Xen timing mode

Xen timing mode

[PBhat]

A question about timing modes in Xen came up after reading the documentation
on 
http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight=(interrupts)|(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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Re: Xen timing mode

[Keir Fraser]

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=(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.

Re: Xen timing mode

[Priya]

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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=(inter<http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight=%28inter>
> > 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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Re: Xen timing mode

[Keir Fraser]

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
>>> 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.
>> 
>> 
>> 
> 
> 

Re: Xen timing mode

[Priya]

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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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Re: Xen timing mode

[Priya]

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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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Re: Xen timing mode

[Jeremy Fitzhardinge]

On 02/23/2010 09:29 AM, Priya 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.

/proc/stat publishes stolen time for each vcpu (second-last column on 
the "cpuX" lines); you can subtract that from monotonic time to work out 
how much vcpu the domain has got.

     J

Re: Xen timing mode

[Priya]

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Thank you very much!



On Tue, Feb 23, 2010 at 3:28 PM, Jeremy Fitzhardinge <jeremy@goop.org>wrote:

> On 02/23/2010 09:29 AM, Priya 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.
>>
>
> /proc/stat publishes stolen time for each vcpu (second-last column on the
> "cpuX" lines); you can subtract that from monotonic time to work out how
> much vcpu the domain has got.
>
>
>    J
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
>
>

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