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387b14684f
Convert the locking documents to ReST and add them to the kernel development book where it belongs. Most of the stuff here is just to make Sphinx to properly parse the text file, as they're already in good shape, not requiring massive changes in order to be parsed. The conversion is actually: - add blank lines and identation in order to identify paragraphs; - fix tables markups; - add some lists markups; - mark literal blocks; - adjust title markups. At its new index.rst, let's add a :orphan: while this is not linked to the main index.rst file, in order to avoid build warnings. Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org> Acked-by: Federico Vaga <federico.vaga@vaga.pv.it>
205 lines
12 KiB
ReStructuredText
205 lines
12 KiB
ReStructuredText
===============
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Lock Statistics
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===============
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What
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====
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As the name suggests, it provides statistics on locks.
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Why
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===
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Because things like lock contention can severely impact performance.
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How
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===
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Lockdep already has hooks in the lock functions and maps lock instances to
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lock classes. We build on that (see Documentation/locking/lockdep-design.rst).
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The graph below shows the relation between the lock functions and the various
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hooks therein::
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__acquire
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lock _____
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| \
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| __contended
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| <wait>
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| _______/
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|/
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__acquired
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.
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<hold>
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.
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__release
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unlock
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lock, unlock - the regular lock functions
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__* - the hooks
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<> - states
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With these hooks we provide the following statistics:
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con-bounces
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- number of lock contention that involved x-cpu data
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contentions
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- number of lock acquisitions that had to wait
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wait time
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min
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- shortest (non-0) time we ever had to wait for a lock
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max
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- longest time we ever had to wait for a lock
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total
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- total time we spend waiting on this lock
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avg
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- average time spent waiting on this lock
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acq-bounces
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- number of lock acquisitions that involved x-cpu data
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acquisitions
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- number of times we took the lock
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hold time
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min
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- shortest (non-0) time we ever held the lock
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max
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- longest time we ever held the lock
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total
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- total time this lock was held
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avg
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- average time this lock was held
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These numbers are gathered per lock class, per read/write state (when
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applicable).
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It also tracks 4 contention points per class. A contention point is a call site
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that had to wait on lock acquisition.
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Configuration
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-------------
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Lock statistics are enabled via CONFIG_LOCK_STAT.
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Usage
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-----
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Enable collection of statistics::
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# echo 1 >/proc/sys/kernel/lock_stat
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Disable collection of statistics::
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# echo 0 >/proc/sys/kernel/lock_stat
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Look at the current lock statistics::
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( line numbers not part of actual output, done for clarity in the explanation
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below )
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# less /proc/lock_stat
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01 lock_stat version 0.4
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02-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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03 class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
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04-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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05
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06 &mm->mmap_sem-W: 46 84 0.26 939.10 16371.53 194.90 47291 2922365 0.16 2220301.69 17464026916.32 5975.99
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07 &mm->mmap_sem-R: 37 100 1.31 299502.61 325629.52 3256.30 212344 34316685 0.10 7744.91 95016910.20 2.77
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08 ---------------
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09 &mm->mmap_sem 1 [<ffffffff811502a7>] khugepaged_scan_mm_slot+0x57/0x280
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10 &mm->mmap_sem 96 [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510
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11 &mm->mmap_sem 34 [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd0
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12 &mm->mmap_sem 17 [<ffffffff81127e71>] vm_munmap+0x41/0x80
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13 ---------------
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14 &mm->mmap_sem 1 [<ffffffff81046fda>] dup_mmap+0x2a/0x3f0
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15 &mm->mmap_sem 60 [<ffffffff81129e29>] SyS_mprotect+0xe9/0x250
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16 &mm->mmap_sem 41 [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510
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17 &mm->mmap_sem 68 [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd0
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18
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19.............................................................................................................................................................................................................................
