1fprof(3) Erlang Module Definition fprof(3)
2
6 fprof - A Time Profiling Tool using trace to file for minimal runtime
7 performance impact.
8
10 This module is used to profile a program to find out how the execution
11 time is used. Trace to file is used to minimize runtime performance
12 impact.
13 The fprof module uses tracing to collect profiling data, hence there is
15 no need for special compilation of any module to be profiled. When it
16 starts tracing, fprof will erase all previous tracing in the node and
17 set the necessary trace flags on the profiling target processes as well
18 as local call trace on all functions in all loaded modules and all mod‐
19 ules to be loaded. fprof erases all tracing in the node when it stops
20 tracing.
21 fprof presents both own time i.e how much time a function has used for
23 its own execution, and accumulated time i.e including called functions.
24 All presented times are collected using trace timestamps. fprof tries
25 to collect cpu time timestamps, if the host machine OS supports it.
26 Therefore the times may be wallclock times and OS scheduling will ran‐
27 domly strike all called functions in a presumably fair way.
28 If, however, the profiling time is short, and the host machine OS does
30 not support high resolution cpu time measurements, some few OS schedul‐
31 ings may show up as ridiculously long execution times for functions
32 doing practically nothing. An example of a function more or less just
33 composing a tuple in about 100 times the normal execution time has been
34 seen, and when the tracing was repeated, the execution time became nor‐
35 mal.
36 Profiling is essentially done in 3 steps:
38 1:
40 Tracing; to file, as mentioned in the previous paragraph. The trace
41 contains entries for function calls, returns to function, process
42 scheduling, other process related (spawn, etc) events, and garbage
43 collection. All trace entries are timestamped.
44 2:
46 Profiling; the trace file is read, the execution call stack is sim‐
47 ulated, and raw profile data is calculated from the simulated call
48 stack and the trace timestamps. The profile data is stored in the
49 fprof server state. During this step the trace data may be dumped
50 in text format to file or console.
51 3:
53 Analysing; the raw profile data is sorted, filtered and dumped in
54 text format either to file or console. The text format intended to
55 be both readable for a human reader, as well as parsable with the
56 standard erlang parsing tools.
57 Since fprof uses trace to file, the runtime performance degradation is
59 minimized, but still far from negligible, especially for programs that
60 use the filesystem heavily by themselves. Where you place the trace
61 file is also important, e.g on Solaris /tmp is usually a good choice
62 since it is essentially a RAM disk, while any NFS (network) mounted
63 disk is a bad idea.
64 fprof can also skip the file step and trace to a tracer process that
66 does the profiling in runtime.
67
69 start() -> {ok, Pid} | {error, {already_started, Pid}}
70 Types:
72 Pid = pid()
74 Starts the fprof server.
76 Note that it seldom needs to be started explicitly since it is
78 automatically started by the functions that need a running
79 server.
80 stop() -> ok
82 Same as stop(normal).
84 stop(Reason) -> ok
86 Types:
88 Reason = term()
90 Stops the fprof server.
92 The supplied Reason becomes the exit reason for the server
94 process. Default Any Reason other than kill sends a request to
95 the server and waits for it to clean up, reply and exit. If Rea‐
96 son is kill, the server is bluntly killed.
97 If the fprof server is not running, this function returns imme‐
99 diately with the same return value.
100 Note:
102 When the fprof server is stopped the collected raw profile data
103 is lost.
104 apply(Func, Args) -> term()
107 Types:
109 Func = function() | {Module, Function}
111 Args = [term()]
112 Module = atom()
113 Function = atom()
114 Same as apply(Func, Args, []).
116 apply(Module, Function, Args) -> term()
118 Types:
120 Args = [term()]
122 Module = atom()
123 Function = atom()
124 Same as apply({Module, Function}, Args, []).
126 apply(Func, Args, OptionList) -> term()
128 Types:
130 Func = function() | {Module, Function}
132 Args = [term()]
133 OptionList = [Option]
134 Module = atom()
135 Function = atom()
136 Option = continue | start | {procs, PidList} | TraceStartOp‐
137 tion
138 Calls erlang:apply(Func, Args) surrounded by trace([start, ...])
140 and trace(stop).
