Re: [Xen-devel] [PATCH] Deferrable Timer

[Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>]

Yu, Ke wrote:

> Keir Fraser wrote:
>  
>
> > On 17/7/08 13:54, "Yu, Ke" <ke.yu@xxxxxxxxx> wrote:
> >
> >    
> >
> > > This patch adds new ac timer API set_timer_deferrable for the
> > > deferrable timer.
> > >      
> > Whether this is worthwhile depends on what likely users there are.
> > platform-timer overflow is not very compelling since if you're using
> > anything other than PIT (which ought to be likely on a modern system
> > supporting deep sleep) the overflow period should be multiple seconds.
> >
> > -- Keir
> >    
> True. Another user is the Px state sampling timer, which is 20ms. Other
> potential timers (e.g. sched timer, hvm pt timers) are also under
> evaluation, the principle is to make sure there is no performance
> downgrade.
>  
Ke,

One would think that hpet or vpt support for the guest-handles-missed-ticks
policy would be a good application for a deferrable timer.
If a deferrable timer were used, then the comparator (cmp) would have to
be warped to a non-integer multiple of the period. This is because Linux
reads the comparator register to estimate the delay since the interrupt 
was posted.
I don't think warping like this will be a problem. At some point, I can test
this.

I think we could use the deferrable timer for the 
guest-does-not-handle-missed-ticks
policy as well.

Any investigation that you want to do in the platform timer area would 
be fine.
Or I can do it, but that will probably be after I do the vpt.c/hpet.c 
integration
work.

thanks,
Dave

> Best Regards
> Ke
>
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