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Diffstat (limited to 'Documentation/lockstat.txt')
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diff --git a/Documentation/lockstat.txt b/Documentation/lockstat.txt new file mode 100644 index 000000000000..4ba4664ce5c3 --- /dev/null +++ b/Documentation/lockstat.txt @@ -0,0 +1,120 @@ + +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 |