tools/funclatency_example.txt

Demonstrations of funclatency, the Linux eBPF/bcc version.


Timing the do_sys_open() kernel function until Ctrl-C:

# ./funclatency do_sys_open
Tracing do_sys_open... Hit Ctrl-C to end.
^C
     nsecs           : count     distribution
       0 -> 1        : 0        |                                      |
       2 -> 3        : 0        |                                      |
       4 -> 7        : 0        |                                      |
       8 -> 15       : 0        |                                      |
      16 -> 31       : 0        |                                      |
      32 -> 63       : 0        |                                      |
      64 -> 127      : 0        |                                      |
     128 -> 255      : 0        |                                      |
     256 -> 511      : 0        |                                      |
     512 -> 1023     : 0        |                                      |
    1024 -> 2047     : 0        |                                      |
    2048 -> 4095     : 124      |****************                      |
    4096 -> 8191     : 291      |**************************************|
    8192 -> 16383    : 36       |****                                  |
   16384 -> 32767    : 16       |**                                    |
   32768 -> 65535    : 8        |*                                     |
   65536 -> 131071   : 0        |                                      |
  131072 -> 262143   : 0        |                                      |
  262144 -> 524287   : 0        |                                      |
  524288 -> 1048575  : 0        |                                      |
 1048576 -> 2097151  : 0        |                                      |
 2097152 -> 4194303  : 1        |                                      |

avg = 13746 nsecs, total: 6543360 nsecs, count: 476

Detaching...

The output shows a histogram of function latency (call time), measured from when
the function began executing (was called) to when it finished (returned).

This example output shows that most of the time, do_sys_open() took between
2048 and 65536 nanoseconds (2 to 65 microseconds). The peak of this distribution
shows 291 calls of between 4096 and 8191 nanoseconds. There was also one
occurrence, an outlier, in the 2 to 4 millisecond range.

How this works: the function entry and return are traced using the kernel kprobe
and kretprobe tracer. Timestamps are collected, the delta time calculated, which
is the bucketized and stored as an in-kernel histogram for efficiency. The
histogram is visible in the output: it's the "count" column; everything else is
decoration. Only the count column is copied to user-level on output. This is an
efficient way to time kernel functions and examine their latency distribution.


Now trace a user function, pthread_mutex_lock in libpthread, to determine if
there is considerable lock contention:

# ./funclatency pthread:pthread_mutex_lock -p $(pidof contentions)
Tracing 1 function for "pthread:pthread_mutex_lock"... Hit Ctrl-C to end.
     nsecs               : count     distribution
         0 -> 1          : 0        |                                        |
         2 -> 3          : 0        |                                        |
         4 -> 7          : 0        |                                        |
         8 -> 15         : 0        |                                        |
        16 -> 31         : 0        |                                        |
        32 -> 63         : 0        |                                        |
        64 -> 127        : 0        |                                        |
       128 -> 255        : 0        |                                        |
       256 -> 511        : 0        |                                        |
       512 -> 1023       : 0        |                                        |
      1024 -> 2047       : 0        |                                        |
      2048 -> 4095       : 508967   |****************************************|
      4096 -> 8191       : 70072    |*****                                   |
      8192 -> 16383      : 27686    |**                                      |
     16384 -> 32767      : 5075     |                                        |
     32768 -> 65535      : 2318     |                                        |
     65536 -> 131071     : 581      |                                        |
    131072 -> 262143     : 38       |                                        |
    262144 -> 524287     : 5        |                                        |
    524288 -> 1048575    : 1        |                                        |
   1048576 -> 2097151    : 9        |                                        |

avg = 4317 nsecs, total: 2654426112 nsecs, count: 614752

Detaching...

It seems that most calls to pthread_mutex_lock completed rather quickly (in
under 4us), but there were some cases of considerable contention, sometimes
over a full millisecond.


