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     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

     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


     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.