NAME¶
LOCK_PROFILING
—
kernel lock profiling support
SYNOPSIS¶
options LOCK_PROFILING
DESCRIPTION¶
The
LOCK_PROFILING
kernel option adds support
for measuring and reporting lock use and contention statistics. These
statistics are collated by “acquisition point”. Acquisition
points are distinct places in the kernel source code (identified by source
file name and line number) where a lock is acquired.
For each acquisition point, the following statistics are accumulated:
- The longest time the lock was ever continuously held after being acquired
at this point.
- The total time the lock was held after being acquired at this point.
- The total time that threads have spent waiting to acquire the lock.
- The total number of non-recursive acquisitions.
- The total number of times the lock was already held by another thread when
this point was reached, requiring a spin or a sleep.
- The total number of times another thread tried to acquire the lock while
it was held after having been acquired at this point.
In addition, the average hold time and average wait time are derived from the
total hold time and total wait time respectively and the number of
acquisitions.
The
LOCK_PROFILING
kernel option also adds
the following
sysctl(8) variables to control and
monitor the profiling code:
- debug.lock.prof.enable
- Enable or disable the lock profiling code. This defaults to 0 (off).
- debug.lock.prof.reset
- Reset the current lock profiling buffers.
- debug.lock.prof.stats
- The actual profiling statistics in plain text. The columns are as follows,
from left to right:
- max
- The longest continuous hold time in microseconds.
- wait_max
- The longest continuous wait time in microseconds.
- total
- The total (accumulated) hold time in microseconds.
- wait_total
- The total (accumulated) wait time in microseconds.
- count
- The total number of acquisitions.
- avg
- The average hold time in microseconds, derived from the total hold
time and the number of acquisitions.
- wait_avg
- The average wait time in microseconds, derived from the total wait
time and the number of acquisitions.
- cnt_hold
- The number of times the lock was held and another thread attempted to
acquire the lock.
- cnt_lock
- The number of times the lock was already held when this point was
reached.
- name
- The name of the acquisition point, derived from the source file name
and line number, followed by the name of the lock in parentheses.
- debug.lock.prof.rejected
- The number of acquisition points that were ignored after the table filled
up.
- debug.lock.prof.skipspin
- Disable or enable the lock profiling code for the spin locks. This
defaults to 0 (do profiling for the spin locks).
- debug.lock.prof.skipcount
- Do sampling approximately every N lock acquisitions.
SEE ALSO¶
sysctl(8),
mutex(9)
HISTORY¶
Mutex profiling support appeared in
FreeBSD 5.0.
Generalized lock profiling support appeared in
FreeBSD
7.0.
AUTHORS¶
The
MUTEX_PROFILING
code was written by
Eivind Eklund
⟨eivind@FreeBSD.org⟩,
Dag-Erling
Smørgrav ⟨des@FreeBSD.org⟩ and
Robert Watson
⟨rwatson@FreeBSD.org⟩. The
LOCK_PROFILING
code was written by
Kip Macy ⟨kmacy@FreeBSD.org⟩.
This manual page was written by
Dag-Erling
Smørgrav ⟨des@FreeBSD.org⟩.
NOTES¶
The
LOCK_PROFILING
option increases the size
of
struct lock_object, so a kernel built with
that option will not work with modules built without it.
The
LOCK_PROFILING
option also prevents
inlining of the mutex code, which can result in a fairly severe performance
penalty. This is, however, not always the case.
LOCK_PROFILING
can introduce a substantial
performance overhead that is easily monitorable using other profiling tools,
so combining profiling tools with
LOCK_PROFILING
is not recommended.
Measurements are made and stored in nanoseconds using
nanotime(9), (on architectures without a
synchronized TSC) but are presented in microseconds. This should still be
sufficient for the locks one would be most interested in profiling (those that
are held long and/or acquired often).
LOCK_PROFILING
should generally not be used
in combination with other debugging options, as the results may be strongly
affected by interactions between the features. In particular,
LOCK_PROFILING
will report higher than
normal
uma(9) lock contention when run with
INVARIANTS
due to extra locking that occurs
when
INVARIANTS
is present; likewise, using
it in combination with
WITNESS
will lead to
much higher lock hold times and contention in profiling output.