linux-kernel - Re: RFC: A revised timerfd API

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Message-Id: <1190480717.4035.45.camel@chaos>
Date:	Sat, 22 Sep 2007 19:05:17 +0200
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Michael Kerrisk <mtk.linux.lists@...il.com>
Cc:	Bernd Eckenfels <ecki@...a.inka.de>, linux-kernel@...r.kernel.org,
	Ulrich Drepper <drepper@...hat.com>, geoff@...are.org.uk,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>,
	Jonathan Corbet <corbet@....net>,
	Randy Dunlap <rdunlap@...otime.net>, vda.linux@...glemail.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	David Härdeman <david@...deman.nu>,
	mtk-manpages@....net
Subject: Re: RFC: A revised timerfd API

On Sat, 2007-09-22 at 18:07 +0200, Michael Kerrisk wrote:
> Hello Bernd,
> 
> Please don't trim the CC list when replying!  I nearly did not see
> your reply, and others will have missed it also.

Yup.

> On 9/22/07, Bernd Eckenfels <ecki@...a.inka.de> wrote:
> > In article <46F514C9.5010208@....net> you wrote:
> > >  1. This design stretches the POSIX timers API in strange
> > >     ways.
> >
> > Maybe it is possible to reimplement the POSIX API in usermode using the
> > kernel's FD implementation?

Yikes.

> It's a clever idea...  Without thinking on it too long, I'm not sure
> whether or not there might be some details which would make this
> difficult.

You'd need be quite masochistic to start such a project. The POSIX timer
API consists mostly of corner cases and I doubt that you get them even
halfway under control in a pure user space implementation.

It would be a rather huge performance penalty as well. You need at least
two user space context switches to get the most simple cases resolved.

> > (and drop the posix support from kernel)
> 
> However we couldn't drop POSIX support from the kernel, because that
> would break the ABI.

True. So there is no point in reinventing the wheel.

	tglx


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