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- 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
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