141 Some effort is made to keep the trace clean from unnecessary
143 trace messages; tracing is started and stopped from a spawned
144 process while the erlang:apply/2 call is made in the current
145 process, only surrounded by receive and send statements towards
146 the trace starting process. The trace starting process exits
147 when not needed any more.
148 The TraceStartOption is any option allowed for trace/1. The
150 options [start, {procs, [self() | PidList]} | OptList] are given
151 to trace/1, where OptList is OptionList with continue, start and
152 {procs, _} options removed.
153 The continue option inhibits the call to trace(stop) and leaves
155 it up to the caller to stop tracing at a suitable time.
156 apply(Module, Function, Args, OptionList) -> term()
158 Types:
160 Module = atom()
162 Function = atom()
163 Args = [term()]
164 Same as apply({Module, Function}, Args, OptionList).
166 OptionList is an option list allowed for apply/3.
168 trace(start, Filename) -> ok | {error, Reason} | {'EXIT', ServerPid,
170 Reason}
171 Types:
173 Reason = term()
175 Same as trace([start, {file, Filename}]).
177 trace(verbose, Filename) -> ok | {error, Reason} | {'EXIT', ServerPid,
179 Reason}
180 Types:
182 Reason = term()
184 Same as trace([start, verbose, {file, Filename}]).
186 trace(OptionName, OptionValue) -> ok | {error, Reason} | {'EXIT',
188 ServerPid, Reason}
189 Types:
191 OptionName = atom()
193 OptionValue = term()
194 Reason = term()
195 Same as trace([{OptionName, OptionValue}]).
197 trace(verbose) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}
199 Types:
201 Reason = term()
203 Same as trace([start, verbose]).
205 trace(OptionName) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}
207 Types:
209 OptionName = atom()
211 Reason = term()
212 Same as trace([OptionName]).
214 trace({OptionName, OptionValue}) -> ok | {error, Reason} | {'EXIT',
216 ServerPid, Reason}
217 Types:
219 OptionName = atom()
221 OptionValue = term()
222 Reason = term()
223 Same as trace([{OptionName, OptionValue}]).
225 trace([Option]) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}
227 Types:
229 Option = start | stop | {procs, PidSpec} | {procs, [PidSpec]}
231 | verbose | {verbose, bool()} | file | {file, Filename} |
232 {tracer, Tracer}
233 PidSpec = pid() | atom()
234 Tracer = pid() | port()
235 Reason = term()
236 Starts or stops tracing.
238 PidSpec and Tracer are used in calls to erlang:trace(PidSpec,
240 true, [{tracer, Tracer} | Flags]), and Filename is used to call
241 dbg:trace_port(file, Filename). Please see the appropriate docu‐
242 mentation.
243 Option description:
245 stop:
247 Stops a running fprof trace and clears all tracing from the
248 node. Either option stop or start must be specified, but not
249 both.
250 start:
252 Clears all tracing from the node and starts a new fprof
253 trace. Either option start or stop must be specified, but
254 not both.
255 verbose| {verbose, bool()}:
257 The options verbose or {verbose, true} adds some trace flags
258 that fprof does not need, but that may be interesting for
259 general debugging purposes. This option is only allowed with
260 the start option.
261 cpu_time| {cpu_time, bool()}:
263 The options cpu_time or {cpu_time, true} makes the time‐
264 stamps in the trace be in CPU time instead of wallclock time
265 which is the default. This option is only allowed with the
266 start option.
267 Warning:
269 Getting correct values out of cpu_time can be difficult. The
270 best way to get correct values is to run using a single sched‐
271 uler and bind that scheduler to a specific CPU, i.e. erl +S 1
272 +sbt db.
273 {procs, PidSpec}| {procs, [PidSpec]}:
276 Specifies which processes that shall be traced. If this
277 option is not given, the calling process is traced. All pro‐
278 cesses spawned by the traced processes are also traced. This
279 option is only allowed with the start option.
