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NAME

       fprof - A Time Profiling Tool using trace to file for minimal runtime performance impact.

DESCRIPTION

       This module is used to profile a program to find out how the execution time is used. Trace
       to file is used to minimize runtime performance impact.

       The fprof module uses tracing to collect profiling  data,  hence  there  is  no  need  for
       special compilation of any module to be profiled. When it starts tracing, fprof will erase
       all previous tracing in the node and set the necessary trace flags on the profiling target
       processes  as  well  as  local  call  trace on all functions in all loaded modules and all
       modules to be loaded. fprof erases all tracing in the node when it stops tracing.

       fprof presents both own time i.e how much time a function has used for its own  execution,
       and  accumulated  time  i.e  including called functions. All presented times are collected
       using trace timestamps. fprof tries to collect cpu time timestamps, if the host machine OS
       supports  it.  Therefore  the times may be wallclock times and OS scheduling will randomly
       strike all called functions in a presumably fair way.

       If, however, the profiling time is short, and the host machine OS does  not  support  high
       resolution cpu time measurements, some few OS schedulings may show up as ridiculously long
       execution times for functions doing practically nothing. An example of a function more  or
       less  just  composing  a tuple in about 100 times the normal execution time has been seen,
       and when the tracing was repeated, the execution time became normal.

       Profiling is essentially done in 3 steps:

         1:
           Tracing; to file, as mentioned in the previous paragraph. The trace  contains  entries
           for  function  calls,  returns  to function, process scheduling, other process related
           (spawn, etc) events, and garbage collection. All trace entries are timestamped.

         2:
           Profiling; the trace file is read, the execution call  stack  is  simulated,  and  raw
           profile data is calculated from the simulated call stack and the trace timestamps. The
           profile data is stored in the fprof server state. During this step the trace data  may
           be dumped in text format to file or console.

         3:
           Analysing;  the  raw profile data is sorted, filtered and dumped in text format either
           to file or console. The text format intended to be both readable for a  human  reader,
           as well as parsable with the standard erlang parsing tools.

       Since  fprof  uses  trace  to  file, the runtime performance degradation is minimized, but
       still far from negligible, especially for programs that  use  the  filesystem  heavily  by
       themselves.  Where  you  place  the  trace  file is also important, e.g on Solaris /tmp is
       usually a good choice since it is essentially a RAM disk, while any NFS (network)  mounted
       disk is a bad idea.

       fprof can also skip the file step and trace to a tracer process that does the profiling in
       runtime.

EXPORTS

       start() -> {ok, Pid} | {error, {already_started, Pid}}

              Types:

                 Pid = pid()

              Starts the fprof server.

              Note that it seldom needs to  be  started  explicitly  since  it  is  automatically
              started by the functions that need a running server.

       stop() -> ok

              Same as stop(normal).

       stop(Reason) -> ok

              Types:

                 Reason = term()

              Stops the fprof server.

              The  supplied  Reason  becomes  the exit reason for the server process. Default Any
              Reason other than kill sends a request to the server and waits for it to clean  up,
              reply and exit. If Reason is kill, the server is bluntly killed.

              If the fprof server is not running, this function returns immediately with the same
              return value.

          Note:
              When the fprof server is stopped the collected raw profile data is lost.

       apply(Func, Args) -> term()

              Types:

                 Func = function() | {Module, Function}
                 Args = [term()]
                 Module = atom()
                 Function = atom()

              Same as apply(Func, Args, []).

       apply(Module, Function, Args) -> term()

              Types:

                 Args = [term()]
                 Module = atom()
                 Function = atom()

              Same as apply({Module, Function}, Args, []).

       apply(Func, Args, OptionList) -> term()

              Types:

                 Func = function() | {Module, Function}
                 Args = [term()]
                 OptionList = [Option]
                 Module = atom()
                 Function = atom()
                 Option = continue | start | {procs, PidList} | TraceStartOption

              Calls erlang:apply(Func, Args) surrounded by trace([start, ...]) and trace(stop).

              Some effort is made to keep  the  trace  clean  from  unnecessary  trace  messages;
              tracing is started and stopped from a spawned process while the erlang:apply/2 call
              is made in the current process, only surrounded  by  receive  and  send  statements
              towards  the  trace  starting  process.  The  trace starting process exits when not
              needed any more.

