RE: [Xen-devel] Question about hpet calls to set_timer

[Daniel Magenheimer <dan.magenheimer@xxxxxxxxxx>]

On some Linux time algorithms, if hpet ticks are delivered
too closely together, time advances too rapidly, whereas
if they are late (delivered too far apart), the algorithm
makes adjustments and time is kept properly.

Dave Winchell is the expert in this area and had a patch for
better hpet vpt handling submitted in early June but
Keir requested some rework and it got delayed until after
3.3.0.  Hopefully Dave can both comment on your questions
and submit the reworked patch soon.

> -----Original Message-----
> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> Sent: Sunday, August 17, 2008 4:25 AM
> To: Beth Kon; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Question about hpet calls to set_timer
> 
> 
> I might use 'hg anno' to see if anyone changed it to period 
> for good reason,
> but it seems to me that your suggested approach would be more 
> accurate,
> especially in the long run.
> 
>  -- Keir
> 
> On 15/8/08 19:41, "Beth Kon" <eak@xxxxxxxxxx> wrote:
> 
> > Hello. I'm looking over the Xen hpet.c to see what I can 
> steal for the
> > qemu implementation I'm working on. (Thank you kindly :-)
> > 
> > One thing that doesn't make sense to me is why, in 
> hpet_timer_fn, the
> > timer is set with:
> > 
> > set_timer(&h->timers[tn], NOW() + hpet_tick_to_ns(h, period));
> > 
> > rather than calculating the difference between the final 
> value of the
> > comparator (after the time_after loop) and the current 
> time. This is the
> > approach used in hpet_set_timer:
> > 
> > set_timer(&h->timers[tn], NOW() + hpet_tick_to_ns(h, diff));
> > 
> > and would seem to be the best way to get as close to the 
> desired expire
> > time as possible. Can someone explain why the approaches 
> are different
> > in these 2 places?
> 
> 
> 
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