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<chapter>
  <header>
    <copyright>
      <year>2002</year><year>2009</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>cprof - The Call Count Profiler</title>
    <prepared>Raimo Niskanen</prepared>
    <responsible>nobody</responsible>
    <docno></docno>
    <approved>nobody</approved>
    <checked>no</checked>
    <date>2002-09-11</date>
    <rev>PA1</rev>
    <file>cprof_chapter.xml</file>
  </header>
  <p><c>cprof</c> is a profiling tool that can be used to get a picture of
    how often different functions in the system are called.
    </p>
  <p><c>cprof</c> uses breakpoints similar to local call trace,
    but containing counters, to collect profiling
    data. Therfore there is no need for special compilation of any
    module to be profiled. 
    </p>
  <p><c>cprof</c> presents all profiled modules in decreasing total
    call count order, and for each module presents all profiled
    functions also in decreasing call count order. A call count limit
    can be specified to filter out all functions below the limit.
    </p>
  <p>Profiling is done in the following steps:</p>
  <taglist>
    <tag><c>cprof:start/0..3</c></tag>
    <item>Starts profiling with zeroed call counters for specified
     functions by setting call count breakpoints on them. </item>
    <tag><c>Mod:Fun()</c></tag>
    <item>Runs the code to be profiled.</item>
    <tag><c>cprof:pause/0..3</c></tag>
    <item>Pauses the call counters for specified functions. This minimises
     the impact of code running in the background or in the shell
     that disturbs the profiling. Call counters are automatically
     paused when they  "hit the ceiling" of the host machine word
     size. For a 32 bit host the maximum counter value is
     2147483647.</item>
    <tag><c>cprof:analyse/0..2</c></tag>
    <item>Collects call counters and computes the result.</item>
    <tag><c>cprof:restart/0..3</c></tag>
    <item>Restarts the call counters from zero for specified
     functions. Can be used to collect a new set of counters without 
     having to stop and start call count profiling.</item>
    <tag><c>cprof:stop/0..3</c></tag>
    <item>Stops profiling by removing call count breakpoints from
     specified functions.</item>
  </taglist>
  <p>Functions can be specified as either all in the system, all in one
    module, all arities of one function, one function, or all
    functions in all modules not yet loaded. As for now, BIFs cannot
    be call count traced.
    </p>
  <p>The analysis result can either be for all modules, or for one
    module. In either case a call count limit can be given to filter
    out the functions with a call count below the limit. The all
    modules analysis does <em>not</em> contain the module <c>cprof</c>
    itself, it can only be analysed by specifying it as a single
    module to analyse.
    </p>
  <p>Call count tracing is very lightweight compared to other forms of
    tracing since no trace message has to be generated. Some
    measurements indicates performance degradations in the vicinity of
    10 percent.
    </p>
  <p>The following sections show some examples of profiling with
    <c>cprof</c>. See also 
    <seealso marker="cprof">cprof(3)</seealso>.
    </p>

  <section>
    <title>Example: Background work</title>
    <p>From the Erlang shell:</p>
    <pre>
1> <input>cprof:start(), cprof:pause(). % Stop counters just after start</input>
3476
2> <input>cprof:analyse().</input>
{30,
 [{erl_eval,11,
            [{{erl_eval,expr,3},3},
             {{erl_eval,'-merge_bindings/2-fun-0-',2},2},
             {{erl_eval,expand_module_name,2},1},
             {{erl_eval,merge_bindings,2},1},
             {{erl_eval,binding,2},1},
             {{erl_eval,expr_list,5},1},
             {{erl_eval,expr_list,3},1},
             {{erl_eval,exprs,4},1}]},
  {orddict,8,
           [{{orddict,find,2},6},
            {{orddict,dict_to_list,1},1},
            {{orddict,to_list,1},1}]},
  {packages,7,[{{packages,is_segmented_1,1},6},
               {{packages,is_segmented,1},1}]},
  {lists,4,[{{lists,foldl,3},3},{{lists,reverse,1},1}]}]}
3> <input>cprof:analyse(cprof).</input>
{cprof,3,[{{cprof,tr,2},2},{{cprof,pause,0},1}]}
4> <input>cprof:stop().</input>
3476</pre>
    <p>The example showed the background work that the shell performs
      just to interpret the first command line. Most work is done by
      <c>erl_eval</c> and <c>orddict</c>.
      </p>
    <p>What is captured in this example is the part of the work the
      shell does while interpreting the command line that occurs
      between the actual calls to <c>cprof:start()</c> and
      <c>cprof:analyse()</c>.
      </p>
  </section>

