Re: [Qemu-devel] Rethinking missed tick catchup

[Gleb Natapov <gleb@redhat.com>]

On Wed, Sep 12, 2012 at 06:06:47PM +0300, Gleb Natapov wrote:
> On Wed, Sep 12, 2012 at 09:44:10AM -0500, Anthony Liguori wrote:
> > Jan Kiszka <jan.kiszka@siemens.com> writes:
> > 
> > > On 2012-09-12 15:54, Anthony Liguori wrote:
> > >> 
> > >> Hi,
> > >> 
> > >> We've been running into a lot of problems lately with Windows guests and
> > >> I think they all ultimately could be addressed by revisiting the missed
> > >> tick catchup algorithms that we use.  Mike and I spent a while talking
> > >> about it yesterday and I wanted to take the discussion to the list to
> > >> get some additional input.
> > >> 
> > >> Here are the problems we're seeing:
> > >> 
> > >> 1) Rapid reinjection can lead to time moving faster for short bursts of
> > >>    time.  We've seen a number of RTC watchdog BSoDs and it's possible
> > >>    that at least one cause is reinjection speed.
> > >> 
> > >> 2) When hibernating a host system, the guest gets is essentially paused
> > >>    for a long period of time.  This results in a very large tick catchup
> > >>    while also resulting in a large skew in guest time.
> > >> 
> > >>    I've gotten reports of the tick catchup consuming a lot of CPU time
> > >>    from rapid delivery of interrupts (although I haven't reproduced this
> > >>    yet).
> > >> 
> > >> 3) Windows appears to have a service that periodically syncs the guest
> > >>    time with the hardware clock.  I've been told the resync period is an
> > >>    hour.  For large clock skews, this can compete with reinjection
> > >>    resulting in a positive skew in time (the guest can be ahead of the
> > >>    host).
> > >> 
> > >> I've been thinking about an algorithm like this to address these
> > >> problems:
> > >> 
> > >> A) Limit the number of interrupts that we reinject to the equivalent of
> > >>    a small period of wallclock time.  Something like 60 seconds.
> > >> 
> > >> B) In the event of (A), trigger a notification in QEMU.  This is easy
> > >>    for the RTC but harder for the in-kernel PIT.  Maybe it's a good time to
> > >>    revisit usage of the in-kernel PIT?
> > >> 
> > >> C) On acculumated tick overflow, rely on using a qemu-ga command to
> > >>    force a resync of the guest's time to the hardware wallclock time.
> > >> 
> > >> D) Whenever the guest reads the wallclock time from the RTC, reset all
> > >>    accumulated ticks.
> > >> 
> > >> In order to do (C), we'll need to plumb qemu-ga through QMP.  Mike and I
> > >> discussed a low-impact way of doing this (having a separate dispatch
> > >> path for guest agent commands) and I'm confident we could do this for
> > >> 1.3.
> > >> 
> > >> This would mean that management tools would need to consume qemu-ga
> > >> through QMP.  Not sure if this is a problem for anyone.
> > >> 
> > >> I'm not sure whether it's worth trying to support this with the
> > >> in-kernel PIT or not either.
> > >
> > > As with our current discussion around fixing the PIC and its impact on
> > > the PIT, we should try on the userspace model first and then check if
> > > the design can be adapted to support in-kernel as well.
> > >
> > > For which guests is the PIT important again? Old Linux kernels? Windows
> > > should be mostly happy with the RTC - or the HPET.
> > 
> > I thought that only 64-bit Win2k8+ used the RTC.
> > 
> > I thought win2k3 and even 32-bit win2k8 still used the PIT.
> > 
> Only WindowsXP non-acpi hal uses PIT. Any other windows uses RTC. In
> other words we do not care about PIT.
> 
Small clarification. They use RTC if HPET is not present. I don't know
at what version Windows started to prefer HPET over RTC.

--
			Gleb.