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

[Richard W.M. Jones]

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.

Rich.

References:

libvirt.git/src/qemu/qemu_driver.c qemuGetProcessInfo &
qemuDomainMemoryStats.

libvirt.git/src/qemu/qemu_monitor_json.c qemuMonitorJSONGetBalloonInfo.

qemu.git/hw/virtio/virtio-balloon.c virtio_balloon_stat

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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