gprof Command Summary
gprof's Output
gprof Output
gprof
This manual describes the GNU profiler, gprof, and how you can
use it to determine which parts of a program are taking most of the execution
time. We assume that you know how to write, compile, and execute programs. GNU
gprof was written by Jay Fenlason.
This manual was edited January 1993 by Jeffrey Osier and updated September 1997 by Brent Baccala.
Copyright (C) 1988, 1992, 1997, 1998 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.
Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this manual into another language, under the same conditions as for modified versions.
Profiling allows you to learn where your program spent its time and which functions called which other functions while it was executing. This information can show you which pieces of your program are slower than you expected, and might be candidates for rewriting to make your program execute faster. It can also tell you which functions are being called more or less often than you expected. This may help you spot bugs that had otherwise been unnoticed.
Since the profiler uses information collected during the actual execution of your program, it can be used on programs that are too large or too complex to analyze by reading the source. However, how your program is run will affect the information that shows up in the profile data. If you don't use some feature of your program while it is being profiled, no profile information will be generated for that feature.
Profiling has several steps:
gprof to analyze the profile data. See section
gprof
Command Summary. The next three chapters explain these steps in greater detail.
Several forms of output are available from the analysis.
The flat profile shows how much time your program spent in each function, and how many times that function was called. If you simply want to know which functions burn most of the cycles, it is stated concisely here. See section The Flat Profile.
The call graph shows, for each function, which functions called it, which other functions it called, and how many times. There is also an estimate of how much time was spent in the subroutines of each function. This can suggest places where you might try to eliminate function calls that use a lot of time. See section The Call Graph.
The annotated source listing is a copy of the program's source code, labeled with the number of times each line of the program was executed. See section The Annotated Source Listing.
To better understand how profiling works, you may wish to read a description of its implementation. See section Implementation of Profiling.
The first step in generating profile information for your program is to compile and link it with profiling enabled.
To compile a source file for profiling, specify the `-pg' option when you run the compiler. (This is in addition to the options you normally use.)
To link the program for profiling, if you use a compiler such as
cc to do the linking, simply specify `-pg' in addition
to your usual options. The same option, `-pg', alters either
compilation or linking to do what is necessary for profiling. Here are examples:
cc -g -c myprog.c utils.c -pg cc -o myprog myprog.o utils.o -pg
The `-pg' option also works with a command that both compiles and links:
cc -o myprog myprog.c utils.c -g -pg
If you run the linker ld directly instead of through a compiler
such as cc, you may have to specify a profiling startup file
`gcrt0.o' as the first input file instead of the usual startup file
`crt0.o'. In addition, you would probably want to specify the profiling
C library, `libc_p.a', by writing `-lc_p' instead of the
usual `-lc'. This is not absolutely necessary, but doing this gives
you number-of-calls information for standard library functions such as
read and open. For example:
ld -o myprog /lib/gcrt0.o myprog.o utils.o -lc_p
If you compile only some of the modules of the program with
`-pg', you can still profile the program, but you won't get
complete information about the modules that were compiled without
`-pg'. The only information you get for the functions in those
modules is the total time spent in them; there is no record of how many times
they were called, or from where. This will not affect the flat profile (except
that the calls field for the functions will be blank), but will
greatly reduce the usefulness of the call graph.
If you wish to perform line-by-line profiling, you will also need to specify the `-g' option, instructing the compiler to insert debugging symbols into the program that match program addresses to source code lines. See section Line-by-line Profiling.
In addition to the `-pg' and `-g' options, you may
also wish to specify the `-a' option when compiling. This will
instrument the program to perform basic-block counting. As the program runs, it
will count how many times it executed each branch of each `if'
statement, each iteration of each `do' loop, etc. This will enable
gprof to construct an annotated source code listing showing how
many times each line of code was executed.
Once the program is compiled for profiling, you must run it in order to
generate the information that gprof needs. Simply run the program
as usual, using the normal arguments, file names, etc. The program should run
normally, producing the same output as usual. It will, however, run somewhat
slower than normal because of the time spent collecting and the writing the
profile data.
