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Date: Thu, 8 Feb 2001 13:22:18 +0000
From: "Stephen C. Tweedie" <sct@redhat.com>
To: Pavel Machek <pavel@suse.cz>
Cc: Linus Torvalds <torvalds@transmeta.com>, Jens Axboe <axboe@suse.de>,
        Marcelo Tosatti <marcelo@conectiva.com.br>,
        Manfred Spraul <manfred@colorfullife.com>,
        Ben LaHaise <bcrl@redhat.com>, Ingo Molnar <mingo@elte.hu>,
        "Stephen C. Tweedie" <sct@redhat.com>,
        Alan Cox <alan@lxorguk.ukuu.org.uk>, Steve Lord <lord@sgi.com>,
        Linux Kernel List <linux-kernel@vger.kernel.org>,
        kiobuf-io-devel@lists.sourceforge.net, Ingo Molnar <mingo@redhat.com>
Subject: Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait
Message-ID: <20010208132218.E9130@redhat.com>
In-Reply-To: <20010206230929.K2975@suse.de> <Pine.LNX.4.10.10102061421490.1825-100000@penguin.transmeta.com> <20010208001513.B189@bug.ucw.cz>
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In-Reply-To: <20010208001513.B189@bug.ucw.cz>; from pavel@suse.cz on Thu, Feb 08, 2001 at 12:15:13AM +0100
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Hi,

On Thu, Feb 08, 2001 at 12:15:13AM +0100, Pavel Machek wrote:
> 
> > 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_?

Select might make some sort of sense for sequential access to files,
and for random access via lseek/read but it makes no sense at all for
pread and pwrite where select() has no idea _which_ part of the file
the user is going to want to access next.

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

Select can work if the access is sequential, but async IO is a more
general solution.

Cheers,
 Stephen
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