280 file| {file, Filename}:
282 Specifies the filename of the trace. If the option file is
283 given, or none of these options are given, the file
284 "fprof.trace" is used. This option is only allowed with the
285 start option, but not with the {tracer, Tracer} option.
286 {tracer, Tracer}:
288 Specifies that trace to process or port shall be done
289 instead of trace to file. This option is only allowed with
290 the start option, but not with the {file, Filename} option.
291 profile() -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}
293 Types:
295 Reason = term()
297 Same as profile([]).
299 profile(OptionName, OptionValue) -> ok | {error, Reason} | {'EXIT',
301 ServerPid, Reason}
302 Types:
304 OptionName = atom()
306 OptionValue = term()
307 Reason = term()
308 Same as profile([{OptionName, OptionValue}]).
310 profile(OptionName) -> ok | {error, Reason} | {'EXIT', ServerPid, Rea‐
312 son}
313 Types:
315 OptionName = atom()
317 Reason = term()
318 Same as profile([OptionName]).
320 profile({OptionName, OptionValue}) -> ok | {error, Reason} | {'EXIT',
322 ServerPid, Reason}
323 Types:
325 OptionName = atom()
327 OptionValue = term()
328 Reason = term()
329 Same as profile([{OptionName, OptionValue}]).
331 profile([Option]) -> ok | {ok, Tracer} | {error, Reason} | {'EXIT',
333 ServerPid, Reason}
334 Types:
336 Option = file | {file, Filename} | dump | {dump, Dump} |
338 append | start | stop
339 Dump = pid() | Dumpfile | []
340 Tracer = pid()
341 Reason = term()
342 Compiles a trace into raw profile data held by the fprof server.
344 Dumpfile is used to call file:open/2, and Filename is used to
346 call dbg:trace_port(file, Filename). Please see the appropriate
347 documentation.
348 Option description:
350 file| {file, Filename}:
352 Reads the file Filename and creates raw profile data that is
353 stored in RAM by the fprof server. If the option file is
354 given, or none of these options are given, the file
355 "fprof.trace" is read. The call will return when the whole
356 trace has been read with the return value ok if successful.
357 This option is not allowed with the start or stop options.
358 dump| {dump, Dump}:
360 Specifies the destination for the trace text dump. If this
361 option is not given, no dump is generated, if it is dump the
362 destination will be the caller's group leader, otherwise the
363 destination Dump is either the pid of an I/O device or a
364 filename. And, finally, if the filename is [] - "fprof.dump"
365 is used instead. This option is not allowed with the stop
366 option.
367 append:
369 Causes the trace text dump to be appended to the destination
370 file. This option is only allowed with the {dump, Dumpfile}
371 option.
372 start:
374 Starts a tracer process that profiles trace data in runtime.
375 The call will return immediately with the return value {ok,
376 Tracer} if successful. This option is not allowed with the
377 stop, file or {file, Filename} options.
378 stop:
380 Stops the tracer process that profiles trace data in run‐
381 time. The return value will be value ok if successful. This
382 option is not allowed with the start, file or {file, File‐
383 name} options.
384 analyse() -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}
386 Types:
388 Reason = term()
390 Same as analyse([]).
392 analyse(OptionName, OptionValue) -> ok | {error, Reason} | {'EXIT',
394 ServerPid, Reason}
395 Types:
397 OptionName = atom()
399 OptionValue = term()
400 Reason = term()
401 Same as analyse([{OptionName, OptionValue}]).
403 analyse(OptionName) -> ok | {error, Reason} | {'EXIT', ServerPid, Rea‐
405 son}
406 Types:
408 OptionName = atom()
410 Reason = term()
411 Same as analyse([OptionName]).
413 analyse({OptionName, OptionValue}) -> ok | {error, Reason} | {'EXIT',
415 ServerPid, Reason}
416 Types:
418 OptionName = atom()
420 OptionValue = term()
421 Reason = term()
422 Same as analyse([{OptionName, OptionValue}]).
424 analyse([Option]) -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}
426 Types:
428 Option = dest | {dest, Dest} | append | {cols, Cols} | call‐
430 ers | {callers, bool()} | no_callers | {sort, SortSpec} |
431 totals | {totals, bool()} | details | {details, bool()} |
432 no_details
433 Dest = pid() | Destfile
434 Cols = integer() >= 80
435 SortSpec = acc | own
436 Reason = term()
437 Analyses raw profile data in the fprof server. If called while
439 there is no raw profile data available, {error, no_profile} is
440 returned.