The way you run the program--the arguments and input that you give it--may have a dramatic effect on what the profile information shows. The profile data will describe the parts of the program that were activated for the particular input you use. For example, if the first command you give to your program is to quit, the profile data will show the time used in initialization and in cleanup, but not much else.
Your program will write the profile data into a file called `gmon.out' just before exiting. If there is already a file called `gmon.out', its contents are overwritten. There is currently no way to tell the program to write the profile data under a different name, but you can rename the file afterward if you are concerned that it may be overwritten.
In order to write the `gmon.out' file properly, your program must
exit normally: by returning from main or by calling
exit. Calling the low-level function _exit does not
write the profile data, and neither does abnormal termination due to an
unhandled signal.
The `gmon.out' file is written in the program's current working
directory at the time it exits. This means that if your program calls
chdir, the `gmon.out' file will be left in the last
directory your program chdir'd to. If you don't have permission to
write in this directory, the file is not written, and you will get an error
message.
Older versions of the GNU profiling library may also write a file called
`bb.out'. This file, if present, contains an human-readable listing of
the basic-block execution counts. Unfortunately, the appearance of a
human-readable `bb.out' means the basic-block counts didn't get written
into `gmon.out'. The Perl script bbconv.pl, included with
the gprof source distribution, will convert a `bb.out'
file into a format readable by gprof.
gprof Command SummaryAfter you have a profile data file `gmon.out', you can run
gprof to interpret the information in it. The gprof
program prints a flat profile and a call graph on standard output. Typically you
would redirect the output of gprof into a file with
`>'.
You run gprof like this:
gprof options [executable-file [profile-data-files...]] [> outfile]
Here square-brackets indicate optional arguments.
If you omit the executable file name, the file `a.out' is used. If you give no profile data file name, the file `gmon.out' is used. If any file is not in the proper format, or if the profile data file does not appear to belong to the executable file, an error message is printed.
You can give more than one profile data file by entering all their names after the executable file name; then the statistics in all the data files are summed together.
The order of these options does not matter.
These options specify which of several output formats gprof
should produce.
Many of these options take an optional symspec to specify functions to be included or excluded. These options can be specified multiple times, with different symspecs, to include or exclude sets of symbols. See section Symspecs.
Specifying any of these options overrides the default (`-p -q'), which prints a flat profile and call graph analysis for all functions.
-A[symspec]
--annotated-source[=symspec]
gprof to print annotated
source code. If symspec is specified, print output only for
matching symbols. See section The
Annotated Source Listing.
-b
--brief
gprof doesn't print
the verbose blurbs that try to explain the meaning of all of the fields in the
tables. This is useful if you intend to print out the output, or are tired of
seeing the blurbs.
-C[symspec]
--exec-counts[=symspec]
gprof to print a tally of
functions and the number of times each was called. If symspec is
specified, print tally only for matching symbols. If the profile data file
contains basic-block count records, specifing the `-l' option,
along with `-C', will cause basic-block execution counts to be
tallied and displayed.
-i
--file-info
gprof to display summary
information about the profile data file(s) and then exit. The number of
histogram, call graph, and basic-block count records is displayed.
-I dirs
--directory-path=dirs
-J[symspec]
--no-annotated-source[=symspec]
gprof not to print
annotated source code. If symspec is specified, gprof
prints annotated source, but excludes matching symbols.
-L
--print-path
gprof to print the full
pathname of source filenames, which is determined from symbolic debugging
information in the image file and is relative to the directory in which the
compiler was invoked.
-p[symspec]
--flat-profile[=symspec]
gprof to print a flat
profile. If symspec is specified, print flat profile only for
matching symbols. See section The
Flat Profile.
-P[symspec]
--no-flat-profile[=symspec]
gprof to suppress
printing a flat profile. If symspec is specified,
gprof prints a flat profile, but excludes matching symbols.
-q[symspec]
--graph[=symspec]
gprof to print the call
graph analysis. If symspec is specified, print call graph only for
matching symbols and their children. See section The
Call Graph.
-Q[symspec]
--no-graph[=symspec]
gprof to suppress
printing the call graph. If symspec is specified,
gprof prints a call graph, but excludes matching symbols.
-y
--separate-files
-Z[symspec]
--no-exec-counts[=symspec]
gprof not to print a
tally of functions and the number of times each was called. If
symspec is specified, print tally, but exclude matching symbols.