  <section>
    <title>Example: One module</title>
    <p>From the Erlang shell:</p>
    <pre>
1> <input>cprof:start(),R=calendar:day_of_the_week(1896,4,27),cprof:pause(),R.</input>
1
2> <input>cprof:analyse(calendar).</input>
{calendar,9,
          [{{calendar,df,2},1},
           {{calendar,dm,1},1},
           {{calendar,dy,1},1},
           {{calendar,last_day_of_the_month1,2},1},
           {{calendar,last_day_of_the_month,2},1},
           {{calendar,is_leap_year1,1},1},
           {{calendar,is_leap_year,1},1},
           {{calendar,day_of_the_week,3},1},
           {{calendar,date_to_gregorian_days,3},1}]}
3> <input>cprof:stop().</input>
3271</pre>
    <p>The example tells us that "Aktiebolaget LM Ericsson &amp; Co"
      was registered on a Monday (since the return value
      of the first command is 1), and that the <c>calendar</c> module
      needed 9 function calls to calculate that.
      </p>
    <p>Using <c>cprof:analyse()</c> in this example also shows
      approximately the same background work as in the first example. 
      </p>
  </section>

  <section>
    <title>Example: In the code</title>
    <p>Write a module:</p>
    <pre>
-module(sort).
      
-export([do/1]).
      
do(N) ->
    cprof:stop(),
    cprof:start(),
    do(N, []).
      
do(0, L) ->
    R = lists:sort(L),
    cprof:pause(),
    R;
do(N, L) ->
    do(N-1, [random:uniform(256)-1 | L]).</pre>
    <p>From the Erlang shell:</p>
    <pre>
1> <input>c(sort).</input>
{ok,sort}
2> <input>l(random).</input>
{module,random}
3> <input>sort:do(1000).</input>
[0,0,1,1,1,1,1,1,2,2,2,3,3,3,3,3,4,4,4,5,5,5,5,6,6,6,6,6,6|...]
4> <input>cprof:analyse().</input>
{9050,
 [{lists_sort,6047,
              [{{lists_sort,merge3_2,6},923},
               {{lists_sort,merge3_1,6},879},
               {{lists_sort,split_2,5},661},
               {{lists_sort,rmerge3_1,6},580},
               {{lists_sort,rmerge3_2,6},543},
               {{lists_sort,merge3_12_3,6},531},
               {{lists_sort,merge3_21_3,6},383},
               {{lists_sort,split_2_1,6},338},
               {{lists_sort,rmerge3_21_3,6},299},
               {{lists_sort,rmerge3_12_3,6},205},
               {{lists_sort,rmerge2_2,4},180},
               {{lists_sort,rmerge2_1,4},171},
               {{lists_sort,merge2_1,4},127},
               {{lists_sort,merge2_2,4},121},
               {{lists_sort,mergel,2},79},
               {{lists_sort,rmergel,2},27}]},
  {random,2001,
          [{{random,uniform,1},1000},
           {{random,uniform,0},1000},
           {{random,seed0,0},1}]},
  {sort,1001,[{{sort,do,2},1001}]},
  {lists,1,[{{lists,sort,1},1}]}]}
5> <input>cprof:stop().</input>
5369</pre>
    <p>The example shows some details of how <c>lists:sort/1</c>
      works. It used 6047 function calls in the module
      <c>lists_sort</c> to complete the work.
      </p>
    <p>This time, since the shell was not involved, no other work was
      done in the system during the profiling. If you retry the same
      example with a freshly started Erlang emulator, but omit the
      command <c>l(random)</c>, the analysis will show a lot more
      function calls done by <c>code_server</c> and others to
      automatically load the module <c>random</c>.
      </p>
  </section>
</chapter>

