| LOCK STATISTICS | |
| - WHAT | |
| As the name suggests, it provides statistics on locks. | |
| - WHY | |
| Because things like lock contention can severely impact performance. | |
| - HOW | |
| Lockdep already has hooks in the lock functions and maps lock instances to | |
| lock classes. We build on that. The graph below shows the relation between | |
| the lock functions and the various hooks therein. | |
| __acquire | |
| | | |
| lock _____ | |
| | \ | |
| | __contended | |
| | | | |
| | <wait> | |
| | _______/ | |
| |/ | |
| | | |
| __acquired | |
| | | |
| . | |
| <hold> | |
| . | |
| | | |
| __release | |
| | | |
| unlock | |
| lock, unlock - the regular lock functions | |
| __* - the hooks | |
| <> - states | |
| With these hooks we provide the following statistics: | |
| con-bounces - number of lock contention that involved x-cpu data | |
| contentions - number of lock acquisitions that had to wait | |
| wait time min - shortest (non-0) time we ever had to wait for a lock | |
| max - longest time we ever had to wait for a lock | |
| total - total time we spend waiting on this lock | |
| acq-bounces - number of lock acquisitions that involved x-cpu data | |
| acquisitions - number of times we took the lock | |
| hold time min - shortest (non-0) time we ever held the lock | |
| max - longest time we ever held the lock | |
| total - total time this lock was held | |
| From these number various other statistics can be derived, such as: | |
| hold time average = hold time total / acquisitions | |
| These numbers are gathered per lock class, per read/write state (when | |
| applicable). | |
| It also tracks 4 contention points per class. A contention point is a call site | |
| that had to wait on lock acquisition. | |
| - USAGE | |
| Look at the current lock statistics: | |
| ( line numbers not part of actual output, done for clarity in the explanation | |
| below ) | |
| # less /proc/lock_stat | |
| 01 lock_stat version 0.2 | |
| 02 ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | |
| 03 class name con-bounces contentions waittime-min waittime-max waittime-total acq-bounces acquisitions holdtime-min holdtime-max holdtime-total | |
| 04 ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | |
| 05 | |
| 06 &inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60 | |
| 07 &inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38 | |
| 08 -------------------------- | |
| 09 &inode->i_data.tree_lock 0 [<ffffffff8027c08f>] add_to_page_cache+0x5f/0x190 | |
| 10 | |
| 11 ............................................................................................................................................................................................... | |
| 12 | |
| 13 dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24 | |
| 14 ----------- | |
| 15 dcache_lock 180 [<ffffffff802c0d7e>] sys_getcwd+0x11e/0x230 | |
| 16 dcache_lock 165 [<ffffffff802c002a>] d_alloc+0x15a/0x210 | |
| 17 dcache_lock 33 [<ffffffff8035818d>] _atomic_dec_and_lock+0x4d/0x70 | |
| 18 dcache_lock 1 [<ffffffff802beef8>] shrink_dcache_parent+0x18/0x130 | |
| This excerpt shows the first two lock class statistics. Line 01 shows the | |
| output version - each time the format changes this will be updated. Line 02-04 | |
| show the header with column descriptions. Lines 05-10 and 13-18 show the actual | |
| statistics. These statistics come in two parts; the actual stats separated by a | |
| short separator (line 08, 14) from the contention points. | |
| The first lock (05-10) is a read/write lock, and shows two lines above the | |
| short separator. The contention points don't match the column descriptors, | |
| they have two: contentions and [<IP>] symbol. | |
| View the top contending locks: | |
| # grep : /proc/lock_stat | head | |
| &inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60 | |
| &inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38 | |
| dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24 | |
| &inode->i_mutex: 161 286 18446744073709 62882.54 1244614.55 3653 20598 18446744073709 62318.60 1693822.74 | |
| &zone->lru_lock: 94 94 0.53 7.33 92.10 4366 32690 0.29 59.81 16350.06 | |
| &inode->i_data.i_mmap_lock: 79 79 0.40 3.77 53.03 11779 87755 0.28 116.93 29898.44 | |
| &q->__queue_lock: 48 50 0.52 31.62 86.31 774 13131 0.17 113.08 12277.52 | |
| &rq->rq_lock_key: 43 47 0.74 68.50 170.63 3706 33929 0.22 107.99 17460.62 | |
| &rq->rq_lock_key#2: 39 46 0.75 6.68 49.03 2979 32292 0.17 125.17 17137.63 | |
| tasklist_lock-W: 15 15 1.45 10.87 32.70 1201 7390 0.58 62.55 13648.47 | |
| Clear the statistics: | |
| # echo 0 > /proc/lock_stat |