--function-ordering
gprof to
print a suggested function ordering for the program based on profiling data.
This option suggests an ordering which may improve paging, tlb and cache
behavior for the program on systems which support arbitrary ordering of
functions in an executable. The exact details of how to force the linker to
place functions in a particular order is system dependent and out of the scope
of this manual.
--file-ordering map_file
gprof to
print a suggested .o link line ordering for the program based on profiling
data. This option suggests an ordering which may improve paging, tlb and cache
behavior for the program on systems which do not support arbitrary ordering of
functions in an executable. Use of the `-a' argument is highly
recommended with this option. The map_file argument is a pathname
to a file which provides function name to object file mappings. The format of
the file is similar to the output of the program nm. c-parse.o:00000000 T yyparse c-parse.o:00000004 C yyerrflag c-lang.o:00000000 T maybe_objc_method_name c-lang.o:00000000 T print_lang_statistics c-lang.o:00000000 T recognize_objc_keyword c-decl.o:00000000 T print_lang_identifier c-decl.o:00000000 T print_lang_type ...GNU
nm `--extern-only'
`--defined-only' `-v'
`--print-file-name' can be used to create map_file.
-T
--traditional
gprof to print its output
in "traditional" BSD style.
-w width
--width=width
-x
--all-lines
tcov's
`-a'.
--demangle
--no-demangle
--no-demangle option may be used to turn off demangling. -a
--no-static
gprof to suppress the
printing of statically declared (private) functions. (These are functions
whose names are not listed as global, and which are not visible outside the
file/function/block where they were defined.) Time spent in these functions,
calls to/from them, etc, will all be attributed to the function that was
loaded directly before it in the executable file. This option affects both the
flat profile and the call graph.
-c
--static-call-graph
-D
--ignore-non-functions
gprof to ignore symbols
which are not known to be functions. This option will give more accurate
profile data on systems where it is supported (Solaris and HPUX for example).
-k from/to
-l
--line
gprof, and magnifies statistical inaccuracies. See section Statistical
Sampling Error.
-m num
--min-count=num
-n[symspec]
--time[=symspec]
gprof, in its call graph
analysis, to only propagate times for symbols matching symspec.
-N[symspec]
--no-time[=symspec]
gprof, in its call graph
analysis, not to propagate times for symbols matching symspec.
-z
--display-unused-functions
gprof will mention
all functions in the flat profile, even those that were never called, and that
had no time spent in them. This is useful in conjunction with the
`-c' option for discovering which routines were never called.
-d[num]
--debug[=num]
gprof.
-Oname
--file-format=name
-s
--sum
gprof to summarize the
information in the profile data files it read in, and write out a profile data
file called `gmon.sum', which contains all the information from the
profile data files that gprof read in. The file
`gmon.sum' may be one of the specified input files; the effect of
this is to merge the data in the other input files into `gmon.sum'.
Eventually you can run gprof again without `-s' to
analyze the cumulative data in the file `gmon.sum'.
-v
--version
gprof to print the current
version number, and then exit. -e function_name
gprof
to not print information about the function function_name (and its
children...) in the call graph. The function will still be listed as a child
of any functions that call it, but its index number will be shown as
`[not printed]'. More than one `-e' option may be
given; only one function_name may be indicated with each
`-e' option.
-E function_name
-E function option works like the
-e option, but time spent in the function (and children who were
not called from anywhere else), will not be used to compute the
percentages-of-time for the call graph. More than one `-E' option
may be given; only one function_name may be indicated with each
`-E' option.
-f function_name
gprof
to limit the call graph to the function function_name and its
children (and their children...). More than one `-f' option may
be given; only one function_name may be indicated with each
`-f' option.
-F function_name
-f option, but only time spent in the function and its children
(and their children...) will be used to determine total-time and
percentages-of-time for the call graph. More than one `-F' option
may be given; only one function_name may be indicated with each
`-F' option. The `-F' option overrides the
`-E' option. Note that only one function can be specified with each -e,
-E, -f or -F option. To specify more than
one function, use multiple options. For example, this command:
gprof -e boring -f foo -f bar myprogram > gprof.output
lists in the call graph all functions that were reached from either
foo or bar and were not reachable from
boring.
