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

[Pavel Machek]

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@xxxxxx> 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
-- 
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