[virt-tools-list] How does the memory usage calculated?

[Cole Robinson]

On 11/18/2015 09:44 AM, Richard W.M. Jones wrote:
> On Wed, Nov 18, 2015 at 01:04:32PM +0000, 张强 wrote:
>> Hi all,
>> I’m studying the source code, and I saw this in domain.py around line 164:
>> …
>>         curmem = stats['rss']
>>         totalmem = stats['actual']
>> except libvirt.libvirtError, err:
>>     logging.error("Error reading mem stats: %s", err)
>>
>> pcentCurrMem = curmem * 100.0 / totalmem
>> pcentCurrMem = max(0.0, min(pcentCurrMem, 100.0))
>> …
>>
>> But using this algorithm, I’m always getting results that are greater than 100:
>>>>> dom.memoryStats()
>> {'actual': 16777216L, 'rss': 17062900L}
>>
>> 17062900 * 100 / 16777216 = 101.7028…
>>
>> Is this normal?
> 
> The VIR_DOMAIN_MEMORY_STAT_RSS statistic returned for qemu/KVM guests
> is the resident set size (RSS) of the qemu process.  Of course that
> includes all the overhead of qemu, such as host-side structures used
> when emulating devices.  Plus the guest RAM, which is just a regular
> malloc-style allocation inside qemu and so also contributes to the RSS.
> 
> The 'actual' field seems to come from the libvirt
> VIR_DOMAIN_MEMORY_STAT_ACTUAL_BALLOON statistic which is found by
> sending the query-balloon monitor command to the guest.  The returned
> value is the guest's get_current_ram_size() (or less if the balloon is
> "inflated", but for the majority of guests the balloon is never used).
> 
> Given all that, it seems reasonable that rss > actual, and so you'd
> get a number > 100%.  Sometimes.  It also seems to me that if the
> guest RAM had been allocated but not accessed, you might see rss < actual.
> 
> rss / actual seems like a fairly meaningless number from a
> virt-manager point of view.
> 

Yeah I think this calculation is leftover from when virt-manager's memory
reporting just tracked memory ballooning, so pcentCurrMem was <currentMemory>
/ <memory>. I'll look into cleaning it up

- Cole