On mar, 2013-05-21 at 11:02 +0200, Massimo Canonico wrote:
> Hi,
>
Hi again,

> I sent the following problem on xen-user ML without an answer. I hope 
> I'll get one in this ML.
> 
> My application is written in std C++ and it makes a matrix 
> multiplication: so it uses only CPU and memory (no I/O, no network).
> 
> I'm quite surprise that with CAP = 100% I got my results in about 600 
> seconds and with CAP = 50% I got my results in about 1800 seconds 
> (around 3 times longer).
> 
> For this kind of application I was expecting to get results in about 
> 1200 seconds (2 times longer) for the second scenario with respect to 
> the first one.
> 
> Of course, the HW and SW are exactly the same for the 2 experiments.
> 
> Am I wrong or the CAP mechanism is not working well?
> 
Ok, I found a minute to run your code myself on my test box. It's quite
a large one, but since the VM has only 1 vcpu, that shouldn't really
make much difference.

I configured vcpu-pinning in such a way that there should be no room for
interference of any kind, i.e., dedicating a core to the VM, and making
sure even his fellow thread is not busy (which matters in an
hyperthreaded system):

# xl vcpu-list 
Name                                ID  VCPU   CPU State   Time(s) CPU
Affinity
Domain-0                             0     0    7   -b-      38.7  0-7
Domain-0                             0     1    3   -b-       2.3  0-7
Domain-0                             0     2    2   -b-       3.3  0-7
Domain-0                             0     3    6   -b-       6.8  0-7
Domain-0                             0     4    4   -b-       3.2  0-7
Domain-0                             0     5    2   -b-       3.6  0-7
Domain-0                             0     6    4   -b-       2.1  0-7
Domain-0                             0     7    1   -b-       1.8  0-7
Domain-0                             0     8    0   -b-       2.2  0-7
Domain-0                             0     9    7   -b-       1.7  0-7
Domain-0                             0    10    1   -b-       1.8  0-7
Domain-0                             0    11    5   r--      10.4  0-7
Domain-0                             0    12    1   -b-       3.5  0-7
Domain-0                             0    13    2   -b-       3.5  0-7
Domain-0                             0    14    3   -b-       2.7  0-7
Domain-0                             0    15    0   -b-       1.9  0-7
vm1                                  1     0   11   -b-     677.0  11

The numbers I'm getting are, I think, much more consistent with the
expectations:

 * no cap:
  Client served in 299.024
  Client served in 298.783
  Client served in 298.445
 * cap 50%:
  Client served in 643.668
  Client served in 643.372
  Client served in 644.342

Which means time roughly doubles.

I tried without pinning as well, and I'm getting pretty much the same
values.

At this point, I'm not sure what could be going on on your side. If you
want to try producing some traces, we can help inspect them, looking for
something weird. You can find some information about how to produce and
better interpret traces in this blog post:

http://blog.xen.org/index.php/2012/09/27/tracing-with-xentrace-and-xenalyze/

Perhaps you can share your VM config file and Dom0 configuration
(basically, Xen and Linux boot command lines), to check whether there is
something strange there. Also, you might have said this already (in
which case I forgot), what versions of Xen and Linux are we talking
about?

I really am out of good ideas... George, any clue?

Regards,
Dario