xref: /freebsd/share/man/man9/LOCK_PROFILING.9 (revision 9336e0699bda8a301cd2bfa37106b6ec5e32012e)
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3.\" Copyright (c) 2005 Robert N. M. Watson
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30.\" $FreeBSD$
31.\"
32.Dd November 11, 2006
33.Dt LOCK_PROFILING 9
34.Os
35.Sh NAME
36.Nm LOCK_PROFILING
37.Nd kernel lock profiling support
38.Sh SYNOPSIS
39.Cd "options LOCK_PROFILING"
40.Sh DESCRIPTION
41The
42.Dv LOCK_PROFILING
43kernel option adds support for measuring and reporting lock use and
44contention statistics.
45These statistics are collated by
46.Dq acquisition point .
47Acquisition points are
48distinct places in the kernel source code (identified by source file
49name and line number) where a lock is acquired.
50.Pp
51For each acquisition point, the following statistics are accumulated:
52.Bl -bullet
53.It
54The longest time the lock was ever continuously held after being
55acquired at this point.
56.It
57The total time the lock was held after being acquired at this point.
58.It
59The total time that threads have spent waiting to acquire the lock.
60.It
61The total number of non-recursive acquisitions.
62.It
63The total number of times the lock was already held by another thread
64when this point was reached, requiring a spin or a sleep.
65.It
66The total number of times another thread tried to acquire the lock
67while it was held after having been acquired at this point.
68.El
69.Pp
70In addition, the average hold time and average wait time are derived
71from the total hold time
72and total wait time respectively and the number of acquisitions.
73.Pp
74The
75.Dv LOCK_PROFILING
76kernel option also adds the following
77.Xr sysctl 8
78variables to control and monitor the profiling code:
79.Bl -tag -width indent
80.It Va debug.lock.prof.enable
81Enable or disable the lock profiling code.
82This defaults to 0 (off).
83.It Va debug.lock.prof.reset
84Reset the current lock profiling buffers.
85.It Va debug.lock.prof.acquisitions
86The total number of lock acquisitions recorded.
87.It Va debug.lock.prof.records
88The total number of acquisition points recorded.
89Note that only active acquisition points (i.e., points that have been
90reached at least once) are counted.
91.It Va debug.lock.prof.maxrecords
92The maximum number of acquisition points the profiling code is capable
93of monitoring.
94Since it would not be possible to call
95.Xr malloc 9
96from within the lock profiling code, this is a static limit.
97The number of records can be changed with the
98.Dv LPROF_BUFFERS
99kernel option.
100.It Va debug.lock.prof.rejected
101The number of acquisition points that were ignored after the table
102filled up.
103.It Va debug.lock.prof.hashsize
104The size of the hash table used to map acquisition points to
105statistics records.
106The hash size can be changed with the
107.Dv LPROF_HASH_SIZE
108kernel option.
109.It Va debug.lock.prof.collisions
110The number of hash collisions in the acquisition point hash table.
111.It Va debug.lock.prof.stats
112The actual profiling statistics in plain text.
113The columns are as follows, from left to right:
114.Bl -tag -width ".Va cnt_hold"
115.It Va max
116The longest continuous hold time in microseconds.
117.It Va total
118The total (accumulated) hold time in microseconds.
119.It Va wait_total
120The total (accumulated) wait time in microseconds.
121.It Va count
122The total number of acquisitions.
123.It Va avg
124The average hold time in microseconds, derived from the total hold time
125and the number of acquisitions.
126.It Va wait_avg
127The average wait time in microseconds, derived from the total wait time
128and the number of acquisitions.
129.It Va cnt_hold
130The number of times the lock was held and another thread attempted to
131acquire the lock.
132.It Va cnt_lock
133The number of times the lock was already held when this point was
134reached.
135.It Va name
136The name of the acquisition point, derived from the source file name
137and line number, followed by the name of the lock in parentheses.
138.El
139.El
140.Sh SEE ALSO
141.Xr sysctl 8 ,
142.Xr mutex 9
143.Sh HISTORY
144Mutex profiling support appeared in
145.Fx 5.0 .
146Generalized lock profiling support appeared in
147.Fx 7.0 .
148.Sh AUTHORS
149.An -nosplit
150The
151.Nm MUTEX_PROFILING
152code was written by
153.An Eivind Eklund Aq eivind@FreeBSD.org ,
154.An Dag-Erling Sm\(/orgrav Aq des@FreeBSD.org
155and
156.An Robert Watson Aq rwatson@FreeBSD.org .
157The
158.Nm
159code was written by
160.An Kip Macy Aq kmacy@FreeBSD.org .
161This manual page was written by
162.An Dag-Erling Sm\(/orgrav Aq des@FreeBSD.org .
163.Sh NOTES
164The
165.Dv LOCK_PROFILING
166option increases the size of
167.Vt "struct lock_object" ,
168so a kernel built with that option will not work with modules built
169without it.
170.Pp
171The
172.Dv LOCK_PROFILING
173option also prevents inlining of the mutex code, which can result in a
174fairly severe performance penalty.
175This is, however, not always the case.
176.Dv LOCK_PROFILING
177can introduce a substantial performance overhead that is easily
178monitorable using other profiling tools, so combining profiling tools
179with
180.Dv LOCK_PROFILING
181is not recommended.
182.Pp
183Measurements are made and stored in nanoseconds using
184.Xr nanotime 9 ,
185(on architectures without a synchronized TSC) but are presented in microseconds.
186This should still be sufficient for the locks one would be most
187interested in profiling (those that are held long and/or acquired
188often).
189.Pp
190.Dv LOCK_PROFILING
191should generally not be used in combination with other debugging options, as
192the results may be strongly affected by interactions between the features.
193In particular,
194.Dv LOCK_PROFILING
195will report higher than normal
196.Xr uma 9
197lock contention when run with
198.Dv INVARIANTS
199due to extra locking that occurs when
200.Dv INVARIANTS
201is present; likewise, using it in combination with
202.Dv WITNESS
203will lead to much higher lock hold times and contention in profiling output.
204