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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">
<chapter>
<header>
<copyright>
<year>2002</year><year>2013</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions 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 & 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>
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