Archipelago » qemu

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

interval.

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": {} }