linux-kernel - Re: [patch 6/9] signalfd/timerfd v1

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Date:	Sat, 10 Mar 2007 13:01:13 -0800
From:	Nicholas Miell <nmiell@...cast.net>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 6/9] signalfd/timerfd v1 - timerfd core ...

On Sat, 2007-03-10 at 12:41 -0800, Davide Libenzi wrote:
> On Sat, 10 Mar 2007, Nicholas Miell wrote:
> 
> > Try reading the timer_create man page.
> > 
> > In short, you're limited to a single clock, so you can't set timers
> > based on wall-clock time (subject to NTP correction), monotomic time
> > (not subject to NTP, will not ever go backwards or skip ticks), the
> > high-res versions of the previous two clocks, per-thread or per-process
> > CPU usage time, or any other clocks that may get introduced in the
> > future.
> 
> One timer per fd yes. So?

I never complained about one timer per fd (although, now that you
mention it, that would get a bit excessive if you have thousands of
outstanding timers).

> The real-time and monotonic selection can be added. 

IOW, the timerfd patch is not suitable for inclusion as-is. (While
you're at it, you should probably add a flags argument for future
expansion.)

> If you look at the posix timers code, that's a bunch of code over the real 
> meat of it, that is hrtimer.c. The timerfd interface goes straight to 
> that, without adding yet another meaning to the sigevent structure,

That's what the sigevent structure is for -- to describe how events
should be signaled to userspace, whether by signal delivery, thread
creation, or queuing to event completion ports. If if you think
extending it would be bad, I can show you the line in POSIX where it
encourages the contrary.

>  and 
> yet another case inside the posix timers trigger functions. That will be 
> as unstandard as timerfd is, and even more, since you cannot use that 
> interface and hope to be portable in any case.

If Linux were to do a wholesale theft of the Solaris interface (warts
and all), you'd be portable (and, now that I think of it, more
efficient).

Two major unixes using the same interface would probably make it a
shoe-in for the next POSIX, too. (c.f. openat(2) and friends)

> On top of that, handing over files to the posix timers will creates 
> problems with references kept around.
> The timerfd code is just a *really* thin layer (if you exclude the 
> includes, the structure definitions and the fd setup code, there's 
> basically *nothing*) over hrtimer.c and does not mess up with other kernel 
> code in any way, and offers the same functionalities. I'd like to keep it 
> that way.

-- 
Nicholas Miell <nmiell@...cast.net>

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