Run a quick-and-dirty profiler over all the functions in an executable:
# ./funclatency /home/user/primes:* -p $(pidof primes) -F
Tracing 15 functions for "/home/user/primes:*"... Hit Ctrl-C to end.
^C

Function = is_prime [6556]
     nsecs               : count     distribution
         0 -> 1          : 0        |                                        |
         2 -> 3          : 0        |                                        |
         4 -> 7          : 0        |                                        |
         8 -> 15         : 0        |                                        |
        16 -> 31         : 0        |                                        |
        32 -> 63         : 0        |                                        |
        64 -> 127        : 0        |                                        |
       128 -> 255        : 0        |                                        |
       256 -> 511        : 0        |                                        |
       512 -> 1023       : 0        |                                        |
      1024 -> 2047       : 0        |                                        |
      2048 -> 4095       : 1495322  |****************************************|
      4096 -> 8191       : 95744    |**                                      |
      8192 -> 16383      : 9926     |                                        |
     16384 -> 32767      : 3070     |                                        |
     32768 -> 65535      : 1415     |                                        |
     65536 -> 131071     : 112      |                                        |
    131072 -> 262143     : 9        |                                        |
    262144 -> 524287     : 3        |                                        |
    524288 -> 1048575    : 0        |                                        |
   1048576 -> 2097151    : 8        |                                        |

Function = insert_result [6556]
     nsecs               : count     distribution
         0 -> 1          : 0        |                                        |
         2 -> 3          : 0        |                                        |
         4 -> 7          : 0        |                                        |
         8 -> 15         : 0        |                                        |
        16 -> 31         : 0        |                                        |
        32 -> 63         : 0        |                                        |
        64 -> 127        : 0        |                                        |
       128 -> 255        : 0        |                                        |
       256 -> 511        : 0        |                                        |
       512 -> 1023       : 0        |                                        |
      1024 -> 2047       : 0        |                                        |
      2048 -> 4095       : 111047   |****************************************|
      4096 -> 8191       : 3998     |*                                       |
      8192 -> 16383      : 720      |                                        |
     16384 -> 32767      : 238      |                                        |
     32768 -> 65535      : 106      |                                        |
     65536 -> 131071     : 5        |                                        |
    131072 -> 262143     : 4        |                                        |

avg = 3404 nsecs, total: 5862276096 nsecs, count: 1721727

Detaching...

From the results, we can see that the is_prime function has something resembling
an exponential distribution -- very few primes take a very long time to test,
while most numbers are verified as prime or composite in less than 4us. The
insert_result function exhibits a similar phenomenon, likely due to contention
over a shared results container.


Now vfs_read() is traced, and a microseconds histogram printed:

# ./funclatency -u vfs_read
Tracing vfs_read... Hit Ctrl-C to end.
^C
     usecs           : count     distribution
       0 -> 1        : 1143     |**************************************|
       2 -> 3        : 420      |*************                         |
       4 -> 7        : 159      |*****                                 |
       8 -> 15       : 295      |*********                             |
      16 -> 31       : 25       |                                      |
      32 -> 63       : 5        |                                      |
      64 -> 127      : 1        |                                      |
     128 -> 255      : 0        |                                      |
     256 -> 511      : 0        |                                      |
     512 -> 1023     : 0        |                                      |
    1024 -> 2047     : 1        |                                      |
    2048 -> 4095     : 0        |                                      |
    4096 -> 8191     : 5        |                                      |
    8192 -> 16383    : 0        |                                      |
   16384 -> 32767    : 0        |                                      |
   32768 -> 65535    : 0        |                                      |
   65536 -> 131071   : 7        |                                      |
  131072 -> 262143   : 7        |                                      |
  262144 -> 524287   : 3        |                                      |
  524288 -> 1048575  : 7        |                                      |

avg = 4229 nsecs, total: 8789145 nsecs, count: 2078

Detaching...

This shows a bimodal distribution. Many vfs_read() calls were faster than 15
microseconds, however, there was also a small handful between 65 milliseconds
and 1 second, seen at the bottom of the table. These are likely network reads
from SSH, waiting on interactive keystrokes.


