qemu-devel
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Qemu-devel] Rethinking missed tick catchup


From: Stefan Weil
Subject: Re: [Qemu-devel] Rethinking missed tick catchup
Date: Wed, 12 Sep 2012 19:30:08 +0200
User-agent: Mozilla/5.0 (X11; Linux i686; rv:15.0) Gecko/20120827 Thunderbird/15.0

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,
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.

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.




reply via email to

[Prev in Thread] Current Thread [Next in Thread]