XenAPI: vm_metrics sometimes does not return valid data

[Joanna Rutkowska <joanna@invisiblethingslab.com>]

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Hello,

In order to get per-VM CPU utilization I use a code similar to this one
[1] (simplified version):

uuids = xend_session.session.xenapi.VM.get_by_name_label (name)
session_uuid = uuids[0]
session_metrics = session.xenapi.VM.get_metrics(session_uuid)
cpus_util = session.xenapi.VM_metrics.get_VCPUs_utilisation
(session_metrics)

Now, this works well, except for when it doesn't... Namely, very rarely,
XenAPI seems to break down and starts to return 0.0s for CPU load for
all the VMs for all the VCPUs.

I have verified that all the other objects are not "None", i.e. that
XenAPI keeps returning valid uuids, and session_metrics objects, but the
cpus_util list is always this:

cpus_util =  {'0': 0.0}

(For the case with 1 VCPU assigned to a VM).

I keep getting this for all the VMs in the system, despite the fact that
those VMs actually do some work (e.g. xentop correctly reports some load
on their VCPUs).

The above doesn't apply to Dom0 CPUs utilization, which I obtain using
the following code [2]:

session_hosts = session.xenapi.host.get_all()
session_cpus = session.xenapi.host.get_host_CPUs(session_hosts[0])

for cpu in self.session_cpus:
    cpu_total_load += session.xenapi.host_cpu.get_utilisation(cpu)

cpu_total_load /= len(session_cpus)

In that case I always keep getting the following values (Dom0 uses 2 CPUs):

0.361701816398
0.299435554096

Once I reboot the system the problems vanish.

Have no idea how to reproduce it -- occurs very rarely, but still
annoying. Anybody run into a similar problem?

I'm running Xen 3.4.3-rc3, but I don't think saw any commit that would
be dealing with such a problem in the more recent rc's.

joanna.

[1] The actual code is here, in case you're interested:
http://qubes-os.org/gitweb/?p=mainstream/core.git;a=blob;f=dom0/qvm-core/qubes.py;h=6e5a686b4a43b3b1b3c2cfba04b63232a6ef6edf;hb=HEAD#l326

[2]
http://qubes-os.org/gitweb/?p=mainstream/core.git;a=blob;f=dom0/qvm-core/qubes.py;h=6e5a686b4a43b3b1b3c2cfba04b63232a6ef6edf;hb=HEAD#l867


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