Documentation: add pointer to name_to_dev_t for root= values
[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 (see Documentation/lockdep-design.txt).
16 The graph below shows the relation between the lock functions and the various
17 hooks therein.
18
19         __acquire
20             |
21            lock _____
22             |        \
23             |    __contended
24             |         |
25             |       <wait>
26             | _______/
27             |/
28             |
29        __acquired
30             |
31             .
32           <hold>
33             .
34             |
35        __release
36             |
37          unlock
38
39 lock, unlock    - the regular lock functions
40 __*             - the hooks
41 <>              - states
42
43 With these hooks we provide the following statistics:
44
45  con-bounces       - number of lock contention that involved x-cpu data
46  contentions       - number of lock acquisitions that had to wait
47  wait time min     - shortest (non-0) time we ever had to wait for a lock
48            max     - longest time we ever had to wait for a lock
49            total   - total time we spend waiting on this lock
50  acq-bounces       - number of lock acquisitions that involved x-cpu data
51  acquisitions      - number of times we took the lock
52  hold time min     - shortest (non-0) time we ever held the lock
53            max     - longest time we ever held the lock
54            total   - total time this lock was held
55
56 From these number various other statistics can be derived, such as:
57
58  hold time average = hold time total / acquisitions
59
60 These numbers are gathered per lock class, per read/write state (when
61 applicable).
62
63 It also tracks 4 contention points per class. A contention point is a call site
64 that had to wait on lock acquisition.
65
66  - CONFIGURATION
67
68 Lock statistics are enabled via CONFIG_LOCK_STATS.
69
70  - USAGE
71
72 Enable collection of statistics:
73
74 # echo 1 >/proc/sys/kernel/lock_stat
75
76 Disable collection of statistics:
77
78 # echo 0 >/proc/sys/kernel/lock_stat
79
80 Look at the current lock statistics:
81
82 ( line numbers not part of actual output, done for clarity in the explanation
83   below )
84
85 # less /proc/lock_stat
86
87 01 lock_stat version 0.3
88 02 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
89 03                               class name    con-bounces    contentions   waittime-min   waittime-max waittime-total    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total
90 04 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
91 05
92 06                          &mm->mmap_sem-W:           233            538 18446744073708       22924.27      607243.51           1342          45806           1.71        8595.89     1180582.34
93 07                          &mm->mmap_sem-R:           205            587 18446744073708       28403.36      731975.00           1940         412426           0.58      187825.45     6307502.88
94 08                          ---------------
95 09                            &mm->mmap_sem            487          [<ffffffff8053491f>] do_page_fault+0x466/0x928
96 10                            &mm->mmap_sem            179          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
97 11                            &mm->mmap_sem            279          [<ffffffff80210a57>] sys_mmap+0x75/0xce
98 12                            &mm->mmap_sem             76          [<ffffffff802a490b>] sys_munmap+0x32/0x59
99 13                          ---------------
100 14                            &mm->mmap_sem            270          [<ffffffff80210a57>] sys_mmap+0x75/0xce
101 15                            &mm->mmap_sem            431          [<ffffffff8053491f>] do_page_fault+0x466/0x928
102 16                            &mm->mmap_sem            138          [<ffffffff802a490b>] sys_munmap+0x32/0x59
103 17                            &mm->mmap_sem            145          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
104 18
105 19 ...............................................................................................................................................................................................
106 20
107 21                              dcache_lock:           621            623           0.52         118.26        1053.02           6745          91930           0.29         316.29      118423.41
108 22                              -----------
109 23                              dcache_lock            179          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
110 24                              dcache_lock            113          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
111 25                              dcache_lock             99          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
112 26                              dcache_lock            104          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
113 27                              -----------
114 28                              dcache_lock            192          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
115 29                              dcache_lock             98          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
116 30                              dcache_lock             72          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
117 31                              dcache_lock            112          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
118
119 This excerpt shows the first two lock class statistics. Line 01 shows the
120 output version - each time the format changes this will be updated. Line 02-04
121 show the header with column descriptions. Lines 05-18 and 20-31 show the actual
122 statistics. These statistics come in two parts; the actual stats separated by a
123 short separator (line 08, 13) from the contention points.
124
125 The first lock (05-18) is a read/write lock, and shows two lines above the
126 short separator. The contention points don't match the column descriptors,
127 they have two: contentions and [<IP>] symbol. The second set of contention
128 points are the points we're contending with.
129
130 The integer part of the time values is in us.
131
132 Dealing with nested locks, subclasses may appear:
133
134 32...............................................................................................................................................................................................
135 33
136 34                               &rq->lock:         13128          13128           0.43         190.53      103881.26          97454        3453404           0.00         401.11    13224683.11
137 35                               ---------
138 36                               &rq->lock            645          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
139 37                               &rq->lock            297          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
140 38                               &rq->lock            360          [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a
141 39                               &rq->lock            428          [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb
142 40                               ---------
143 41                               &rq->lock             77          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
144 42                               &rq->lock            174          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
145 43                               &rq->lock           4715          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
146 44                               &rq->lock            893          [<ffffffff81340524>] schedule+0x157/0x7b8
147 45
148 46...............................................................................................................................................................................................
149 47
150 48                             &rq->lock/1:         11526          11488           0.33         388.73      136294.31          21461          38404           0.00          37.93      109388.53
151 49                             -----------
152 50                             &rq->lock/1          11526          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
153 51                             -----------
154 52                             &rq->lock/1           5645          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
155 53                             &rq->lock/1           1224          [<ffffffff81340524>] schedule+0x157/0x7b8
156 54                             &rq->lock/1           4336          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
157 55                             &rq->lock/1            181          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
158
159 Line 48 shows statistics for the second subclass (/1) of &rq->lock class
160 (subclass starts from 0), since in this case, as line 50 suggests,
161 double_rq_lock actually acquires a nested lock of two spinlocks.
162
163 View the top contending locks:
164
165 # grep : /proc/lock_stat | head
166               &inode->i_data.tree_lock-W:            15          21657           0.18     1093295.30 11547131054.85             58          10415           0.16          87.51        6387.60
167               &inode->i_data.tree_lock-R:             0              0           0.00           0.00           0.00          23302         231198           0.25           8.45       98023.38
168                              dcache_lock:          1037           1161           0.38          45.32         774.51           6611         243371           0.15         306.48       77387.24
169                          &inode->i_mutex:           161            286 18446744073709       62882.54     1244614.55           3653          20598 18446744073709       62318.60     1693822.74
170                          &zone->lru_lock:            94             94           0.53           7.33          92.10           4366          32690           0.29          59.81       16350.06
171               &inode->i_data.i_mmap_mutex:            79             79           0.40           3.77          53.03          11779          87755           0.28         116.93       29898.44
172                         &q->__queue_lock:            48             50           0.52          31.62          86.31            774          13131           0.17         113.08       12277.52
173                         &rq->rq_lock_key:            43             47           0.74          68.50         170.63           3706          33929           0.22         107.99       17460.62
174                       &rq->rq_lock_key#2:            39             46           0.75           6.68          49.03           2979          32292           0.17         125.17       17137.63
175                          tasklist_lock-W:            15             15           1.45          10.87          32.70           1201           7390           0.58          62.55       13648.47
176
177 Clear the statistics:
178
179 # echo 0 > /proc/lock_stat