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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