docs/virtio-balloon-stats.txt

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