know_ when to stop. It asks politely using

I'm "In my country we have almost anarchy and I don't care."
Panos Katsaloulis describing me w.r.t. patents at
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This archive was generated by hypermail 2b29 : Thu Feb 15 2001 - 21:00:11 EST