[Xen-users] Need for precisions on SAR measures

[Nicolas80 <dedestri@xxxxxxxxxxx>]

Hi,

Encountering some difficulties to exploit SAR results in a virtualized
environment, I need some precisions about measures given by the command "sar
-u" (sysstat version : 8.1.8).
I hope you will have time to answer my few questions.

Hereafter are usefull information to present the problem.

Context
-------
My test run on a system virtualized with Xen 3.0.1.
5 OS are installed on this system : Dom0 and 4 identical virtual machines.
The virtual CPU of each VM is caped to 25% of the physical CPU. Dom0 has no
CPU limitation.

The command "sar -u" is executed on each OS during the test.

(1)Average values given by the test are :

    user    nice    system    iowait    steal    idle
Dom0    0,02    0    0,58    18,82    1,57    79,01
vm1    14,93    0    2,18    13,43    33,38    36,08
vm2    0,01    0    0,02    5,43    0,63    93,9
vm3    0,01    0    0,02    2,64    0,64    96,7
vm4    0,01    0    0,01    4,22    0,14    95,63

(2)Next values are an extract from vm1 results :

user       nice      system   iowait    steal     idle
...
13,47      0,00      0,00     11,72     55,86     18,95
51,85      0,00      1,23      7,41     36,79      2,72
29,46      0,00      0,33     15,55     40,59     14,08
...


Questions
---------
 - Is the CPU load of a virtual machine equal to the sum : %user + %system +
%iowait ? Otherwise which sum could be representative ?

 - As the CPU allocated to each VM is 25% of the physical CPU, is the SAR
measure related to 25% of the physical CPU or to 100% of the physical CPU ?

 - Inside a VM, is it normal to get measures overriding the fixed CPU limit
(25%) (cf. (2))?

 - Does the "iowait" measure correspond to a process wait for hard drive IO
response ?

 - Is the CPU idle during "iowait" (ie : available for other uses) ?

 - What does "steal" measure mean ? Does it figure :
   * a physical CPU ressource needed by the VM beyond the 25% threshold and
refused to the VM ?
   * a fraction of the 25% physical CPU ressource allocated to the VM and
which is not disposable for the VM processes ?


Despite our researches (manual, sites, forums), we didn't find a complete
answer. I'd be grateful if you could provide me some.

Thanks.
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