Tracing do_nanosleep() in milliseconds:

# ./funclatency -m do_nanosleep
Tracing do_nanosleep... Hit Ctrl-C to end.
^C
     msecs           : count     distribution
       0 -> 1        : 0        |                                      |
       2 -> 3        : 0        |                                      |
       4 -> 7        : 0        |                                      |
       8 -> 15       : 0        |                                      |
      16 -> 31       : 0        |                                      |
      32 -> 63       : 0        |                                      |
      64 -> 127      : 0        |                                      |
     128 -> 255      : 0        |                                      |
     256 -> 511      : 0        |                                      |
     512 -> 1023     : 328      |**************************************|
    1024 -> 2047     : 0        |                                      |
    2048 -> 4095     : 0        |                                      |
    4096 -> 8191     : 32       |***                                   |
    8192 -> 16383    : 0        |                                      |
   16384 -> 32767    : 0        |                                      |
   32768 -> 65535    : 2        |                                      |

avg = 1510 nsecs, total: 546816 nsecs, count: 326

Detaching...

This looks like it has found threads that are sleeping every 1, 5, and 60
seconds.


An interval can be provided using -i, and timestamps added using -T. For
example, tracing vfs_read() latency in milliseconds and printing output
every 5 seconds:

# ./funclatency -mTi 5 vfs_read
Tracing vfs_read... Hit Ctrl-C to end.

20:10:08
     msecs           : count     distribution
       0 -> 1        : 1500     |*************************************+|
       2 -> 3        : 3        |                                      |
       4 -> 7        : 1        |                                      |
       8 -> 15       : 2        |                                      |
      16 -> 31       : 0        |                                      |
      32 -> 63       : 0        |                                      |
      64 -> 127      : 4        |                                      |
     128 -> 255      : 3        |                                      |
     256 -> 511      : 1        |                                      |
     512 -> 1023     : 7        |                                      |

avg = 5 nsecs, total: 8259 nsecs, count: 1521

20:10:13
     msecs           : count     distribution
       0 -> 1        : 1251     |*************************************+|
       2 -> 3        : 3        |                                      |
       4 -> 7        : 2        |                                      |
       8 -> 15       : 0        |                                      |
      16 -> 31       : 2        |                                      |
      32 -> 63       : 3        |                                      |
      64 -> 127      : 5        |                                      |
     128 -> 255      : 5        |                                      |
     256 -> 511      : 3        |                                      |
     512 -> 1023     : 6        |                                      |
    1024 -> 2047     : 2        |                                      |

avg = 9 nsecs, total: 11736 nsecs, count: 1282

20:10:18
     msecs           : count     distribution
       0 -> 1        : 1265     |*************************************+|
       2 -> 3        : 0        |                                      |
       4 -> 7        : 5        |                                      |
       8 -> 15       : 9        |                                      |
      16 -> 31       : 7        |                                      |
      32 -> 63       : 1        |                                      |
      64 -> 127      : 2        |                                      |
     128 -> 255      : 3        |                                      |
     256 -> 511      : 5        |                                      |
     512 -> 1023     : 5        |                                      |
    1024 -> 2047     : 0        |                                      |
    2048 -> 4095     : 1        |                                      |

avg = 8 nsecs, total: 11219 nsecs, count: 1303

^C
20:10:20
     msecs           : count     distribution
       0 -> 1        : 249      |*************************************+|
       2 -> 3        : 0        |                                      |
       4 -> 7        : 0        |                                      |
       8 -> 15       : 1        |                                      |
      16 -> 31       : 0        |                                      |
      32 -> 63       : 0        |                                      |
      64 -> 127      : 0        |                                      |
     128 -> 255      : 0        |                                      |
     256 -> 511      : 0        |                                      |
     512 -> 1023     : 1        |                                      |

avg = 4 nsecs, total: 1029 nsecs, count: 251

Detaching...


