Re: [Xen-devel] [PATCH] Add a timer mode that disables pending missed ticks

[Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>]

oops, you're right. I missed the  ( (NOW() - pt->scheduled) >= 0 ).

This means I will have to come up with another explanation.

-Dave


Keir Fraser wrote:

> Thanks for the explanation. I think you are mis-describing the current
> behaviour of vpt.c though (If I understand it correctly myself, which I may
> not!). As I understand it, we do *not* unconditionally deliver a tick and
> reset the time space when a vcpu is scheduled onto a cpu. We only do that if
> (NOW() - pt->scheduled) >= 0 -- that is if more than a tick period has
> passed. Otherwise we wait to deliver the next tick exactly one period after
> the previous tick was delivered. The remainder of your explanation seems to
> be predicated on the (impossible?) case that we deliver a tick less than one
> period after the previous one.
>
> Am I confused? :-) Also, what version of Linux are we talking about here,
> and what periodic timer, and x86/64 or i386? There are lots of different
> Linux timer-handling logics out there in the wild!
>
> -- Keir
>
> On 2/11/07 15:51, "Dave Winchell" <dwinchell@xxxxxxxxxxxxxxx> wrote:
>
>  
>
> > Let D be the time that the clock vcpu is descheduled and P be
> > the clock period.  When D < P, I think there is an issue.
> >
> > The reason is that Linux's offset calculation, which affects the
> > last clock interrupt tsc that is recorded, doesn't kick in if the
> > time since the last interrupt is less than P. In this case it sets
> > offset to zero. Linux records the tsc of the current (last) clock interrupt
> > as (current tsc - offset).
> >
> > Interrupt delivery method AS delivers a clock interrupt
> > at context switch (in) time. When D < P, Linux records the
> > interrupt delivery time as current tsc and bumps jiffies.
> > This results in a gain of time equal to P-D over wall time.
> >
> > For method S this never happens because the interrupt isn't
> > delivered until the next boundary.
> >    
>  
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