1 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 0.2 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 &inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60 8007 &inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38 8108 -------------------------- 8209 &inode->i_data.tree_lock 0 [<ffffffff8027c08f>] add_to_page_cache+0x5f/0x190 8310 8411 ............................................................................................................................................................................................... 8512 8613 dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24 8714 ----------- 8815 dcache_lock 180 [<ffffffff802c0d7e>] sys_getcwd+0x11e/0x230 8916 dcache_lock 165 [<ffffffff802c002a>] d_alloc+0x15a/0x210 9017 dcache_lock 33 [<ffffffff8035818d>] _atomic_dec_and_lock+0x4d/0x70 9118 dcache_lock 1 [<ffffffff802beef8>] shrink_dcache_parent+0x18/0x130 92 93This excerpt shows the first two lock class statistics. Line 01 shows the 94output version - each time the format changes this will be updated. Line 02-04 95show the header with column descriptions. Lines 05-10 and 13-18 show the actual 96statistics. These statistics come in two parts; the actual stats separated by a 97short separator (line 08, 14) from the contention points. 98 99The first lock (05-10) is a read/write lock, and shows two lines above the 100short separator. The contention points don't match the column descriptors, 101they have two: contentions and [<IP>] symbol. 102 103 104View 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 118Clear the statistics: 119 120# echo 0 > /proc/lock_stat 121