441 Destfile is used to call file:open/2. Please see the appropriate
443 documentation.
444 Option description:
446 dest| {dest, Dest}:
448 Specifies the destination for the analysis. If this option
449 is not given or it is dest, the destination will be the
450 caller's group leader, otherwise the destination Dest is
451 either the pid() of an I/O device or a filename. And,
452 finally, if the filename is [] - "fprof.analysis" is used
453 instead.
454 append:
456 Causes the analysis to be appended to the destination file.
457 This option is only allowed with the {dest, Destfile}
458 option.
459 {cols, Cols}:
461 Specifies the number of columns in the analysis text. If
462 this option is not given the number of columns is set to 80.
463 callers| {callers, true}:
465 Prints callers and called information in the analysis. This
466 is the default.
467 {callers, false}| no_callers:
469 Suppresses the printing of callers and called information in
470 the analysis.
471 {sort, SortSpec}:
473 Specifies if the analysis should be sorted according to the
474 ACC column, which is the default, or the OWN column. See
475 Analysis Format below.
476 totals| {totals, true}:
478 Includes a section containing call statistics for all calls
479 regardless of process, in the analysis.
480 {totals, false}:
482 Supresses the totals section in the analysis, which is the
483 default.
484 details| {details, true}:
486 Prints call statistics for each process in the analysis.
487 This is the default.
488 {details, false}| no_details:
490 Suppresses the call statistics for each process from the
491 analysis.
492
494 This section describes the output format of the analyse command. See
495 analyse/0.
496 The format is parsable with the standard Erlang parsing tools erl_scan
498 and erl_parse, file:consult/1 or io:read/2. The parse format is not
499 explained here - it should be easy for the interested to try it out.
500 Note that some flags to analyse/1 will affect the format.
501 The following example was run on OTP/R8 on Solaris 8, all OTP internals
503 in this example are very version dependent.
504 As an example, we will use the following function, that you may recog‐
506 nise as a slightly modified benchmark function from the manpage
507 file(3):
508 -module(foo).
510 -export([create_file_slow/2]).
511 create_file_slow(Name, N) when is_integer(N), N >= 0 ->
513 {ok, FD} =
514 file:open(Name, [raw, write, delayed_write, binary]),
515 if N > 256 ->
516 ok = file:write(FD,
517 lists:map(fun (X) -> <<X:32/unsigned>> end,
518 lists:seq(0, 255))),
519 ok = create_file_slow(FD, 256, N);
520 true ->
521 ok = create_file_slow(FD, 0, N)
522 end,
523 ok = file:close(FD).
524 create_file_slow(FD, M, M) ->
526 ok;
527 create_file_slow(FD, M, N) ->
528 ok = file:write(FD, <<M:32/unsigned>>),
529 create_file_slow(FD, M+1, N).
530 Let us have a look at the printout after running:
532 1> fprof:apply(foo, create_file_slow, [junk, 1024]).
534 2> fprof:profile().
535 3> fprof:analyse().
536 The printout starts with:
538 %% Analysis results:
540 { analysis_options,
541 [{callers, true},
542 {sort, acc},
543 {totals, false},
544 {details, true}]}.
545 % CNT ACC OWN
547 [{ totals, 9627, 1691.119, 1659.074}]. %%%
548 The CNT column shows the total number of function calls that was found
550 in the trace. In the ACC column is the total time of the trace from
551 first timestamp to last. And in the OWN column is the sum of the execu‐
552 tion time in functions found in the trace, not including called func‐
553 tions. In this case it is very close to the ACC time since the emulator
554 had practically nothing else to do than to execute our test program.
