Re: Strange interdependace between domains

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 14/02/14 12:02, Simon Martin wrote:
> Hello Simon,
>
> Thanks everyone and especially Ian! It was the hyperthreading that was
> causing the problem.
>
> Here's my current configuration:
>
> # xl cpupool-list -c
> Name               CPU list
> Pool-0             0,1
> pv499              2,3
> # xl vcpu-list
> Name                                ID  VCPU   CPU State   Time(s) CPU Affinity
> Domain-0                             0     0    0   r--      16.6  0
> Domain-0                             0     1    1   -b-       7.3  1
> win7x64                              1     0    1   -b-      82.5  all
> win7x64                              1     1    0   -b-      18.6  all
> pv499                                2     0    3   r--     226.1  3
>
> I  have  pinned  dom0 as I wasn't sure whether it belongs to Pool-0 (I
> assume it does, can you confirm please).
>
> Dario, if you are going to look at the
>
> Looking at my timings with this configuration I am seeing a 1%
> variation (945 milliseconds +/- 5). I think this can be attributable
> to  RAM  contention, at the end of the day all cores are competing for
> the same bus.
>

There is also things such as the Xen time calibration rendezvous which a
synchronisation point of all online cpus, once per second.  Having any
single cpu slow to enter the rendezvous will delay all others which have
already entered.

This will likely add a bit of jitter if one cpu in xen is doing a
lengthy operation with interrupts disabled at the point at which the
rendezvous is triggered.

~Andrew