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Luiz Capitulino045a7082012-12-04 12:04:39 -02001virtio balloon memory statistics
2================================
3
4The virtio balloon driver supports guest memory statistics reporting. These
5statistics are available to QEMU users as QOM (QEMU Object Model) device
6properties via a polling mechanism.
7
8Before querying the available stats, clients first have to enable polling.
9This is done by writing a time interval value (in seconds) to the
10guest-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
18Once polling is enabled, the virtio-balloon device in QEMU will start
19polling the guest's balloon driver for new stats in the specified time
20interval.
21
22To retrieve those stats, clients have to query the guest-stats property,
23which 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,
Ján Tomko38dbd482014-05-21 11:03:47 +020038 a buggy guest can't influence its value. The value is 0 if the guest
39 has not updated the stats (yet).
Luiz Capitulino045a7082012-12-04 12:04:39 -020040
41It'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
Ján Tomko38dbd482014-05-21 11:03:47 +020053 and stats are queried, last-update will be 0.
Luiz Capitulino045a7082012-12-04 12:04:39 -020054
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
60Here are a few examples. QEMU is started with '-balloon virtio', which
61generates '/machine/peripheral-anon/device[1]' as the QOM path for the
62balloon device.
63
64Enable 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
72Change 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
80Get 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
99Disable polling:
100
101{ "execute": "qom-set",
102 "arguments": { "path": "/machine/peripheral-anon/device[1]",
103 "property": "stats-polling-interval", "value": 0 } }
104
105{ "return": {} }