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virtio balloon memory statistics
The virtio balloon driver supports guest memory statistics reporting. These
statistics are available to QEMU users as QOM (QEMU Object Model) device
properties via a polling mechanism.
Before querying the available stats, clients first have to enable polling.
This is done by writing a time interval value (in seconds) to the
guest-stats-polling-interval property. This value can be:
> 0 enables polling in the specified interval. If polling is already
enabled, the polling time interval is changed to the new value
0 disables polling. Previous polled statistics are still valid and
can be queried.
Once polling is enabled, the virtio-balloon device in QEMU will start
polling the guest's balloon driver for new stats in the specified time
To retrieve those stats, clients have to query the guest-stats property,
which will return a dictionary containing:
o A key named 'stats', containing all available stats. If the guest
doesn't support a particular stat, or if it couldn't be retrieved,
its value will be -1. Currently, the following stats are supported:
- stat-swap-in
- stat-swap-out
- stat-major-faults
- stat-minor-faults
- stat-free-memory
- stat-total-memory
o A key named last-update, which contains the last stats update
timestamp in seconds. Since this timestamp is generated by the host,
a buggy guest can't influence its value
It's also important to note the following:
- Previously polled statistics remain available even if the polling is
later disabled
- As noted above, if a guest doesn't support a particular stat its value
will always be -1. However, it's also possible that a guest temporarily
couldn't update one or even all stats. If this happens, just wait for
the next update
- Polling can be enabled even if the guest doesn't have stats support
or the balloon driver wasn't loaded in the guest. If this is the case
and stats are queried, an error will be returned
- The polling timer is only re-armed when the guest responds to the
statistics request. This means that if a (buggy) guest doesn't ever
respond to the request the timer will never be re-armed, which has
the same effect as disabling polling
Here are a few examples. QEMU is started with '-balloon virtio', which
generates '/machine/peripheral-anon/device[1]' as the QOM path for the
balloon device.
Enable polling with 2 seconds interval:
{ "execute": "qom-set",
"arguments": { "path": "/machine/peripheral-anon/device[1]",
"property": "guest-stats-polling-interval", "value": 2 } }
{ "return": {} }
Change polling to 10 seconds:
{ "execute": "qom-set",
"arguments": { "path": "/machine/peripheral-anon/device[1]",
"property": "guest-stats-polling-interval", "value": 10 } }
{ "return": {} }
Get stats:
{ "execute": "qom-get",
"arguments": { "path": "/machine/peripheral-anon/device[1]",
"property": "guest-stats" } }
"return": {
"stats": {
"stat-swap-out": 0,
"stat-free-memory": 844943360,
"stat-minor-faults": 219028,
"stat-major-faults": 235,
"stat-total-memory": 1044406272,
"stat-swap-in": 0
"last-update": 1358529861
Disable polling:
{ "execute": "qom-set",
"arguments": { "path": "/machine/peripheral-anon/device[1]",
"property": "stats-polling-interval", "value": 0 } }
{ "return": {} }