RE: [Xen-ia64-devel] Question about migration

["Xu, Anthony" <anthony.xu@xxxxxxxxx>]

Kouya Shimura wrote:
> Hi,
> 
> In a SMP machine, complete synchronizing ITCs is impossible.
Agree,

But the difference is limited in a small range,
So, when application is migrated to another CPU, it will see ITC > what
it saw last time.
That means, itc difference have to be less than the itc cycle spent on
process switch.
Otherwise, linux kernel needs to re sync the ITC.

> (i.e. each cpu's ITC has some error)
> So small backward jump might occur in native SMP linux
> except that a process is pinned to a cpu.
> Does its DB perform pinning?
I didn't run any DB, just know it.

Anthony



> 
> Thanks,
> Kouya
> 
> Xu, Anthony writes:
>> Isaku Yamahata wrote:
>>> On Fri, Feb 01, 2008 at 03:08:24PM +0800, Xu, Anthony wrote:
>>>> In DomU, ar.itc is not virtualized, so application can get itc
>>>> directly. When DomU is migrated, how do you prevent vitc jump a
>>>> lot( foreward/backward)?
>>> 
>>> Currently nothing prevents it. So a big itc jump can happen.
>>> Anyway user application can be stopped/continued by SIGSTOP/SIGCONT
>>> so that user application should be tolerable with such a big jump of
>>> itc.
>> Foreward jump may be OK,
>> How about back-ward jump? Application may be confused.
>> 
>>> 
>>> So in theory it's ok. But I'm not sure in practical.
>>> Do you have any concrete examples/scenario in your mind?
>> No, But I know some DB use ITC,
>> 
>> Anthony
>> 
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