Re: Strange interdependace between domains

[Dario Faggioli <dario.faggioli@citrix.com>]

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On ven, 2014-02-14 at 12:02 +0000, Simon Martin wrote:
> Thanks everyone and especially Ian! It was the hyperthreading that was
> causing the problem.
> 
Good to hear, and at the same time, sorry to hear that. :-)

I mean, I'm glad you nailed it, but at the same time, I'm sorry that the
solution is to 'waste' a core! :-( I reiterate and restate here, without
any problem doing so, the fact that Xen should be doing at least a bit
better in these circumstances, if we want to properly address use cases
like yours. However, there are limits on how far we can go, and hardware
design is certainly among them!

All this to say that, it should be possible to get a bit more of
isolation, by tweaking the proper Xen code path appropriately, but if
the amount of interference that comes from two hypethreads sharing
registers, pipeline stages, and whatever it is that they share, is
enough for disturbing your workload, then, I'm afraid we never get much
farther from the 'don't use hyperthread' solution! :-(

Anyways, with respect to the first part of this reasoning, would you
mind (when you've got the time of course) one more test? If no, I'd say,
configure the system as I was suggesting in my first reply, i.e., using
also core #2 (or, in general, all the cores). Also, make sure you add
this parameter, to the Xen boot command line:

 sched_smt_power_savings=1

(some background here:
http://lists.xen.org/archives/html/xen-devel/2009-03/msg01335.html)

And then run the bench with disk activity on.

> 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).
> 
Actually, you are right. It looks like there is no command or command
parameter telling explicitly to which pool a domain belong [BTW, adding
Juergen, who knows that for sure].

If that is the case, we really should add one.

BTW, If you boot the system and then create the (new) pool(s), all the
existing domains, including Dom0, at pool(s) creation time will stay in
the "original" pool, while the new pool(s) will be empty. To change
that, you'd have to either migrate existing domain into specific pools
with `xl cpupool-migrate', or create them specifying the proper option
and the name of the target pool in the config file (which is probably
what you're doing for your DomUs).

I guess a workaround to confirm where a domain is, you can (try to)
migrate it around with `xl cpupool-migrate', and see what happens.

> Dario, if you are going to look at the
> 
Is something missing here... ?

Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)


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