[ALSA] cmipci: clean up struct cmipci_pcm
[linux-2.6.git] / Documentation / lockstat.txt
1
2 LOCK STATISTICS
3
4 - WHAT
5
6 As the name suggests, it provides statistics on locks.
7
8 - WHY
9
10 Because things like lock contention can severely impact performance.
11
12 - HOW
13
14 Lockdep already has hooks in the lock functions and maps lock instances to
15 lock classes. We build on that. The graph below shows the relation between
16 the lock functions and the various hooks therein.
17
18         __acquire
19             |
20            lock _____
21             |        \
22             |    __contended
23             |         |
24             |       <wait>
25             | _______/
26             |/
27             |
28        __acquired
29             |
30             .
31           <hold>
32             .
33             |
34        __release
35             |
36          unlock
37
38 lock, unlock    - the regular lock functions
39 __*             - the hooks
40 <>              - states
41
42 With these hooks we provide the following statistics:
43
44  con-bounces       - number of lock contention that involved x-cpu data
45  contentions       - number of lock acquisitions that had to wait
46  wait time min     - shortest (non-0) time we ever had to wait for a lock
47            max     - longest time we ever had to wait for a lock
48            total   - total time we spend waiting on this lock
49  acq-bounces       - number of lock acquisitions that involved x-cpu data
50  acquisitions      - number of times we took the lock
51  hold time min     - shortest (non-0) time we ever held the lock
52            max     - longest time we ever held the lock
53            total   - total time this lock was held
54
55 From these number various other statistics can be derived, such as:
56
57  hold time average = hold time total / acquisitions
58
59 These numbers are gathered per lock class, per read/write state (when
60 applicable).
61
62 It also tracks 4 contention points per class. A contention point is a call site
63 that had to wait on lock acquisition.
64
65  - USAGE
66
67 Look at the current lock statistics:
68
69 ( line numbers not part of actual output, done for clarity in the explanation
70   below )
71
72 # less /proc/lock_stat
73
74 01 lock_stat version 0.2
75 02 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
76 03                               class name    con-bounces    contentions   waittime-min   waittime-max waittime-total    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total
77 04 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
78 05
79 06               &inode->i_data.tree_lock-W:            15          21657           0.18     1093295.30 11547131054.85             58          10415           0.16          87.51        6387.60
80 07               &inode->i_data.tree_lock-R:             0              0           0.00           0.00           0.00          23302         231198           0.25           8.45       98023.38
81 08               --------------------------
82 09                 &inode->i_data.tree_lock              0          [<ffffffff8027c08f>] add_to_page_cache+0x5f/0x190
83 10
84 11 ...............................................................................................................................................................................................
85 12
86 13                              dcache_lock:          1037           1161           0.38          45.32         774.51           6611         243371           0.15         306.48       77387.24
87 14                              -----------
88 15                              dcache_lock            180          [<ffffffff802c0d7e>] sys_getcwd+0x11e/0x230
89 16                              dcache_lock            165          [<ffffffff802c002a>] d_alloc+0x15a/0x210
90 17                              dcache_lock             33          [<ffffffff8035818d>] _atomic_dec_and_lock+0x4d/0x70
91 18                              dcache_lock              1          [<ffffffff802beef8>] shrink_dcache_parent+0x18/0x130
92
93 This excerpt shows the first two lock class statistics. Line 01 shows the
94 output version - each time the format changes this will be updated. Line 02-04
95 show the header with column descriptions. Lines 05-10 and 13-18 show the actual
96 statistics. These statistics come in two parts; the actual stats separated by a
97 short separator (line 08, 14) from the contention points.
98
99 The first lock (05-10) is a read/write lock, and shows two lines above the
100 short separator. The contention points don't match the column descriptors,
101 they have two: contentions and [<IP>] symbol.
102
103
104 View the top contending locks:
105
106 # grep : /proc/lock_stat | head
107               &inode->i_data.tree_lock-W:            15          21657           0.18     1093295.30 11547131054.85             58          10415           0.16          87.51        6387.60
108               &inode->i_data.tree_lock-R:             0              0           0.00           0.00           0.00          23302         231198           0.25           8.45       98023.38
109                              dcache_lock:          1037           1161           0.38          45.32         774.51           6611         243371           0.15         306.48       77387.24
110                          &inode->i_mutex:           161            286 18446744073709       62882.54     1244614.55           3653          20598 18446744073709       62318.60     1693822.74
111                          &zone->lru_lock:            94             94           0.53           7.33          92.10           4366          32690           0.29          59.81       16350.06
112               &inode->i_data.i_mmap_lock:            79             79           0.40           3.77          53.03          11779          87755           0.28         116.93       29898.44
113                         &q->__queue_lock:            48             50           0.52          31.62          86.31            774          13131           0.17         113.08       12277.52
114                         &rq->rq_lock_key:            43             47           0.74          68.50         170.63           3706          33929           0.22         107.99       17460.62
115                       &rq->rq_lock_key#2:            39             46           0.75           6.68          49.03           2979          32292           0.17         125.17       17137.63
116                          tasklist_lock-W:            15             15           1.45          10.87          32.70           1201           7390           0.58          62.55       13648.47
117
118 Clear the statistics:
119
120 # echo 0 > /proc/lock_stat