555 All time values in the printout are in milliseconds.
557 The printout continues:
559 % CNT ACC OWN
561 [{ "<0.28.0>", 9627,undefined, 1659.074}]. %%
562 This is the printout header of one process. The printout contains only
564 this one process since we did fprof:apply/3 which traces only the cur‐
565 rent process. Therefore the CNT and OWN columns perfectly matches the
566 totals above. The ACC column is undefined since summing the ACC times
567 of all calls in the process makes no sense - you would get something
568 like the ACC value from totals above multiplied by the average depth of
569 the call stack, or something.
570 All paragraphs up to the next process header only concerns function
572 calls within this process.
573 Now we come to something more interesting:
575 {[{undefined, 0, 1691.076, 0.030}],
577 { {fprof,apply_start_stop,4}, 0, 1691.076, 0.030}, %
578 [{{foo,create_file_slow,2}, 1, 1691.046, 0.103},
579 {suspend, 1, 0.000, 0.000}]}.
580 {[{{fprof,apply_start_stop,4}, 1, 1691.046, 0.103}],
582 { {foo,create_file_slow,2}, 1, 1691.046, 0.103}, %
583 [{{file,close,1}, 1, 1398.873, 0.019},
584 {{foo,create_file_slow,3}, 1, 249.678, 0.029},
585 {{file,open,2}, 1, 20.778, 0.055},
586 {{lists,map,2}, 1, 16.590, 0.043},
587 {{lists,seq,2}, 1, 4.708, 0.017},
588 {{file,write,2}, 1, 0.316, 0.021}]}.
589 The printout consists of one paragraph per called function. The func‐
591 tion marked with '%' is the one the paragraph concerns - foo:cre‐
592 ate_file_slow/2. Above the marked function are the calling functions -
593 those that has called the marked, and below are those called by the
594 marked function.
595 The paragraphs are per default sorted in decreasing order of the ACC
597 column for the marked function. The calling list and called list within
598 one paragraph are also per default sorted in decreasing order of their
599 ACC column.
600 The columns are: CNT - the number of times the function has been
602 called, ACC - the time spent in the function including called func‐
603 tions, and OWN - the time spent in the function not including called
604 functions.
605 The rows for the calling functions contain statistics for the marked
607 function with the constraint that only the occasions when a call was
608 made from the row's function to the marked function are accounted for.
609 The row for the marked function simply contains the sum of all calling
611 rows.
612 The rows for the called functions contains statistics for the row's
614 function with the constraint that only the occasions when a call was
615 made from the marked to the row's function are accounted for.
616 So, we see that foo:create_file_slow/2 used very little time for its
618 own execution. It spent most of its time in file:close/1. The function
619 foo:create_file_slow/3 that writes 3/4 of the file contents is the sec‐
620 ond biggest time thief.
621 We also see that the call to file:write/2 that writes 1/4 of the file
623 contents takes very little time in itself. What takes time is to build
624 the data (lists:seq/2 and lists:map/2).
625 The function 'undefined' that has called fprof:apply_start_stop/4 is an
627 unknown function because that call was not recorded in the trace. It
628 was only recorded that the execution returned from
629 fprof:apply_start_stop/4 to some other function above in the call
630 stack, or that the process exited from there.
631 Let us continue down the printout to find:
633 {[{{foo,create_file_slow,2}, 1, 249.678, 0.029},
635 {{foo,create_file_slow,3}, 768, 0.000, 23.294}],
636 { {foo,create_file_slow,3}, 769, 249.678, 23.323}, %
637 [{{file,write,2}, 768, 220.314, 14.539},
638 {suspend, 57, 6.041, 0.000},
639 {{foo,create_file_slow,3}, 768, 0.000, 23.294}]}.
640 If you compare with the code you will see there also that foo:cre‐
642 ate_file_slow/3 was called only from foo:create_file_slow/2 and itself,
643 and called only file:write/2, note the number of calls to file:write/2.
644 But here we see that suspend was called a few times. This is a pseudo
645 function that indicates that the process was suspended while executing
646 in foo:create_file_slow/3, and since there is no receive or
647 erlang:yield/0 in the code, it must be Erlang scheduling suspensions,
648 or the trace file driver compensating for large file write operations
649 (these are regarded as a schedule out followed by a schedule in to the
650 same process).
