NAME

       funclatency - Time functions and print latency as a histogram.

SYNOPSIS

       funclatency [-h] [-p PID] [-i INTERVAL] [-T] [-u] [-m] [-F] [-r] [-v] pattern

DESCRIPTION

       This  tool traces function calls and times their duration (latency), and shows the latency
       distribution as a histogram. The time is measured from when the function is called to when
       it returns, and is inclusive of both on-CPU time and time spent blocked.

       This  tool  uses  in-kernel  eBPF  maps  for  storing  timestamps  and  the histogram, for
       efficiency.

       Currently nested or recursive functions are not supported properly, and timestamps will be
       overwritten,  creating  dubious  output.  Try  to  match  single  functions,  or groups of
       functions that run at the same stack layer, and don't ultimately call each other.

       WARNING: This uses dynamic tracing of (what can be many) functions, an activity  that  has
       had  issues  on  some kernel versions (risk of panics or freezes). Test, and know what you
       are doing, before use.

       Since this uses BPF, only the root user can use this tool.

REQUIREMENTS

       CONFIG_BPF and bcc.

OPTIONS

       pattern Function name or search pattern. Supports "*" wildcards. See  EXAMPLES.   You  can
       also use -r for regular expressions.  -h Print usage message.

       -p PID Trace this process ID only.

       -i INTERVAL
              Print output every interval seconds.

       -T     Include timestamps on output.

       -u     Output histogram in microseconds.

       -m     Output histogram in milliseconds.

       -F     Print a separate histogram per function matched.

       -r     Use regular expressions for the search pattern.

       -v     Print the BPF program (for debugging purposes).

EXAMPLES

       Time the do_sys_open() kernel function, and print the distribution as a histogram:
              # funclatency do_sys_open

       Time the read() function in libc across all processes on the system:
              # funclatency c:read

       Time vfs_read(), and print the histogram in units of microseconds:
              # funclatency -u vfs_read

       Time do_nanosleep(), and print the histogram in units of milliseconds:
              # funclatency -m do_nanosleep

       Time vfs_read(), and print output every 5 seconds, with timestamps:
              # funclatency -mTi 5 vfs_read

       Time vfs_read() for process ID 181 only:
              # funclatency -p 181 vfs_read:

       Time both vfs_fstat() and vfs_fstatat() calls, by use of a wildcard:
              # funclatency 'vfs_fstat*'

       Time both vfs_fstat* calls, and print a separate histogram for each:
              # funclatency -F 'vfs_fstat*'

FIELDS

       necs   Nanosecond range

       usecs  Microsecond range

       msecs  Millisecond range

       count  How many calls fell into this range

       distribution
              An ASCII bar chart to visualize the distribution (count column)

OVERHEAD

       This traces kernel functions and maintains in-kernel timestamps and a histogram, which are
       asynchronously copied to user-space. While this method is  very  efficient,  the  rate  of
       kernel  functions can also be very high (>1M/sec), at which point the overhead is expected
       to be measurable. Measure in a test environment and understand overheads before  use.  You
       can  also  use funccount to measure the rate of kernel functions over a short duration, to
       set some expectations before use.

SOURCE

       This is from bcc.

              https://github.com/iovisor/bcc

       Also look in the bcc distribution for a companion _examples.txt  file  containing  example
       usage, output, and commentary for this tool.

OS

       Linux

STABILITY

       Unstable - in development.

AUTHOR

       Brendan Gregg, Sasha Goldshtein

SEE ALSO

       funccount(8)