              The TraceStartOption is any option allowed for trace/1. The options [start, {procs,
              [self()  |  PidList]}  | OptList] are given to trace/1, where OptList is OptionList
              with continue, start and {procs, _} options removed.

              The continue option inhibits the call to trace(stop) and leaves it up to the caller
              to stop tracing at a suitable time.

       apply(Module, Function, Args, OptionList) -> term()

              Types:

                 Module = atom()
                 Function = atom()
                 Args = [term()]

              Same as apply({Module, Function}, Args, OptionList).

              OptionList is an option list allowed for apply/3.

       trace(start, Filename) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 Reason = term()

              Same as trace([start, {file, Filename}]).

       trace(verbose, Filename) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 Reason = term()

              Same as trace([start, verbose, {file, Filename}]).

       trace(OptionName, OptionValue) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 OptionName = atom()
                 OptionValue = term()
                 Reason = term()

              Same as trace([{OptionName, OptionValue}]).

       trace(verbose) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 Reason = term()

              Same as trace([start, verbose]).

       trace(OptionName) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 OptionName = atom()
                 Reason = term()

              Same as trace([OptionName]).

       trace({OptionName, OptionValue}) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 OptionName = atom()
                 OptionValue = term()
                 Reason = term()

              Same as trace([{OptionName, OptionValue}]).

       trace([Option]) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 Option  =  start  |  stop  |  {procs,  PidSpec} | {procs, [PidSpec]} | verbose |
                 {verbose, bool()} | file | {file, Filename} | {tracer, Tracer}
                 PidSpec = pid() | atom()
                 Tracer = pid() | port()
                 Reason = term()

              Starts or stops tracing.

              PidSpec and Tracer are used  in  calls  to  erlang:trace(PidSpec,  true,  [{tracer,
              Tracer}  |  Flags]),  and  Filename is used to call dbg:trace_port(file, Filename).
              Please see the appropriate documentation.

              Option description:

                stop:
                  Stops a running fprof trace and clears all tracing from the node. Either option
                  stop or start must be specified, but not both.

                start:
                  Clears  all  tracing  from the node and starts a new fprof trace. Either option
                  start or stop must be specified, but not both.

                verbose| {verbose, bool()}:
                  The options verbose or {verbose, true} adds some trace flags  that  fprof  does
                  not  need,  but  that  may  be interesting for general debugging purposes. This
                  option is only allowed with the start option.

                cpu_time| {cpu_time, bool()}:
                  The options cpu_time or {cpu_time, true> makes the timestamps in the  trace  be
                  in CPU time instead of wallclock time which is the default. This option is only
                  allowed with the start option.

                {procs, PidSpec}| {procs, [PidSpec]}:
                  Specifies which processes that shall be traced. If this option  is  not  given,
                  the  calling  process  is traced. All processes spawned by the traced processes
                  are also traced. This option is only allowed with the start option.

                file| {file, Filename}:
                  Specifies the filename of the trace. If the option file is given,  or  none  of
                  these  options  are  given, the file "fprof.trace" is used. This option is only
                  allowed with the start option, but not with the {tracer, Tracer} option.

                {tracer, Tracer}:
                  Specifies that trace to process or port shall be done instead of trace to file.
                  This  option  is  only  allowed  with the start option, but not with the {file,
                  Filename} option.

       profile() -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 Reason = term()

              Same as profile([]).

       profile(OptionName, OptionValue) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 OptionName = atom()
                 OptionValue = term()
                 Reason = term()

              Same as profile([{OptionName, OptionValue}]).

       profile(OptionName) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 OptionName = atom()
                 Reason = term()

              Same as profile([OptionName]).

       profile({OptionName, OptionValue}) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 OptionName = atom()
                 OptionValue = term()
                 Reason = term()

              Same as profile([{OptionName, OptionValue}]).

       profile([Option]) -> ok | {ok, Tracer} | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 Option = file | {file, Filename} | dump | {dump, Dump} | append | start | stop
                 Dump = pid() | Dumpfile | []
                 Tracer = pid()
                 Reason = term()

              Compiles a trace into raw profile data held by the fprof server.

              Dumpfile  is  used  to  call  file:open/2,   and   Filename   is   used   to   call
              dbg:trace_port(file, Filename). Please see the appropriate documentation.