651 Let us find the suspend entry:
653 {[{{file,write,2}, 53, 6.281, 0.000},
655 {{foo,create_file_slow,3}, 57, 6.041, 0.000},
656 {{prim_file,drv_command,4}, 50, 4.582, 0.000},
657 {{prim_file,drv_get_response,1}, 34, 2.986, 0.000},
658 {{lists,map,2}, 10, 2.104, 0.000},
659 {{prim_file,write,2}, 17, 1.852, 0.000},
660 {{erlang,port_command,2}, 15, 1.713, 0.000},
661 {{prim_file,drv_command,2}, 22, 1.482, 0.000},
662 {{prim_file,translate_response,2}, 11, 1.441, 0.000},
663 {{prim_file,'-drv_command/2-fun-0-',1}, 15, 1.340, 0.000},
664 {{lists,seq,4}, 3, 0.880, 0.000},
665 {{foo,'-create_file_slow/2-fun-0-',1}, 5, 0.523, 0.000},
666 {{erlang,bump_reductions,1}, 4, 0.503, 0.000},
667 {{prim_file,open_int_setopts,3}, 1, 0.165, 0.000},
668 {{prim_file,i32,4}, 1, 0.109, 0.000},
669 {{fprof,apply_start_stop,4}, 1, 0.000, 0.000}],
670 { suspend, 299, 32.002, 0.000}, %
671 [ ]}.
672 We find no particulary long suspend times, so no function seems to have
674 waited in a receive statement. Actually, prim_file:drv_command/4 con‐
675 tains a receive statement, but in this test program, the message lies
676 in the process receive buffer when the receive statement is entered. We
677 also see that the total suspend time for the test run is small.
678 The suspend pseudo function has got an OWN time of zero. This is to
680 prevent the process total OWN time from including time in suspension.
681 Whether suspend time is really ACC or OWN time is more of a philosophi‐
682 cal question.
683 Now we look at another interesting pseudo function, garbage_collect:
685 {[{{prim_file,drv_command,4}, 25, 0.873, 0.873},
687 {{prim_file,write,2}, 16, 0.692, 0.692},
688 {{lists,map,2}, 2, 0.195, 0.195}],
689 { garbage_collect, 43, 1.760, 1.760}, %
690 [ ]}.
691 Here we see that no function distinguishes itself considerably, which
693 is very normal.
694 The garbage_collect pseudo function has not got an OWN time of zero
696 like suspend, instead it is equal to the ACC time.
697 Garbage collect often occurs while a process is suspended, but fprof
699 hides this fact by pretending that the suspended function was first
700 unsuspended and then garbage collected. Otherwise the printout would
701 show garbage_collect being called from suspend but not which function
702 that might have caused the garbage collection.
703 Let us now get back to the test code:
705 {[{{foo,create_file_slow,3}, 768, 220.314, 14.539},
707 {{foo,create_file_slow,2}, 1, 0.316, 0.021}],
708 { {file,write,2}, 769, 220.630, 14.560}, %
709 [{{prim_file,write,2}, 769, 199.789, 22.573},
710 {suspend, 53, 6.281, 0.000}]}.
711 Not unexpectedly, we see that file:write/2 was called from foo:cre‐
713 ate_file_slow/3 and foo:create_file_slow/2. The number of calls in each
714 case as well as the used time are also just confirms the previous
715 results.
716 We see that file:write/2 only calls prim_file:write/2, but let us
718 refrain from digging into the internals of the kernel application.
719 But, if we nevertheless do dig down we find the call to the linked in
721 driver that does the file operations towards the host operating system:
722 {[{{prim_file,drv_command,4}, 772, 1458.356, 1456.643}],
724 { {erlang,port_command,2}, 772, 1458.356, 1456.643}, %
725 [{suspend, 15, 1.713, 0.000}]}.