Many of the output options allow functions to be included or excluded using symspecs (symbol specifications), which observe the following syntax:
filename_containing_a_dot | funcname_not_containing_a_dot | linenumber | ( [ any_filename ] `:' ( any_funcname | linenumber ) )
Here are some sample symspecs:
main.c
main
main.c:main
main.c:134
gprof's Outputgprof can produce several different output styles, the most
important of which are described below. The simplest output styles (file
information, execution count, and function and file ordering) are not described
here, but are documented with the respective options that trigger them. See
section Output
Options.
The flat profile shows the total amount of time your program spent executing each function. Unless the `-z' option is given, functions with no apparent time spent in them, and no apparent calls to them, are not mentioned. Note that if a function was not compiled for profiling, and didn't run long enough to show up on the program counter histogram, it will be indistinguishable from a function that was never called.
This is part of a flat profile for a small program:
Flat profile: Each sample counts as 0.01 seconds. % cumulative self self total time seconds seconds calls ms/call ms/call name 33.34 0.02 0.02 7208 0.00 0.00 open 16.67 0.03 0.01 244 0.04 0.12 offtime 16.67 0.04 0.01 8 1.25 1.25 memccpy 16.67 0.05 0.01 7 1.43 1.43 write 16.67 0.06 0.01 mcount 0.00 0.06 0.00 236 0.00 0.00 tzset 0.00 0.06 0.00 192 0.00 0.00 tolower 0.00 0.06 0.00 47 0.00 0.00 strlen 0.00 0.06 0.00 45 0.00 0.00 strchr 0.00 0.06 0.00 1 0.00 50.00 main 0.00 0.06 0.00 1 0.00 0.00 memcpy 0.00 0.06 0.00 1 0.00 10.11 print 0.00 0.06 0.00 1 0.00 0.00 profil 0.00 0.06 0.00 1 0.00 50.00 report ...
The functions are sorted by first by decreasing run-time spent in them, then by decreasing number of calls, then alphabetically by name. The functions `mcount' and `profil' are part of the profiling aparatus and appear in every flat profile; their time gives a measure of the amount of overhead due to profiling.
Just before the column headers, a statement appears indicating how much time each sample counted as. This sampling period estimates the margin of error in each of the time figures. A time figure that is not much larger than this is not reliable. In this example, each sample counted as 0.01 seconds, suggesting a 100 Hz sampling rate. The program's total execution time was 0.06 seconds, as indicated by the `cumulative seconds' field. Since each sample counted for 0.01 seconds, this means only six samples were taken during the run. Two of the samples occured while the program was in the `open' function, as indicated by the `self seconds' field. Each of the other four samples occured one each in `offtime', `memccpy', `write', and `mcount'. Since only six samples were taken, none of these values can be regarded as particularly reliable. In another run, the `self seconds' field for `mcount' might well be `0.00' or `0.02'. See section Statistical Sampling Error, for a complete discussion.
The remaining functions in the listing (those whose `self seconds' field is `0.00') didn't appear in the histogram samples at all. However, the call graph indicated that they were called, so therefore they are listed, sorted in decreasing order by the `calls' field. Clearly some time was spent executing these functions, but the paucity of histogram samples prevents any determination of how much time each took.
Here is what the fields in each line mean:
% time
cumulative seconds
self seconds
calls
self ms/call
total ms/call
name
The call graph shows how much time was spent in each function and its children. From this information, you can find functions that, while they themselves may not have used much time, called other functions that did use unusual amounts of time.
