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fprof(3erl)			     Erlang Module Definition			      fprof(3erl)

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  spe-
       cial  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 mod-
       ules 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 pro-
	   file  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 usu-
       ally 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;  trac-
	      ing  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 pro-
	      file 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 des-
		  tination 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 for-
       mat.

       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 con-
       straint 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 state-
       ment 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  ran-
       domly burn virtual machine time.

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

Ericsson AB				  tools 2.6.6.3 			      fprof(3erl)
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