              Option description:

                file| {file, Filename}:
                  Reads  the  file Filename and creates raw profile data that is stored in RAM by
                  the fprof server. If the option file is given, or none  of  these  options  are
                  given,  the  file  "fprof.trace"  is  read. The call will return when the whole
                  trace has been read with the return value ok if successful. This option is  not
                  allowed with the start or stop options.

                dump| {dump, Dump}:
                  Specifies the destination for the trace text dump. If this option is not given,
                  no dump is generated, if it is dump the destination will be the caller's  group
                  leader,  otherwise the destination Dump is either the pid of an I/O device or a
                  filename. And, finally, if the filename is [] - "fprof.dump" is  used  instead.
                  This option is not allowed with the stop option.

                append:
                  Causes  the trace text dump to be appended to the destination file. This option
                  is only allowed with the {dump, Dumpfile} option.

                start:
                  Starts a tracer process that profiles trace data  in  runtime.  The  call  will
                  return  immediately  with  the  return  value  {ok, Tracer} if successful. This
                  option is not allowed with the stop, file or {file, Filename} options.

                stop:
                  Stops the tracer process that profiles trace data in runtime. The return  value
                  will be value ok if successful. This option is not allowed with the start, file
                  or {file, Filename} options.

       analyse() -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 Reason = term()

              Same as analyse([]).

       analyse(OptionName, OptionValue) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 OptionName = atom()
                 OptionValue = term()
                 Reason = term()

              Same as analyse([{OptionName, OptionValue}]).

       analyse(OptionName) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 OptionName = atom()
                 Reason = term()

              Same as analyse([OptionName]).

       analyse({OptionName, OptionValue}) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 OptionName = atom()
                 OptionValue = term()
                 Reason = term()

              Same as analyse([{OptionName, OptionValue}]).

       analyse([Option]) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}

              Types:

                 Option = dest | {dest, Dest} | append |  {cols,  Cols}  |  callers  |  {callers,
                 bool()}  | no_callers | {sort, SortSpec} | totals | {totals, bool()} | details |
                 {details, bool()} | no_details
                 Dest = pid() | Destfile
                 Cols = integer() >= 80
                 SortSpec = acc | own
                 Reason = term()

              Analyses raw profile data in the fprof server. If called  while  there  is  no  raw
              profile data available, {error, no_profile} is returned.

              Destfile is used to call file:open/2. Please see the appropriate documentation.

              Option description:

                dest| {dest, Dest}:
                  Specifies  the  destination for the analysis. If this option is not given or it
                  is dest, the destination will be  the  caller's  group  leader,  otherwise  the
                  destination  Dest  is  either  the  pid()  of an I/O device or a filename. And,
                  finally, if the filename is [] - "fprof.analysis" is used instead.

                append:
                  Causes the analysis to be appended to the destination file. This option is only
                  allowed with the {dest, Destfile} option.

                {cols, Cols}:
                  Specifies  the  number  of  columns in the analysis text. If this option is not
                  given the number of columns is set to 80.

                callers| {callers, true}:
                  Prints callers and called information in the analysis. This is the default.

                {callers, false}| no_callers:
                  Suppresses the printing of callers and called information in the analysis.

                {sort, SortSpec}:
                  Specifies if the analysis should be sorted according to the ACC  column,  which
                  is the default, or the OWN column. See Analysis Format below.

                totals| {totals, true}:
                  Includes  a  section  containing  call  statistics  for all calls regardless of
                  process, in the analysis.

                {totals, false}:
                  Supresses the totals section in the analysis, which is the default.

                details| {details, true}:
                  Prints call statistics for each process in the analysis. This is the default.

                {details, false}| no_details:
                  Suppresses the call statistics for each process from the analysis.

ANALYSIS FORMAT

       This section describes the output format of the analyse command. See analyse/0.

       The format is parsable with the standard Erlang  parsing  tools  erl_scan  and  erl_parse,
       file:consult/1  or  io:read/2.  The parse format is not explained here - it should be easy
       for the interested to try it out. Note that  some  flags  to  analyse/1  will  affect  the
       format.

       The  following  example  was run on OTP/R8 on Solaris 8, all OTP internals in this example
       are very version dependent.

       As an example, we will use the following function, that you may recognise  as  a  slightly
       modified benchmark function from the manpage file(3erl):

       -module(foo).
       -export([create_file_slow/2]).

       create_file_slow(Name, N) when integer(N), N >= 0 ->
           {ok, FD} =
               file:open(Name, [raw, write, delayed_write, binary]),
           if N > 256 ->
                   ok = file:write(FD,
                                   lists:map(fun (X) -> <<X:32/unsigned>> end,
                                   lists:seq(0, 255))),
                   ok = create_file_slow(FD, 256, N);
              true ->
                   ok = create_file_slow(FD, 0, N)
           end,
           ok = file:close(FD).

       create_file_slow(FD, M, M) ->
           ok;
       create_file_slow(FD, M, N) ->
           ok = file:write(FD, <<M:32/unsigned>>),
           create_file_slow(FD, M+1, N).

