[linux-block.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 0.3
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                          &mm->mmap_sem-W:           233            538 18446744073708       22924.27      607243.51           1342          45806           1.71        8595.89     1180582.34
07                          &mm->mmap_sem-R:           205            587 18446744073708       28403.36      731975.00           1940         412426           0.58      187825.45     6307502.88
08                          ---------------
09                            &mm->mmap_sem            487          [<ffffffff8053491f>] do_page_fault+0x466/0x928
10                            &mm->mmap_sem            179          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
11                            &mm->mmap_sem            279          [<ffffffff80210a57>] sys_mmap+0x75/0xce
12                            &mm->mmap_sem             76          [<ffffffff802a490b>] sys_munmap+0x32/0x59
13                          ---------------
14                            &mm->mmap_sem            270          [<ffffffff80210a57>] sys_mmap+0x75/0xce
15                            &mm->mmap_sem            431          [<ffffffff8053491f>] do_page_fault+0x466/0x928
16                            &mm->mmap_sem            138          [<ffffffff802a490b>] sys_munmap+0x32/0x59
17                            &mm->mmap_sem            145          [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
18
19 ...............................................................................................................................................................................................
20
21                              dcache_lock:           621            623           0.52         118.26        1053.02           6745          91930           0.29         316.29      118423.41
22                              -----------
23                              dcache_lock            179          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
24                              dcache_lock            113          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
25                              dcache_lock             99          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
26                              dcache_lock            104          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
27                              -----------
28                              dcache_lock            192          [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
29                              dcache_lock             98          [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
30                              dcache_lock             72          [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
31                              dcache_lock            112          [<ffffffff802cbca0>] d_instantiate+0x36/0x8a

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 05-18 and 20-31 show the actual
statistics. These statistics come in two parts; the actual stats separated by a
short separator (line 08, 13) from the contention points.

The first lock (05-18) 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. The second set of contention
points are the points we're contending with.

The integer part of the time values is in us.

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
c7e78cff	74	01 lock_stat version 0.3
a560aa48 PZ	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
c7e78cff PZ	79	06 &mm->mmap_sem-W: 233 538 18446744073708 22924.27 607243.51 1342 45806 1.71 8595.89 1180582.34
	80	07 &mm->mmap_sem-R: 205 587 18446744073708 28403.36 731975.00 1940 412426 0.58 187825.45 6307502.88
	81	08 ---------------
	82	09 &mm->mmap_sem 487 [<ffffffff8053491f>] do_page_fault+0x466/0x928
	83	10 &mm->mmap_sem 179 [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
	84	11 &mm->mmap_sem 279 [<ffffffff80210a57>] sys_mmap+0x75/0xce
	85	12 &mm->mmap_sem 76 [<ffffffff802a490b>] sys_munmap+0x32/0x59
	86	13 ---------------
	87	14 &mm->mmap_sem 270 [<ffffffff80210a57>] sys_mmap+0x75/0xce
	88	15 &mm->mmap_sem 431 [<ffffffff8053491f>] do_page_fault+0x466/0x928
	89	16 &mm->mmap_sem 138 [<ffffffff802a490b>] sys_munmap+0x32/0x59
	90	17 &mm->mmap_sem 145 [<ffffffff802a6200>] sys_mprotect+0xcd/0x21d
	91	18
	92	19 ...............................................................................................................................................................................................
	93	20
	94	21 dcache_lock: 621 623 0.52 118.26 1053.02 6745 91930 0.29 316.29 118423.41
	95	22 -----------
	96	23 dcache_lock 179 [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
	97	24 dcache_lock 113 [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
	98	25 dcache_lock 99 [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
	99	26 dcache_lock 104 [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
	100	27 -----------
	101	28 dcache_lock 192 [<ffffffff80378274>] _atomic_dec_and_lock+0x34/0x54
	102	29 dcache_lock 98 [<ffffffff802ca0dc>] d_rehash+0x1b/0x44
	103	30 dcache_lock 72 [<ffffffff802cc17b>] d_alloc+0x19a/0x1eb
	104	31 dcache_lock 112 [<ffffffff802cbca0>] d_instantiate+0x36/0x8a
a560aa48 PZ	105
	106	This excerpt shows the first two lock class statistics. Line 01 shows the
	107	output version - each time the format changes this will be updated. Line 02-04
c7e78cff	108	show the header with column descriptions. Lines 05-18 and 20-31 show the actual
a560aa48	109	statistics. These statistics come in two parts; the actual stats separated by a
c7e78cff	110	short separator (line 08, 13) from the contention points.
a560aa48	111
c7e78cff	112	The first lock (05-18) is a read/write lock, and shows two lines above the
a560aa48	113	short separator. The contention points don't match the column descriptors,
c7e78cff PZ	114	they have two: contentions and [<IP>] symbol. The second set of contention
c7e78cff PZ	115	points are the points we're contending with.
a560aa48	116
cdad7220	117	The integer part of the time values is in us.
a560aa48 PZ	118
	119	View the top contending locks:
	120
	121	# grep : /proc/lock_stat \| head
	122	&inode->i_data.tree_lock-W: 15 21657 0.18 1093295.30 11547131054.85 58 10415 0.16 87.51 6387.60
	123	&inode->i_data.tree_lock-R: 0 0 0.00 0.00 0.00 23302 231198 0.25 8.45 98023.38
	124	dcache_lock: 1037 1161 0.38 45.32 774.51 6611 243371 0.15 306.48 77387.24
	125	&inode->i_mutex: 161 286 18446744073709 62882.54 1244614.55 3653 20598 18446744073709 62318.60 1693822.74
	126	&zone->lru_lock: 94 94 0.53 7.33 92.10 4366 32690 0.29 59.81 16350.06
	127	&inode->i_data.i_mmap_lock: 79 79 0.40 3.77 53.03 11779 87755 0.28 116.93 29898.44
	128	&q->__queue_lock: 48 50 0.52 31.62 86.31 774 13131 0.17 113.08 12277.52
	129	&rq->rq_lock_key: 43 47 0.74 68.50 170.63 3706 33929 0.22 107.99 17460.62
	130	&rq->rq_lock_key#2: 39 46 0.75 6.68 49.03 2979 32292 0.17 125.17 17137.63
	131	tasklist_lock-W: 15 15 1.45 10.87 32.70 1201 7390 0.58 62.55 13648.47
	132
	133	Clear the statistics:
	134
	135	# echo 0 > /proc/lock_stat