fprof - A Time Profiling Tool using trace to file for minimal runtime performance impact.
Please see following description for synopsis
fprof(3) Erlang Module Definition fprof(3)
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 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 ran-
domly 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 schedul-
ings 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 nor-
mal.
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 sim-
ulated, 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 Rea-
son is kill, the server is bluntly killed.
If the fprof server is not running, this function returns imme-
diately 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 :: module(), Function :: atom()}
Args = [term()]
Same as apply(Func, Args, []).
apply(Module, Function, Args) -> term()
Types:
Module = module()
Function = atom()
Args = [term()]
Same as apply({Module, Function}, Args, []).
apply(Func, Args, OptionList) -> term()
Types:
Func = function() | {Module :: module(), Function :: atom()}
Args = [term()]
OptionList = [Option]
Option = apply_option()
apply_option() =
continue |
{procs, PidList :: [pid()]} |
start |
(TraceStartOption :: trace_option())
trace_option() =
cpu_time |
{cpu_time, boolean()} |
file |
{file, Filename :: file:filename()} |
{procs, PidSpec :: pid_spec()} |
{procs, [PidSpec :: pid_spec()]} |
start | stop |
{tracer, Tracer :: pid() | port()} |
verbose |
{verbose, boolean()}
pid_spec() = pid() | atom()
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 = module()
Function = atom()
Args = [term()]
OptionList = [Option]
Option = apply_option()
apply_option() =
continue |
{procs, PidList :: [pid()]} |
start |
(TraceStartOption :: trace_option())
trace_option() =
cpu_time |
{cpu_time, boolean()} |
file |
{file, Filename :: file:filename()} |
{procs, PidSpec :: pid_spec()} |
{procs, [PidSpec :: pid_spec()]} |
start | stop |
{tracer, Tracer :: pid() | port()} |
verbose |
{verbose, boolean()}
pid_spec() = pid() | atom()
Same as apply({Module, Function}, Args, OptionList).
OptionList is an option list allowed for apply/3.
trace(OptionName :: start, Filename) ->
ok | {error, Reason} | {'EXIT', ServerPid, Reason}
Types:
Filename = file:filename()
ServerPid = pid()
Reason = term()
Same as trace([start, {file, Filename}]).
trace(OptionName :: verbose, Filename) ->
ok | {error, Reason} | {'EXIT', ServerPid, Reason}
Types:
Filename = file:filename()
ServerPid = pid()
Reason = term()
Same as trace([start, verbose, {file, Filename}]).
trace(OptionName, OptionValue) ->
ok | {error, Reason} | {'EXIT', ServerPid, Reason}
Types:
OptionName = atom()
OptionValue = term()
ServerPid = pid()
Reason = term()
Same as trace([{OptionName, OptionValue}]).
trace(Option :: verbose) ->
ok | {error, Reason} | {'EXIT', ServerPid, Reason}
Types:
ServerPid = pid()
Reason = term()
Same as trace([start, verbose]).
trace(OptionName) ->
ok | {error, Reason} | {'EXIT', ServerPid, Reason}
Types:
OptionName = atom()
ServerPid = pid()
Reason = term()
Same as trace([OptionName]).
trace(Option :: {OptionName, OptionValue}) ->
ok | {error, Reason} | {'EXIT', ServerPid, Reason}
Types:
OptionName = atom()
OptionValue = term()
ServerPid = pid()
Reason = term()
Same as trace([{OptionName, OptionValue}]).
trace(OptionList) ->
ok | {error, Reason} | {'EXIT', ServerPid, Reason}
Types:
OptionList = [Option]
Option = trace_option()
ServerPid = pid()
Reason = term()
trace_option() =
cpu_time |
{cpu_time, boolean()} |
file |
{file, Filename :: file:filename()} |
{procs, PidSpec :: pid_spec()} |
{procs, [PidSpec :: pid_spec()]} |
start | stop |
{tracer, Tracer :: pid() | port()} |
verbose |
{verbose, boolean()}
pid_spec() = pid() | atom()
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 erlang:trace/3 and
dbg:trace_port/2.
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, boolean()}:
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, boolean()}:
The options cpu_time or {cpu_time, true} makes the time-
stamps in the trace be in CPU time instead of wallclock time
which is the default. This option is only allowed with the
start option.