       Let us have a look at the printout after running:

       1> fprof:apply(foo, create_file_slow, [junk, 1024]).
       2> fprof:profile().
       3> fprof:analyse().

       The printout starts with:

       %% Analysis results:
       {  analysis_options,
        [{callers, true},
         {sort, acc},
         {totals, false},
         {details, true}]}.

       %                                       CNT       ACC       OWN
       [{ totals,                             9627, 1691.119, 1659.074}].  %%%

       The  CNT  column  shows the total number of function calls that was found in the trace. In
       the ACC column is the total time of the trace from first timestamp to last. And in the OWN
       column  is  the  sum  of the execution time in functions found in the trace, not including
       called functions. In this case it is very close to the ACC time  since  the  emulator  had
       practically nothing else to do than to execute our test program.

       All time values in the printout are in milliseconds.

       The printout continues:

       %                                       CNT       ACC       OWN
       [{ "<0.28.0>",                         9627,undefined, 1659.074}].   %%

       This  is  the  printout header of one process. The printout contains only this one process
       since we did fprof:apply/3 which traces only the current process. Therefore  the  CNT  and
       OWN  columns perfectly matches the totals above. The ACC column is undefined since summing
       the ACC times of all calls in the process makes no sense - you would  get  something  like
       the  ACC  value  from  totals  above multiplied by the average depth of the call stack, or
       something.

       All paragraphs up to the next process header only  concerns  function  calls  within  this
       process.

       Now we come to something more interesting:

       {[{undefined,                             0, 1691.076,    0.030}],
        { {fprof,apply_start_stop,4},            0, 1691.076,    0.030},     %
        [{{foo,create_file_slow,2},              1, 1691.046,    0.103},
         {suspend,                               1,    0.000,    0.000}]}.

       {[{{fprof,apply_start_stop,4},            1, 1691.046,    0.103}],
        { {foo,create_file_slow,2},              1, 1691.046,    0.103},     %
        [{{file,close,1},                        1, 1398.873,    0.019},
         {{foo,create_file_slow,3},              1,  249.678,    0.029},
         {{file,open,2},                         1,   20.778,    0.055},
         {{lists,map,2},                         1,   16.590,    0.043},
         {{lists,seq,2},                         1,    4.708,    0.017},
         {{file,write,2},                        1,    0.316,    0.021}]}.

       The  printout  consists of one paragraph per called function. The function marked with '%'
       is the one the paragraph concerns - foo:create_file_slow/2. Above the marked function  are
       the  calling  functions  - those that has called the marked, and below are those called by
       the marked function.

       The paragraphs are per default sorted in decreasing order of the ACC column for the marked
       function.  The  calling  list  and  called  list within one paragraph are also per default
       sorted in decreasing order of their ACC column.

       The columns are: CNT - the number of times the function has been called, ACC  -  the  time
       spent in the function including called functions, and OWN - the time spent in the function
       not including called functions.

       The rows for the calling functions contain statistics for the  marked  function  with  the
       constraint  that  only  the  occasions when a call was made from the row's function to the
       marked function are accounted for.

       The row for the marked function simply contains the sum of all calling rows.

       The rows for the called functions contains statistics for  the  row's  function  with  the
       constraint  that  only  the  occasions  when  a call was made from the marked to the row's
       function are accounted for.

       So, we see that foo:create_file_slow/2 used very little time for  its  own  execution.  It
       spent  most  of  its time in file:close/1. The function foo:create_file_slow/3 that writes
       3/4 of the file contents is the second biggest time thief.

       We also see that the call to file:write/2 that writes 1/4 of the file contents takes  very
       little time in itself. What takes time is to build the data (lists:seq/2 and lists:map/2).

       The  function  'undefined' that has called fprof:apply_start_stop/4 is an unknown function
       because that call was not recorded in the trace. It was only recorded that  the  execution
       returned  from fprof:apply_start_stop/4 to some other function above in the call stack, or
       that the process exited from there.