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20
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21 unix_table_lock: 110 112 0.21 49.24 163.91 1.46 21094 66312 0.12 624.42 31589.81 0.48
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22 ---------------
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23 unix_table_lock 45 [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0
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24 unix_table_lock 47 [<ffffffff8150b111>] unix_release_sock+0x31/0x250
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25 unix_table_lock 15 [<ffffffff8150ca37>] unix_find_other+0x117/0x230
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26 unix_table_lock 5 [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0
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27 ---------------
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28 unix_table_lock 39 [<ffffffff8150b111>] unix_release_sock+0x31/0x250
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29 unix_table_lock 49 [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0
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30 unix_table_lock 20 [<ffffffff8150ca37>] unix_find_other+0x117/0x230
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31 unix_table_lock 4 [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0
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This excerpt shows the first two lock class statistics. Line 01 shows the
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output version - each time the format changes this will be updated. Line 02-04
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show the header with column descriptions. Lines 05-18 and 20-31 show the actual
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statistics. These statistics come in two parts; the actual stats separated by a
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short separator (line 08, 13) from the contention points.
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Lines 09-12 show the first 4 recorded contention points (the code
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which tries to get the lock) and lines 14-17 show the first 4 recorded
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contended points (the lock holder). It is possible that the max
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con-bounces point is missing in the statistics.
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The first lock (05-18) is a read/write lock, and shows two lines above the
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short separator. The contention points don't match the column descriptors,
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they have two: contentions and [<IP>] symbol. The second set of contention
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points are the points we're contending with.
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The integer part of the time values is in us.
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Dealing with nested locks, subclasses may appear::
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32...........................................................................................................................................................................................................................
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33
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34 &rq->lock: 13128 13128 0.43 190.53 103881.26 7.91 97454 3453404 0.00 401.11 13224683.11 3.82
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35 ---------
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36 &rq->lock 645 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
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37 &rq->lock 297 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
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38 &rq->lock 360 [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a
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39 &rq->lock 428 [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb
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40 ---------
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41 &rq->lock 77 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
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42 &rq->lock 174 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
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43 &rq->lock 4715 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
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44 &rq->lock 893 [<ffffffff81340524>] schedule+0x157/0x7b8
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45
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46...........................................................................................................................................................................................................................
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47
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48 &rq->lock/1: 1526 11488 0.33 388.73 136294.31 11.86 21461 38404 0.00 37.93 109388.53 2.84
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49 -----------
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50 &rq->lock/1 11526 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
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51 -----------
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52 &rq->lock/1 5645 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
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53 &rq->lock/1 1224 [<ffffffff81340524>] schedule+0x157/0x7b8
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54 &rq->lock/1 4336 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
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55 &rq->lock/1 181 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
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Line 48 shows statistics for the second subclass (/1) of &rq->lock class
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(subclass starts from 0), since in this case, as line 50 suggests,
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double_rq_lock actually acquires a nested lock of two spinlocks.
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View the top contending locks::
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# grep : /proc/lock_stat | head
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clockevents_lock: 2926159 2947636 0.15 46882.81 1784540466.34 605.41 3381345 3879161 0.00 2260.97 53178395.68 13.71
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tick_broadcast_lock: 346460 346717 0.18 2257.43 39364622.71 113.54 3642919 4242696 0.00 2263.79 49173646.60 11.59
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&mapping->i_mmap_mutex: 203896 203899 3.36 645530.05 31767507988.39 155800.21 3361776 8893984 0.17 2254.15 14110121.02 1.59
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&rq->lock: 135014 136909 0.18 606.09 842160.68 6.15 1540728 10436146 0.00 728.72 17606683.41 1.69
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&(&zone->lru_lock)->rlock: 93000 94934 0.16 59.18 188253.78 1.98 1199912 3809894 0.15 391.40 3559518.81 0.93
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tasklist_lock-W: 40667 41130 0.23 1189.42 428980.51 10.43 270278 510106 0.16 653.51 3939674.91 7.72
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tasklist_lock-R: 21298 21305 0.20 1310.05 215511.12 10.12 186204 241258 0.14 1162.33 1179779.23 4.89
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rcu_node_1: 47656 49022 0.16 635.41 193616.41 3.95 844888 1865423 0.00 764.26 1656226.96 0.89
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&(&dentry->d_lockref.lock)->rlock: 39791 40179 0.15 1302.08 88851.96 2.21 2790851 12527025 0.10 1910.75 3379714.27 0.27
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rcu_node_0: 29203 30064 0.16 786.55 1555573.00 51.74 88963 244254 0.00 398.87 428872.51 1.76
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Clear the statistics::
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# echo 0 > /proc/lock_stat
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