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Re: [Xen-devel] host information in domU over /proc or /sys filesystem

To: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] host information in domU over /proc or /sys filesystem
From: Hannes Kuehnemund <hannes.kuehnemund@xxxxxxx>
Date: Tue, 19 Jun 2007 16:51:32 +0200
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Mark Williamson wrote:
Rather than trying to sample how many pCPUs the domain gets to run on, perhaps it would be better to monitor the total time its vCPUs are being scheduled *somewhere*. Then you can just compare whether your pCPU time allowance corresponds to your SLA.

This way the domain still doesn't need to know which specific CPUs it gets scheduled on, but you can get all the information you need?

VMware for example handles this information in a similar way. Having the total consumed CPU time (cpu_time) for a VM inside the VM (which is only available in dom0 at the moment) one can calculate how many computing power is available.

Lets consider, that my VM has 2 VCPUs configured. The VM furthermore is configured to run on 2 physical CPUs, and this is also the SLA between the hosting provider and the customer. But, another VM is also running on these 2 physical CPU's. Having the possibility to check the cpu_time on a frequent base, e.g. seconds, one would be able to check, if the guaranteed phsical CPUs are available. To check this, simply generate a high load inside the VM, and if the cpu_time increases every second by the value of 2, both physical CPUs are available. In case the cpu_time only increases by the value on 1 or 1,5 it is clear, that the SLA is not fullfilled.

Is it possbile to provide this information to the domU (if explicitly enabled in dom0)?

The third possibility is to setup a dedicated VM which interacts between
all available dom0's and all available domU's. But this is the last
possible solution I'd prefer, because a third instance to manage is a
third source of errors.
I'm still not sure why the domU would need to know the host's hostname though? Or is it just as a fallback if the CPU data isn't directly available?

This is meant to be the fallback, in case it is not possible to get any data from the hypervisor. I'm not sure, but having HVM setups running, getting data might be very difficult. Therefore, having a fallback (which can be turned on, but will be turned off by default) can be very convenient.

Cheers,
Mark

Cheers,
  Hannes

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