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       - stat-available-memory
  36       - stat-disk-caches
  37       - stat-htlb-pgalloc
  38       - stat-htlb-pgfail
  39
  40   o A key named last-update, which contains the last stats update
  41     timestamp in seconds. Since this timestamp is generated by the host,
  42     a buggy guest can't influence its value. The value is 0 if the guest
  43     has not updated the stats (yet).
  44
  45 It's also important to note the following:
  46
  47  - Previously polled statistics remain available even if the polling is
  48    later disabled
  49
  50  - As noted above, if a guest doesn't support a particular stat its value
  51    will always be -1. However, it's also possible that a guest temporarily
  52    couldn't update one or even all stats. If this happens, just wait for
  53    the next update
  54
  55  - Polling can be enabled even if the guest doesn't have stats support
  56    or the balloon driver wasn't loaded in the guest. If this is the case
  57    and stats are queried, last-update will be 0.
  58
  59  - The polling timer is only re-armed when the guest responds to the
  60    statistics request. This means that if a (buggy) guest doesn't ever
  61    respond to the request the timer will never be re-armed, which has
  62    the same effect as disabling polling
  63
  64 Here are a few examples. QEMU is started with '-device virtio-balloon',
  65 which generates '/machine/peripheral-anon/device[1]' as the QOM path for
  66 the balloon device.
  67
  68 Enable polling with 2 seconds interval:
  69
  70 { "execute": "qom-set",
  71              "arguments": { "path": "/machine/peripheral-anon/device[1]",
  72                          "property": "guest-stats-polling-interval", "value": 2 } }
  73
  74 { "return": {} }
  75
  76 Change polling to 10 seconds:
  77
  78 { "execute": "qom-set",
  79              "arguments": { "path": "/machine/peripheral-anon/device[1]",
  80                          "property": "guest-stats-polling-interval", "value": 10 } }
  81
  82 { "return": {} }
  83
  84 Get stats:
  85
  86 { "execute": "qom-get",
  87   "arguments": { "path": "/machine/peripheral-anon/device[1]",
  88   "property": "guest-stats" } }
  89 {
  90     "return": {
  91         "stats": {
  92             "stat-swap-out": 0,
  93             "stat-free-memory": 844943360,
  94             "stat-minor-faults": 219028,
  95             "stat-major-faults": 235,
  96             "stat-total-memory": 1044406272,
  97             "stat-swap-in": 0
  98         },
  99         "last-update": 1358529861
 100     }
 101 }
 102
 103 Disable polling:
 104
 105 { "execute": "qom-set",
 106              "arguments": { "path": "/machine/peripheral-anon/device[1]",
 107                          "property": "stats-polling-interval", "value": 0 } }
 108
 109 { "return": {} }