       Let us continue down the printout to find:

       {[{{foo,create_file_slow,2},              1,  249.678,    0.029},
         {{foo,create_file_slow,3},            768,    0.000,   23.294}],
        { {foo,create_file_slow,3},            769,  249.678,   23.323},     %
        [{{file,write,2},                      768,  220.314,   14.539},
         {suspend,                              57,    6.041,    0.000},
         {{foo,create_file_slow,3},            768,    0.000,   23.294}]}.

       If you compare with the code you will  see  there  also  that  foo:create_file_slow/3  was
       called only from foo:create_file_slow/2 and itself, and called only file:write/2, note the
       number of calls to file:write/2. But here we see that suspend was called a few times. This
       is  a  pseudo  function  that  indicates that the process was suspended while executing in
       foo:create_file_slow/3, and since there is no receive or erlang:yield/0 in  the  code,  it
       must  be  Erlang  scheduling  suspensions, or the trace file driver compensating for large
       file write operations (these are regarded as a schedule out followed by a schedule  in  to
       the same process).

       Let us find the suspend entry:

       {[{{file,write,2},                       53,    6.281,    0.000},
         {{foo,create_file_slow,3},             57,    6.041,    0.000},
         {{prim_file,drv_command,4},            50,    4.582,    0.000},
         {{prim_file,drv_get_response,1},       34,    2.986,    0.000},
         {{lists,map,2},                        10,    2.104,    0.000},
         {{prim_file,write,2},                  17,    1.852,    0.000},
         {{erlang,port_command,2},              15,    1.713,    0.000},
         {{prim_file,drv_command,2},            22,    1.482,    0.000},
         {{prim_file,translate_response,2},     11,    1.441,    0.000},
         {{prim_file,'-drv_command/2-fun-0-',1},  15,    1.340,    0.000},
         {{lists,seq,4},                         3,    0.880,    0.000},
         {{foo,'-create_file_slow/2-fun-0-',1},   5,    0.523,    0.000},
         {{erlang,bump_reductions,1},            4,    0.503,    0.000},
         {{prim_file,open_int_setopts,3},        1,    0.165,    0.000},
         {{prim_file,i32,4},                     1,    0.109,    0.000},
         {{fprof,apply_start_stop,4},            1,    0.000,    0.000}],
        { suspend,                             299,   32.002,    0.000},     %
        [ ]}.

       We  find  no  particulary  long  suspend  times,  so no function seems to have waited in a
       receive statement. Actually, prim_file:drv_command/4 contains a receive statement, but  in
       this  test  program,  the  message  lies  in  the  process receive buffer when the receive
       statement is entered. We also see that the total suspend time for the test run is small.

       The suspend pseudo function has got an OWN time of zero. This is to  prevent  the  process
       total  OWN  time  from including time in suspension. Whether suspend time is really ACC or
       OWN time is more of a philosophical question.

       Now we look at another interesting pseudo function, garbage_collect:

       {[{{prim_file,drv_command,4},            25,    0.873,    0.873},
         {{prim_file,write,2},                  16,    0.692,    0.692},
         {{lists,map,2},                         2,    0.195,    0.195}],
        { garbage_collect,                      43,    1.760,    1.760},     %
        [ ]}.

       Here we see that no function distinguishes itself considerably, which is very normal.

       The garbage_collect pseudo function has not got an OWN time of zero like suspend,  instead
       it is equal to the ACC time.

       Garbage  collect  often  occurs while a process is suspended, but fprof hides this fact by
       pretending that the suspended function was first unsuspended and then  garbage  collected.
       Otherwise  the  printout  would show garbage_collect being called from suspend but not not
       which function that might have caused the garbage collection.

       Let us now get back to the test code:

       {[{{foo,create_file_slow,3},            768,  220.314,   14.539},
         {{foo,create_file_slow,2},              1,    0.316,    0.021}],
        { {file,write,2},                      769,  220.630,   14.560},     %
        [{{prim_file,write,2},                 769,  199.789,   22.573},
         {suspend,                              53,    6.281,    0.000}]}.

       Not unexpectedly, we see that file:write/2  was  called  from  foo:create_file_slow/3  and
       foo:create_file_slow/2. The number of calls in each case as well as the used time are also
       just confirms the previous results.

       We see that file:write/2 only calls prim_file:write/2, but let  us  refrain  from  digging
       into the internals of the kernel application.