726 This is 86 % of the total run time, and as we saw before it is the
728 close operation the absolutely biggest contributor. We find a compari‐
729 son ratio a little bit up in the call stack:
730 {[{{prim_file,close,1}, 1, 1398.748, 0.024},
732 {{prim_file,write,2}, 769, 174.672, 12.810},
733 {{prim_file,open_int,4}, 1, 19.755, 0.017},
734 {{prim_file,open_int_setopts,3}, 1, 0.147, 0.016}],
735 { {prim_file,drv_command,2}, 772, 1593.322, 12.867}, %
736 [{{prim_file,drv_command,4}, 772, 1578.973, 27.265},
737 {suspend, 22, 1.482, 0.000}]}.
738 The time for file operations in the linked in driver distributes itself
740 as 1 % for open, 11 % for write and 87 % for close. All data is proba‐
741 bly buffered in the operating system until the close.
742 The unsleeping reader may notice that the ACC times for
744 prim_file:drv_command/2 and prim_file:drv_command/4 is not equal
745 between the paragraphs above, even though it is easy to believe that
746 prim_file:drv_command/2 is just a passthrough function.
747 The missing time can be found in the paragraph for prim_file:drv_com‐
749 mand/4 where it is evident that not only prim_file:drv_command/2 is
750 called but also a fun:
751 {[{{prim_file,drv_command,2}, 772, 1578.973, 27.265}],
753 { {prim_file,drv_command,4}, 772, 1578.973, 27.265}, %
754 [{{erlang,port_command,2}, 772, 1458.356, 1456.643},
755 {{prim_file,'-drv_command/2-fun-0-',1}, 772, 87.897, 12.736},
756 {suspend, 50, 4.582, 0.000},
757 {garbage_collect, 25, 0.873, 0.873}]}.
758 And some more missing time can be explained by the fact that
760 prim_file:open_int/4 both calls prim_file:drv_command/2 directly as
761 well as through prim_file:open_int_setopts/3, which complicates the
762 picture.
763 {[{{prim_file,open,2}, 1, 20.309, 0.029},
765 {{prim_file,open_int,4}, 1, 0.000, 0.057}],
766 { {prim_file,open_int,4}, 2, 20.309, 0.086}, %
767 [{{prim_file,drv_command,2}, 1, 19.755, 0.017},
768 {{prim_file,open_int_setopts,3}, 1, 0.360, 0.032},
769 {{prim_file,drv_open,2}, 1, 0.071, 0.030},
770 {{erlang,list_to_binary,1}, 1, 0.020, 0.020},
771 {{prim_file,i32,1}, 1, 0.017, 0.017},
772 {{prim_file,open_int,4}, 1, 0.000, 0.057}]}.
773 {[{{prim_file,open_int,4}, 1, 0.360, 0.032},
774 {{prim_file,open_int_setopts,3}, 1, 0.000, 0.016}],
775 { {prim_file,open_int_setopts,3}, 2, 0.360, 0.048}, %
776 [{suspend, 1, 0.165, 0.000},
777 {{prim_file,drv_command,2}, 1, 0.147, 0.016},
778 {{prim_file,open_int_setopts,3}, 1, 0.000, 0.016}]}.
779
781 The actual supervision of execution times is in itself a CPU intensive
782 activity. A message is written on the trace file for every function
783 call that is made by the profiled code.
784 The ACC time calculation is sometimes difficult to make correct, since
786 it is difficult to define. This happens especially when a function
787 occurs in several instances in the call stack, for example by calling
788 itself perhaps through other functions and perhaps even non-tail recur‐
789 sively.
790 To produce sensible results, fprof tries not to charge any function
792 more than once for ACC time. The instance highest up (with longest
793 duration) in the call stack is chosen.
794 Sometimes a function may unexpectedly waste a lot (some 10 ms or more
796 depending on host machine OS) of OWN (and ACC) time, even functions
797 that does practically nothing at all. The problem may be that the OS
798 has chosen to schedule out the Erlang runtime system process for a
799 while, and if the OS does not support high resolution cpu time measure‐
800 ments fprof will use wallclock time for its calculations, and it will
801 appear as functions randomly burn virtual machine time.
802
804 dbg(3), eprof(3), erlang(3), io(3), Tools User's Guide
805Ericsson AB tools 3.4.4 fprof(3)