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KTR(4)                 FreeBSD Kernel Interfaces Manual                 KTR(4)

     ktr - kernel tracing facility

     options KTR
     options ALQ
     options KTR_ALQ
     options KTR_CPUMASK=0x3
     options KTR_ENTRIES=8192
     options KTR_VERBOSE

     The ktr facility allows kernel events to be logged while the kernel
     executes so that they can be examined later when debugging.  The only
     mandatory option to enable ktr is ``options KTR''.

     The KTR_ENTRIES option sets the size of the buffer of events.  The size
     of the buffer in the currently running kernel can be found via the sysctl
     debug.ktr.entries.  By default the buffer contains 1024 entries.

   Event Masking
     Event levels can be enabled or disabled to trim excessive and overly
     verbose logging.  First, a mask of events is specified at compile time
     via the KTR_COMPILE option to limit which events are actually compiled
     into the kernel.  The default value for this option is for all events to
     be enabled.

     Secondly, the actual events logged while the kernel runs can be further
     masked via the run time event mask.  The KTR_MASK option sets the default
     value of the run time event mask.  The runtime event mask can also be set
     by the loader(8) via the debug.ktr.mask environment variable.  It can
     also be examined and set after booting via the debug.ktr.mask sysctl.  By
     default the run time mask is set to block any tracing.  The definitions
     of the event mask bits can be found in <sys/ktr.h>.

     Furthermore, there is a CPU event mask whose default value can be changed
     via the KTR_CPUMASK option.  When two or more parameters to KTR_CPUMASK,
     are used, it is important they are not separated by whitespace.  A CPU
     must have the bit corresponding to its logical id set in this bitmask for
     events that occur on it to be logged.  This mask can be set by the
     loader(8) via the debug.ktr.cpumask environment variable.  It can also be
     examined and set after booting via the debug.ktr.cpumask sysctl.  By
     default, only CPUs specified in KTR_CPUMASK will log events.  See
     sys/conf/NOTES for more information.

   Verbose Mode
     By default, events are only logged to the internal buffer for examination
     later, but if the verbose flag is set then they are dumped to the kernel
     console as well.  This flag can also be set from the loader via the
     debug.ktr.verbose environment variable, or it can be examined and set
     after booting via the debug.ktr.verbose sysctl.  If the flag is set to
     zero, which is the default, then verbose output is disabled.  If the flag
     is set to one, then the contents of the log message and the CPU number
     are printed to the kernel console.  If the flag is greater than one, then
     the filename and line number of the event are output to the console in
     addition to the log message and the CPU number.  The KTR_VERBOSE option
     sets the flag to one.

   Examining the Events
     The KTR buffer can be examined from within ddb(4) via the show ktr [/vV]
     command.  This command displays the contents of the trace buffer one page
     at a time.  At the ``--more--'' prompt, the Enter key displays one more
     entry and prompts again.  The spacebar displays another page of entries.
     Any other key quits.  By default the timestamp, filename, and line number
     are not displayed with each log entry.  If the /v modifier is specified,
     then they are displayed in addition to the normal output.  If the /V
     modifier is specified, then just the timestamp is displayed in addition
     to the normal output.  Note that the events are displayed in reverse
     chronological order.  That is, the most recent events are displayed

   Logging ktr to Disk
     The KTR_ALQ option can be used to log ktr entries to disk for post
     analysis using the ktrdump(8) utility.  This option depends on the ALQ
     option.  Due to the potentially high volume of trace messages the trace
     mask should be selected carefully.  This feature is configured through a
     group of sysctls.

     debug.ktr.alq_file        displays or sets the file that ktr will log to.
                               By default its value is /tmp/ktr.out.  If the
                               file name is changed while ktr is enabled it
                               will not take effect until the next invocation.

     debug.ktr.alq_enable      enables logging of ktr entries to disk if it is
                               set to one.  Setting this to 0 will terminate
                               logging to disk and revert to logging to the
                               normal ktr ring buffer.  Data is not sent to
                               the ring buffer while logging to disk.

     debug.ktr.alq_max         is the maximum number of entries that will be
                               recorded to disk, or 0 for infinite.  This is
                               helpful for limiting the number of particularly
                               high frequency entries that are recorded.

     debug.ktr.alq_depth       determines the number of entries in the write
                               buffer.  This is the buffer that holds entries
                               before they are written to disk and defaults to
                               the value of the KTR_ENTRIES option.

     debug.ktr.alq_failed      records the number of times we failed to write
                               an entry due to overflowing the write buffer.
                               This may happen if the frequency of the logged
                               ktr messages outpaces the depth of the queue.

     debug.ktr.alq_cnt         records the number of entries that have
                               currently been written to disk.

     ktrdump(8), alq(9), ktr(9)

     The KTR kernel tracing facility first appeared in BSD/OS 3.0 and was
     imported into FreeBSD 5.0.

FreeBSD 11.0-PRERELEASE        October 20, 2012        FreeBSD 11.0-PRERELEASE


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