[linux.git] / Documentation / lockstat.txt


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
Commit	Line	Data
a560aa48 PZ	1
	2	LOCK STATISTICS
	3
	4	- WHAT
	5
	6	As the name suggests, it provides statistics on locks.
	7
	8	- WHY
	9
	10	Because things like lock contention can severely impact performance.
	11
	12	- HOW
	13
	14	Lockdep already has hooks in the lock functions and maps lock instances to
	15	lock classes. We build on that. The graph below shows the relation between
	16	the 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
	38	lock, unlock - the regular lock functions
	39	__* - the hooks
	40	<> - states
	41
	42	With 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
	55	From these number various other statistics can be derived, such as:
	56
	57	hold time average = hold time total / acquisitions
	58
	59	These numbers are gathered per lock class, per read/write state (when
	60	applicable).
	61
	62	It also tracks 4 contention points per class. A contention point is a call site
	63	that had to wait on lock acquisition.
	64
65	- USAGE
66
67	Look 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
74	01 lock_stat version 0.2
75	02 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
76	03 class name con-bounces contentions waittime-min waittime-max waittime-total acq-bounces acquisitions holdtime-min holdtime-max holdtime-total
77	04 -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
78	05
79	06 &inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60
80	07 &inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38
81	08 --------------------------
82	09 &inode->i_data.tree_lock 0 [<ffffffff8027c08f>] add_to_page_cache+0x5f/0x190
83	10
84	11 ...............................................................................................................................................................................................
85	12
86	13 dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24
87	14 -----------
88	15 dcache_lock 180 [<ffffffff802c0d7e>] sys_getcwd+0x11e/0x230
89	16 dcache_lock 165 [<ffffffff802c002a>] d_alloc+0x15a/0x210
90	17 dcache_lock 33 [<ffffffff8035818d>] _atomic_dec_and_lock+0x4d/0x70
91	18 dcache_lock 1 [<ffffffff802beef8>] shrink_dcache_parent+0x18/0x130
92
93	This excerpt shows the first two lock class statistics. Line 01 shows the
94	output version - each time the format changes this will be updated. Line 02-04
95	show the header with column descriptions. Lines 05-10 and 13-18 show the actual
96	statistics. These statistics come in two parts; the actual stats separated by a
97	short separator (line 08, 14) from the contention points.
98
99	The first lock (05-10) is a read/write lock, and shows two lines above the
100	short separator. The contention points don't match the column descriptors,
101	they have two: contentions and [<IP>] symbol.
102
103
104	View 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
118	Clear the statistics:
119
120	# echo 0 > /proc/lock_stat