       But, if we nevertheless do dig down we find the call to the linked in driver that does the
       file operations towards the host operating system:

       {[{{prim_file,drv_command,4},           772, 1458.356, 1456.643}],
        { {erlang,port_command,2},             772, 1458.356, 1456.643},     %
        [{suspend,                              15,    1.713,    0.000}]}.

       This is 86 % of the total run time, and as we saw before it is  the  close  operation  the
       absolutely  biggest  contributor.  We  find a comparison ratio a little bit up in the call
       stack:

       {[{{prim_file,close,1},                   1, 1398.748,    0.024},
         {{prim_file,write,2},                 769,  174.672,   12.810},
         {{prim_file,open_int,4},                1,   19.755,    0.017},
         {{prim_file,open_int_setopts,3},        1,    0.147,    0.016}],
        { {prim_file,drv_command,2},           772, 1593.322,   12.867},     %
        [{{prim_file,drv_command,4},           772, 1578.973,   27.265},
         {suspend,                              22,    1.482,    0.000}]}.

       The time for file operations in the linked in driver distributes itself as 1 %  for  open,
       11  %  for write and 87 % for close. All data is probably buffered in the operating system
       until the close.

       The unsleeping reader may notice  that  the  ACC  times  for  prim_file:drv_command/2  and
       prim_file:drv_command/4  is not equal between the paragraphs above, even though it is easy
       to believe that prim_file:drv_command/2 is just a passthrough function.

       The missing time can be found in the paragraph for  prim_file:drv_command/4  where  it  is
       evident that not only prim_file:drv_command/2 is called but also a fun:

       {[{{prim_file,drv_command,2},           772, 1578.973,   27.265}],
        { {prim_file,drv_command,4},           772, 1578.973,   27.265},     %
        [{{erlang,port_command,2},             772, 1458.356, 1456.643},
         {{prim_file,'-drv_command/2-fun-0-',1}, 772,   87.897,   12.736},
         {suspend,                              50,    4.582,    0.000},
         {garbage_collect,                      25,    0.873,    0.873}]}.

       And  some  more  missing  time can be explained by the fact that prim_file:open_int/4 both
       calls prim_file:drv_command/2 directly as well  as  through  prim_file:open_int_setopts/3,
       which complicates the picture.

       {[{{prim_file,open,2},                    1,   20.309,    0.029},
         {{prim_file,open_int,4},                1,    0.000,    0.057}],
        { {prim_file,open_int,4},                2,   20.309,    0.086},     %
        [{{prim_file,drv_command,2},             1,   19.755,    0.017},
         {{prim_file,open_int_setopts,3},        1,    0.360,    0.032},
         {{prim_file,drv_open,2},                1,    0.071,    0.030},
         {{erlang,list_to_binary,1},             1,    0.020,    0.020},
         {{prim_file,i32,1},                     1,    0.017,    0.017},
         {{prim_file,open_int,4},                1,    0.000,    0.057}]}.
       {[{{prim_file,open_int,4},                1,    0.360,    0.032},
         {{prim_file,open_int_setopts,3},        1,    0.000,    0.016}],
        { {prim_file,open_int_setopts,3},        2,    0.360,    0.048},     %
        [{suspend,                               1,    0.165,    0.000},
         {{prim_file,drv_command,2},             1,    0.147,    0.016},
         {{prim_file,open_int_setopts,3},        1,    0.000,    0.016}]}.

NOTES

       The actual supervision of execution times is in itself a CPU intensive activity. A message
       is written on the trace file for every function call that is made by the profiled code.

       The ACC time calculation is sometimes difficult to make correct, since it is difficult  to
       define.  This  happens  especially when a function occurs in several instances in the call
       stack, for example by calling itself perhaps through other functions and perhaps even non-
       tail recursively.

       To produce sensible results, fprof tries not to charge any function more than once for ACC
       time. The instance highest up (with longest duration) in the call stack is chosen.

       Sometimes a function may unexpectedly waste a lot (some 10 ms or more  depending  on  host
       machine  OS)  of  OWN (and ACC) time, even functions that does practically nothing at all.
       The problem may be that the OS has chosen  to  schedule  out  the  Erlang  runtime  system
       process  for a while, and if the OS does not support high resolution cpu time measurements
       fprof will use wallclock time for its  calculations,  and  it  will  appear  as  functions
       randomly burn virtual machine time.

SEE ALSO

       dbg(3erl), eprof(3erl), erlang(3erl), io(3erl), Tools User's Guide