[Xen-devel] Re: change hvm defaults for timer_mode and hpet?

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

I'm happy for the defaults to change. Send a patch.

 -- Keir

On 3/6/08 20:47, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> Due to recent changes in timer handling (specifically building
> hpet emulation on top of Xen system time and ensuring it is
> monotonic), I wonder if it now makes sense to:
> 
> 1) change hvm default for hpet to 1 (was 0)
> 2) change hvm timer_mode default from 0 to 2
> 
> I encouraged adding the hvm hpet parameter and defaulting
> it to 0 because the virtual hpet was not reliable and many
> guests/versions default to using hpet (by default) it if
> it is available. That reliability problem should now be fixed.
> 
> Timer_mode==0 is necessary for guests that do not have
> a monotonic platform timer; each processor in a multi-VCPU
> guest has to do its best using only pit-generated ticks and
> lost ticks mean lost time, so Xen does its best to squirrel
> away any ticks that occur while a VCPU is asleep and deliver
> all of them when the VCPU awakens.  Thus time moves forward
> independently on each VCPU, leading to potential unavoidable
> "Time went backwards" problems.   With virtual hpet
> working properly, timer_mode==0 should rarely be necessary,
> though I think we should leave it around for pre-hpet-capable
> guests.
> 
> Although both of these parameters are easily specified in the
> hvm config file, both are obscure and difficult to explain.
> It would be nice if they were "right" most of the time and
> only needed to be specified/explained for corner cases.
> 
> Comments?  It would be nice to get this all settled (and fully
> tested) prior to 3.3.
> 
> Thanks,
> Dan
> 
> 
> ===================================
> Thanks... for the memory
> I really could use more / My throughput's on the floor
> The balloon is flat / My swap disk's fat / I've OOM's in store
> Overcommitted so much
> (with apologies to the late great Bob Hope)



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