Re: [Qemu-devel] Rethinking missed tick catchup

[Gleb Natapov <gleb@redhat.com>]

On Wed, Sep 12, 2012 at 07:30:08PM +0200, Stefan Weil wrote:
> Am 12.09.2012 18:45, schrieb Gleb Natapov:
> >On Wed, Sep 12, 2012 at 06:27:14PM +0200, Stefan Weil wrote:
> >>Am 12.09.2012 15:54, schrieb Anthony Liguori:
> >>>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).
> >>Nearly each modern OS (including Windows) uses NTP
> >>or some other protocol to get the time via a TCP network.
> >>
> >The drifts we are talking about will take ages for NTP to fix.
> >
> >>If a guest OS detects a small difference of time, it will usually
> >>accelerate or decelerate the OS clock until the time is
> >>synchronised again.
> >>
> >>Large jumps in network time will make the OS time jump, too.
> >>With a little bad luck, QEMU's reinjection will add the
> >>positive skew, no matter whether the guest is Linux or Windows.
> >>
> >As far as I know NTP will never make OS clock jump. The purpose of NTP
> >is to fix time gradually, so apps will not notice. npdate is used to
> >force clock synchronization, but is should be run manually.
> 
> s/npdate/ntpdate. Yes, some Linux distros run it at system start,
Yes, typo.

> and it's also usual to call it every hour (poor man's NTP, uses
> less resources).
> 
> >
> >>>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.
> >>D) makes no sense, see my comment above.
> >>
> >>Injection of additional timer interrupts should not be needed
> >>after a hibernation. The guest must handle that situation
> >>by reading either the hw clock (which must be updated
> >>by QEMU when it resumes from hibernate) or by using
> >>another time reference (like NTP, for example).
> >>
> >He is talking about host hibernation, not guest.
> >
> 
> I also meant host hibernation.
Than I don't see how guest can handle the situation since it has
no idea that it was stopped. Qemu has not idea about host hibernation
either.

> 
> Maybe the host should tell the guest that it is going to
> hibernate (ACPI event), then the guest can use its
> normal hibernate entry and recovery code, too.
Qemu does not emulate Sleep button, but even if it did guest may ignore
it. AFAIK libvirt migrate VM into a file during host hibernation. While
this does not require guest cooperation it have time keeping issues.

--
			Gleb.