Warning:
Getting correct values out of cpu_time can be difficult. The
best way to get correct values is to run using a single sched-
uler and bind that scheduler to a specific CPU, i.e. erl +S 1
+sbt db.
{procs, PidSpec} | {procs, [PidSpec]}:
Specifies which processes that shall be traced. If this
option is not given, the calling process is traced. All pro-
cesses 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:
ServerPid = pid()
Reason = term()
Same as profile([]).
profile(OptionName, OptionValue) ->
ok |
{ok, Tracer} |
{error, Reason} |
{'EXIT', ServerPid, Reason}
Types:
OptionName = atom()
OptionValue = term()
Tracer = ServerPid = pid()
Reason = term()
Same as profile([{OptionName, OptionValue}]).
profile(OptionName) ->
ok |
{ok, Tracer} |
{error, Reason} |
{'EXIT', ServerPid, Reason}
Types:
OptionName = atom()
Tracer = ServerPid = pid()
Reason = term()
Same as profile([OptionName]).
profile(Option :: {OptionName, OptionValue}) ->
ok |
{ok, Tracer} |
{error, Reason} |
{'EXIT', ServerPid, Reason}
Types:
OptionName = atom()
OptionValue = term()
Tracer = ServerPid = pid()
Reason = term()
Same as profile([{OptionName, OptionValue}]).
profile(OptionList) ->
ok |
{ok, Tracer} |
{error, Reason} |
{'EXIT', ServerPid, Reason}
Types:
OptionList = [Option]
Option = profile_option()
Tracer = ServerPid = pid()
Reason = term()
profile_option() =
append | dump |
{dump, pid() | (Dump :: (Dumpfile :: file:filename() | []))} |
file |
{file, Filename :: file:filename()} |
start | stop
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 file:open/2 and
dbg:trace_port/2.
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 run-
time. The return value will be value ok if successful. This
option is not allowed with the start, file or {file, File-
name} options.
analyse() -> ok | {error, Reason} | {'EXIT', ServerPid, Reason}
Types:
ServerPid = pid()
Reason = term()
Same as analyse([]).
analyse(OptionName, OptionValue) ->
ok | {error, Reason} | {'EXIT', ServerPid, Reason}
Types:
OptionName = atom()
OptionValue = term()
ServerPid = pid()
Reason = term()
Same as analyse([{OptionName, OptionValue}]).
analyse(OptionName) ->
ok | {error, Reason} | {'EXIT', ServerPid, Reason}
Types:
OptionName = atom()
ServerPid = pid()
Reason = term()
Same as analyse([OptionName]).
analyse(Option :: {OptionName, OptionValue}) ->
ok | {error, Reason} | {'EXIT', ServerPid, Reason}
Types:
OptionName = atom()
OptionValue = term()
ServerPid = pid()
Reason = term()
Same as analyse([{OptionName, OptionValue}]).
analyse(OptionList) ->
ok | {error, Reason} | {'EXIT', ServerPid, Reason}
Types:
OptionList = [Option]
Option = analyse_option()
ServerPid = pid()
Reason = term()
analyse_option() =
append | callers |
{callers, boolean()} |
{cols, Cols :: integer() >= 0} |
dest |
{dest, Dest :: pid() | (Destfile :: file:filename())} |
details |
{details, boolean()} |
no_callers | no_details |
{sort, SortSpec :: acc | own} |
totals |
{totals, boolean()}
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.
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 recog-
nise as a slightly modified benchmark function from the manpage
file(3):
-module(foo).
-export([create_file_slow/2]).
create_file_slow(Name, N) when is_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 execu-
tion time in functions found in the trace, not including called func-
tions. 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 cur-
rent 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 func-
tion marked with '%' is the one the paragraph concerns - foo:cre-
ate_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 func-
tions, 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 sec-
ond 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:cre-
ate_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 con-
tains 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 philosophi-
cal 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 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:cre-
ate_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 compari-
son 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 proba-
bly 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_com-
mand/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 recur-
sively.
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 do 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(3), eprof(3), erlang(3), io(3), Tools User's Guide
Ericsson AB tools 3.5.2 fprof(3)