Re: Xen timing mode

[Priya <pbhat@acis.ufl.edu>]

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The latest interface manual which contains a similar definition of
domain-virtual time as stated in my last email can be found here:

http://www.cl.cam.ac.uk/research/srg/netos/xen/readmes/interface/interface.html#SECTION00340000000000000000

Priya

On Tue, Feb 23, 2010 at 12:29 PM, Priya <pbhat@acis.ufl.edu> wrote:

> The Xen Interface manual for Xen v2.0 (attached) Page 4 defines Domain -
> virtual time as
>
> " The time that progresses at the same pace as system time, but only while
> a domain is executing -- it stops while a domain is de-scheduled. Therefore
> the share of the CPU that a domain receives is indicated by the rate at
> which its virtual time increases."
>
> I am interesting in finding out if any of the timing system
> calls/instructions like gettimeofday(), hwclock(), rdtsc() can read this
> domain virtual time.
>
> Thanks, again !
>
> PS: I could not find a similar documentation for Xen v3.0 or later
>
>
>
> On Tue, Feb 23, 2010 at 12:18 PM, Keir Fraser <keir.fraser@eu.citrix.com>wrote:
>
>> What do you mean by domain - virtual time?
>>
>>
>> On 23/02/2010 17:12, "Priya" <pbhat@acis.ufl.edu> wrote:
>>
>> > Thanks !
>> >
>> > In that case, how does a para-virtualized user domain find the domain -
>> > virtual time ?
>> >
>> > --Priya
>> >
>> > On Tue, Feb 23, 2010 at 12:08 PM, Keir Fraser <
>> keir.fraser@eu.citrix.com>
>> > wrote:
>> >> Independent_wallclock is specific to Linux PV guests, and is set within
>> the
>> >> guest. Timer_mode is specific to HVM guests and is a domain build-time
>> >> config option.
>> >>
>> >>  -- Keir
>> >>
>> >> On 23/02/2010 16:50, "PBhat" <pbhat@acis.ufl.edu> wrote:
>> >>
>> >>>
>> >>> A question about timing modes in Xen came up after reading the
>> documentation
>> >>> on
>> >>>
>> http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight=(int<http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight=%28int>
>> >>> er
>> >>> <
>> http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight=%28
>> >>> inter>
>> >>> rupts)|(timer)
>> >>> configuration file options  and I was wondering if you could help me
>> with
>> >>> it.
>> >>>
>> >>>     * I know that Xen has a facility on the user domains to change the
>> >>> timing from something called the '
>> http://openskill.info/infobox.php?ID=1451
>> >>> independent wallclock ' to 'dependent clock' and vice versa. The way
>> to do
>> >>> this is to toggle the parameter /proc/sys/xen/independent_wallclock
>> between
>> >>> 0 (dependent) and 1 (independent)
>> >>>
>> >>>
>> >>>     * Now, the documentation on the configuration options for xen
>> virtual
>> >>> machine under the heading of Timers contains no mention of the above
>> >>> mentioned facility, but instead talks about something called a
>> 'timer_mode'
>> >>>
>> >>>
>> >>>  timer_mode: Timer mode (0=delay virtual time when ticks are missed;
>> >>> 1=virtual time is always wallclock time
>> >>>       timer_mode (default=1; Value='TIMER_MODE')
>> >>>
>> >>> Now my question is whether the timer mode  = 1 is equivalent to
>> independent_
>> >>> wallclock?
>> >>>
>> >>> I think the answer is no. While timer_mode = 1 decides whether the
>> virtual
>> >>> machine is able to read the Domain-0 time, the independent_wallclock
>> decides
>> >>> whether you need NTP sychronization on both (user and control) domains
>> or
>> >>> not.
>> >>>
>> >>> Namely, in the timer_mode = 1, the gettimeofday() called from the
>> virtual
>> >>> machine will return the domain virtual time, but time_mode = 0
>> >>> gettimeofday() instruction from the virtual machine will return the
>> system
>> >>> time.
>> >>>
>> >>> The independent and dependent modes have an effect on whether the
>> virtual
>> >>> machines can use the NTP or any other synchronization of Domain-0.
>> >>>
>> >>> Is my understanding correct?
>> >>>
>> >>> I further want to know whether the working remains similar on Xen HVMs
>> also.
>> >>> In the sense that are Hardware based virtual machines capable of
>> reading the
>> >>> Domain-0 system time in an analogous way?
>> >>>
>> >>>
>> >>> Your help would be much appreciated. I am new to this area and I
>> apologize
>> >>> if my questions seem too basic.
>> >>
>> >>
>> >>
>> >
>> >
>>
>>
>>
>>
>

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