linux/Documentation/lockstat.txt
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   2LOCK STATISTICS
   3
   4- WHAT
   5
   6As the name suggests, it provides statistics on locks.
   7
   8- WHY
   9
  10Because things like lock contention can severely impact performance.
  11
  12- HOW
  13
  14Lockdep already has hooks in the lock functions and maps lock instances to
  15lock classes. We build on that. The graph below shows the relation between
  16the 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
  38lock, unlock    - the regular lock functions
  39__*             - the hooks
  40<>              - states
  41
  42With 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
  55From these number various other statistics can be derived, such as:
  56
  57 hold time average = hold time total / acquisitions
  58
  59These numbers are gathered per lock class, per read/write state (when
  60applicable).
  61
  62It also tracks 4 contention points per class. A contention point is a call site
  63that had to wait on lock acquisition.
  64
  65 - USAGE
  66
  67Look 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
  7401 lock_stat version 0.3
  7502 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  7603                               class name    con-bounces    contentions   waittime-min   waittime-max waittime-total    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total
  7704 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  7805
  7906                          &mm->mmap_sem-W:           233            538 18446744073708       22924.27      607243.51           1342          45806           1.71        8595.89     1180582.34
  8007                          &mm->mmap_sem-R:           205            587 18446744073708       28403.36      731975.00           1940         412426           0.58      187825.45     6307502.88
  8108                          ---------------
  8209                            &mm->mmap_sem            487          [<ffffffff8053491f>] do_page_fault+0x466/0x928
  8310                            &mm->mmap_sem            179          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
  8411                            &mm->mmap_sem            279          [<ffffffff80210a57>] sys_mmap+0x75/0xce
  8512                            &mm->mmap_sem             76          [<ffffffff802a490b>] sys_munmap+0x32/0x59
  8613                          ---------------
  8714                            &mm->mmap_sem            270          [<ffffffff80210a57>] sys_mmap+0x75/0xce
  8815                            &mm->mmap_sem            431          [<ffffffff8053491f>] do_page_fault+0x466/0x928
  8916                            &mm->mmap_sem            138          [<ffffffff802a490b>] sys_munmap+0x32/0x59
  9017                            &mm->mmap_sem            145          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
  9118
  9219 ...............................................................................................................................................................................................
  9320
  9421                              dcache_lock:           621            623           0.52         118.26        1053.02           6745          91930           0.29         316.29      118423.41
  9522                              -----------
  9623                              dcache_lock            179          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
  9724                              dcache_lock            113          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
  9825                              dcache_lock             99          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
  9926                              dcache_lock            104          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
 10027                              -----------
 10128                              dcache_lock            192          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
 10229                              dcache_lock             98          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
 10330                              dcache_lock             72          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
 10431                              dcache_lock            112          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
 105
 106This excerpt shows the first two lock class statistics. Line 01 shows the
 107output version - each time the format changes this will be updated. Line 02-04
 108show the header with column descriptions. Lines 05-18 and 20-31 show the actual
 109statistics. These statistics come in two parts; the actual stats separated by a
 110short separator (line 08, 13) from the contention points.
 111
 112The first lock (05-18) is a read/write lock, and shows two lines above the
 113short separator. The contention points don't match the column descriptors,
 114they have two: contentions and [<IP>] symbol. The second set of contention
 115points are the points we're contending with.
 116
 117The integer part of the time values is in us.
 118
 119View the top contending locks:
 120
 121# grep : /proc/lock_stat | head
 122              &inode->i_data.tree_lock-W:            15          21657           0.18     1093295.30 11547131054.85             58          10415           0.16          87.51        6387.60
 123              &inode->i_data.tree_lock-R:             0              0           0.00           0.00           0.00          23302         231198           0.25           8.45       98023.38
 124                             dcache_lock:          1037           1161           0.38          45.32         774.51           6611         243371           0.15         306.48       77387.24
 125                         &inode->i_mutex:           161            286 18446744073709       62882.54     1244614.55           3653          20598 18446744073709       62318.60     1693822.74
 126                         &zone->lru_lock:            94             94           0.53           7.33          92.10           4366          32690           0.29          59.81       16350.06
 127              &inode->i_data.i_mmap_lock:            79             79           0.40           3.77          53.03          11779          87755           0.28         116.93       29898.44
 128                        &q->__queue_lock:            48             50           0.52          31.62          86.31            774          13131           0.17         113.08       12277.52
 129                        &rq->rq_lock_key:            43             47           0.74          68.50         170.63           3706          33929           0.22         107.99       17460.62
 130                      &rq->rq_lock_key#2:            39             46           0.75           6.68          49.03           2979          32292           0.17         125.17       17137.63
 131                         tasklist_lock-W:            15             15           1.45          10.87          32.70           1201           7390           0.58          62.55       13648.47
 132
 133Clear the statistics:
 134
 135# echo 0 > /proc/lock_stat
 136
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