A single process can be traced, which filters in-kernel for efficiency. Here,
the vfs_read() function is timed as milliseconds for PID 17064, which is a
bash shell:

# ./funclatency -mp 17064 vfs_read
Tracing vfs_read... Hit Ctrl-C to end.
^C
     msecs           : count     distribution
       0 -> 1        : 1        |**                                    |
       2 -> 3        : 0        |                                      |
       4 -> 7        : 0        |                                      |
       8 -> 15       : 1        |**                                    |
      16 -> 31       : 2        |*****                                 |
      32 -> 63       : 0        |                                      |
      64 -> 127      : 13       |**************************************|
     128 -> 255      : 10       |*****************************         |
     256 -> 511      : 4        |***********                           |

avg = 153 nsecs, total: 4765 nsecs, count: 31

Detaching...

The distribution between 64 and 511 milliseconds shows keystroke latency.


The -F option can be used to print a histogram per function. Eg:

# ./funclatency -uF 'vfs_r*'
Tracing 5 functions for "vfs_r*"... Hit Ctrl-C to end.
^C

Function = vfs_read
     usecs               : count     distribution
         0 -> 1          : 1044     |****************************************|
         2 -> 3          : 383      |**************                          |
         4 -> 7          : 76       |**                                      |
         8 -> 15         : 41       |*                                       |
        16 -> 31         : 26       |                                        |
        32 -> 63         : 0        |                                        |
        64 -> 127        : 1        |                                        |
       128 -> 255        : 0        |                                        |
       256 -> 511        : 0        |                                        |
       512 -> 1023       : 0        |                                        |
      1024 -> 2047       : 0        |                                        |
      2048 -> 4095       : 4        |                                        |
      4096 -> 8191       : 2        |                                        |
      8192 -> 16383      : 0        |                                        |
     16384 -> 32767      : 0        |                                        |
     32768 -> 65535      : 2        |                                        |
     65536 -> 131071     : 5        |                                        |
    131072 -> 262143     : 5        |                                        |
    262144 -> 524287     : 3        |                                        |
    524288 -> 1048575    : 7        |                                        |

Function = vfs_rename
     usecs               : count     distribution
         0 -> 1          : 2        |****                                    |
         2 -> 3          : 2        |****                                    |
         4 -> 7          : 2        |****                                    |
         8 -> 15         : 0        |                                        |
        16 -> 31         : 6        |*************                           |
        32 -> 63         : 18       |****************************************|

avg = 5087 nsecs, total: 8287001 nsecs, count: 1629

Detaching...



USAGE message:

# ./funclatency -h
usage: funclatency [-h] [-p PID] [-i INTERVAL] [-T] [-u] [-m] [-F] [-r] [-v]
                   pattern

Time functions and print latency as a histogram

positional arguments:
  pattern               search expression for functions

optional arguments:
  -h, --help            show this help message and exit
  -p PID, --pid PID     trace this PID only
  -i INTERVAL, --interval INTERVAL
                        summary interval, in seconds
  -d DURATION, --duration DURATION
			total duration of trace, in seconds
  -T, --timestamp       include timestamp on output
  -u, --microseconds    microsecond histogram
  -m, --milliseconds    millisecond histogram
  -F, --function        show a separate histogram per function
  -r, --regexp          use regular expressions. Default is "*" wildcards
                        only.
  -l LEVEL, --level LEVEL
                        set the level of nested or recursive functions
  -v, --verbose         print the BPF program (for debugging purposes)

examples:
    ./funclatency do_sys_open       # time the do_sys_open() kernel function
    ./funclatency c:read            # time the read() C library function
    ./funclatency -u vfs_read       # time vfs_read(), in microseconds
    ./funclatency -m do_nanosleep   # time do_nanosleep(), in milliseconds
    ./funclatency -i 2 -d 10 c:open # output every 2 seconds, for duration 10s
    ./funclatency -mTi 5 vfs_read   # output every 5 seconds, with timestamps
    ./funclatency -p 181 vfs_read   # time process 181 only
    ./funclatency 'vfs_fstat*'      # time both vfs_fstat() and vfs_fstatat()
    ./funclatency 'c:*printf'       # time the *printf family of functions
    ./funclatency -F 'vfs_r*'       # show one histogram per matched function