docs/interop/virtio-balloon-stats.rst

   1 Virtio balloon memory statistics
   2 ================================
   3
   4 The virtio balloon driver supports guest memory statistics reporting. These
   5 statistics are available to QEMU users as QOM (QEMU Object Model) device
   6 properties via a polling mechanism.
   7
   8 Before querying the available stats, clients first have to enable polling.
   9 This is done by writing a time interval value (in seconds) to the
  10 guest-stats-polling-interval property. This value can be:
  11
  12   > 0
  13        enables polling in the specified interval. If polling is already
  14        enabled, the polling time interval is changed to the new value
  15
  16   0
  17        disables polling. Previous polled statistics are still valid and
  18        can be queried.
  19
  20 Once polling is enabled, the virtio-balloon device in QEMU will start
  21 polling the guest's balloon driver for new stats in the specified time
  22 interval.
  23
  24 To retrieve those stats, clients have to query the guest-stats property,
  25 which will return a dictionary containing:
  26
  27   * A key named 'stats', containing all available stats. If the guest
  28     doesn't support a particular stat, or if it couldn't be retrieved,
  29     its value will be -1. Currently, the following stats are supported:
  30
  31       - stat-swap-in
  32       - stat-swap-out
  33       - stat-major-faults
  34       - stat-minor-faults
  35       - stat-free-memory
  36       - stat-total-memory
  37       - stat-available-memory
  38       - stat-disk-caches
  39       - stat-htlb-pgalloc
  40       - stat-htlb-pgfail
  41
  42   * A key named last-update, which contains the last stats update
  43     timestamp in seconds. Since this timestamp is generated by the host,
  44     a buggy guest can't influence its value. The value is 0 if the guest
  45     has not updated the stats (yet).
  46
  47 It's also important to note the following:
  48
  49  - Previously polled statistics remain available even if the polling is
  50    later disabled
  51
  52  - As noted above, if a guest doesn't support a particular stat its value
  53    will always be -1. However, it's also possible that a guest temporarily
  54    couldn't update one or even all stats. If this happens, just wait for
  55    the next update
  56
  57  - Polling can be enabled even if the guest doesn't have stats support
  58    or the balloon driver wasn't loaded in the guest. If this is the case
  59    and stats are queried, last-update will be 0.
  60
  61  - The polling timer is only re-armed when the guest responds to the
  62    statistics request. This means that if a (buggy) guest doesn't ever
  63    respond to the request the timer will never be re-armed, which has
  64    the same effect as disabling polling
  65
  66 Here are a few examples. QEMU is started with ``-device virtio-balloon``,
  67 which generates ``/machine/peripheral-anon/device[1]`` as the QOM path for
  68 the balloon device.
  69
  70 Enable polling with 2 seconds interval::
  71
  72   { "execute": "qom-set",
  73                "arguments": { "path": "/machine/peripheral-anon/device[1]",
  74                "property": "guest-stats-polling-interval", "value": 2 } }
  75
  76   { "return": {} }
  77
  78 Change polling to 10 seconds::
  79
  80   { "execute": "qom-set",
  81                "arguments": { "path": "/machine/peripheral-anon/device[1]",
  82                "property": "guest-stats-polling-interval", "value": 10 } }
  83
  84   { "return": {} }
  85
  86 Get stats::
  87
  88   { "execute": "qom-get",
  89                "arguments": { "path": "/machine/peripheral-anon/device[1]",
  90                "property": "guest-stats" } }
  91   {
  92     "return": {
  93         "stats": {
  94             "stat-swap-out": 0,
  95             "stat-free-memory": 844943360,
  96             "stat-minor-faults": 219028,
  97             "stat-major-faults": 235,
  98             "stat-total-memory": 1044406272,
  99             "stat-swap-in": 0
 100         },
 101         "last-update": 1358529861
 102     }
 103   }
 104
 105 Disable polling::
 106
 107   { "execute": "qom-set",
 108                "arguments": { "path": "/machine/peripheral-anon/device[1]",
 109                "property": "stats-polling-interval", "value": 0 } }
 110
 111   { "return": {} }