linux-kernel - Re: [patch V2 00/20] timer: Refactor the timer wheel

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Date:	Sun, 26 Jun 2016 22:02:48 +0200
From:	Pavel Machek <pavel@....cz>
To:	Arjan van de Ven <arjanvandeven@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>,
	Arjan van de Ven <arjan@...radead.org>, rt@...utronix.de,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	George Spelvin <linux@...encehorizons.net>,
	Len Brown <lenb@...nel.org>, ltp@...ts.linux.it
Subject: Re: [patch V2 00/20] timer: Refactor the timer wheel

Hi!

On Sun 2016-06-26 12:21:46, Arjan van de Ven wrote:
> On Sun, Jun 26, 2016 at 12:00 PM, Pavel Machek <pavel@....cz> wrote:
> >
> > Umm. I'm not sure if you should be designing kernel...
> >
> > I have alarm clock application. It does sleep(60) many times till its
> > time to wake me up. I'll be very angry if sleep(60) takes 65 seconds
> > without some very, very good reason.
> 
> I'm fairly sure you shouldn't be designing alarm clock applications!
> Because on busy systems you get random (scheduler) delays added to
>your timer.

I'm pretty sure I should not be designing alarm clock applications,
after looking at the timezone stuff. But alarm clock from mate eats 3%
cpu at my cellphone, so I kind of had to.

And yes, I'm aware that scheduler delays would add up. But if it is 79
seconds before alarm, I do sleep(79), and it would be strange to have
alarm fire 5 seconds too late.

> Having said that, your example is completely crooked here, sleep()
> does not use these kernel timers, it uses hrtimers instead.
> (hrtimers also have slack, but an alarm clock application that is this
> broken would have the choice to set such slack to 0)
> 
> What happened here is that these sigtimewait were actually not great,
> it is just about the only application visible interface that's still
> in jiffies/HZ,
> and in the follow-on patch set, Thomas converted them properly to
> hrtimers as well to make them both accurate and CONFIG_HZ
> independent.

So it is going to be fixed, good.

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html