Here is a sample call from a small program. This call came from the same
gprof run as the flat profile example in the previous chapter.
granularity: each sample hit covers 2 byte(s) for 20.00% of 0.05 seconds
index % time self children called name
<spontaneous>
[1] 100.0 0.00 0.05 start [1]
0.00 0.05 1/1 main [2]
0.00 0.00 1/2 on_exit [28]
0.00 0.00 1/1 exit [59]
-----------------------------------------------
0.00 0.05 1/1 start [1]
[2] 100.0 0.00 0.05 1 main [2]
0.00 0.05 1/1 report [3]
-----------------------------------------------
0.00 0.05 1/1 main [2]
[3] 100.0 0.00 0.05 1 report [3]
0.00 0.03 8/8 timelocal [6]
0.00 0.01 1/1 print [9]
0.00 0.01 9/9 fgets [12]
0.00 0.00 12/34 strncmp <cycle 1> [40]
0.00 0.00 8/8 lookup [20]
0.00 0.00 1/1 fopen [21]
0.00 0.00 8/8 chewtime [24]
0.00 0.00 8/16 skipspace [44]
-----------------------------------------------
[4] 59.8 0.01 0.02 8+472 <cycle 2 as a whole> [4]
0.01 0.02 244+260 offtime <cycle 2> [7]
0.00 0.00 236+1 tzset <cycle 2> [26]
-----------------------------------------------
The lines full of dashes divide this table into entries, one for each function. Each entry has one or more lines.
In each entry, the primary line is the one that starts with an index number in square brackets. The end of this line says which function the entry is for. The preceding lines in the entry describe the callers of this function and the following lines describe its subroutines (also called children when we speak of the call graph).
The entries are sorted by time spent in the function and its subroutines.
The internal profiling function mcount (see section The
Flat Profile) is never mentioned in the call graph.
The primary line in a call graph entry is the line that describes the function which the entry is about and gives the overall statistics for this function.
For reference, we repeat the primary line from the entry for function
report in our main example, together with the heading line that
shows the names of the fields:
index % time self children called name ... [3] 100.0 0.00 0.05 1 report [3]
Here is what the fields in the primary line mean:
index
% time
self
seconds field for this
function in the flat profile.
children
self
and children entries of the children listed directly below this
function.
called
report
was called once from main.
name
gnurr is part of cycle number one, and has index number twelve,
its primary line would be end like this: gnurr <cycle 1> [12]
A function's entry has a line for each function it was called by. These lines' fields correspond to the fields of the primary line, but their meanings are different because of the difference in context.
For reference, we repeat two lines from the entry for the function
report, the primary line and one caller-line preceding it, together
with the heading line that shows the names of the fields:
index % time self children called name
...
0.00 0.05 1/1 main [2]
[3] 100.0 0.00 0.05 1 report [3]
Here are the meanings of the fields in the caller-line for
report called from main:
self
report itself when
it was called from main.
children
report when report was called from
main. The sum of the self and children
fields is an estimate of the amount of time spent within calls to
report from main.
called
report was called from
main, followed by the total number of nonrecursive calls to
report from all its callers.
name and index number
report to which this line applies,
followed by the caller's index number. Not all functions have entries in the
call graph; some options to gprof request the omission of certain
functions. When a caller has no entry of its own, it still has caller-lines in
the entries of the functions it calls. If the caller is part of a recursion
cycle, the cycle number is printed between the name and the index number.
If the identity of the callers of a function cannot be determined, a dummy caller-line is printed which has `<spontaneous>' as the "caller's name" and all other fields blank. This can happen for signal handlers.
A function's entry has a line for each of its subroutines--in other words, a line for each other function that it called. These lines' fields correspond to the fields of the primary line, but their meanings are different because of the difference in context.
For reference, we repeat two lines from the entry for the function
main, the primary line and a line for a subroutine, together with
the heading line that shows the names of the fields:
index % time self children called name
...
[2] 100.0 0.00 0.05 1 main [2]
0.00 0.05 1/1 report [3]
Here are the meanings of the fields in the subroutine-line for
main calling report:
self
report when report was called from
main.
children
report when report was called from
main. The sum of the self and children
fields is an estimate of the total time spent in calls to report
from main.
called
report from
main followed by the total number of nonrecursive calls to
report. This ratio is used to determine how much of
report's self and children time gets
credited to main. See section Estimating
children Times.
name
main to which this line
applies, followed by the subroutine's index number. If the caller is part of a
recursion cycle, the cycle number is printed between the name and the index
number. The graph may be complicated by the presence of cycles of recursion
in the call graph. A cycle exists if a function calls another function that
(directly or indirectly) calls (or appears to call) the original function. For
example: if a calls b, and b calls
a, then a and b form a cycle.
Whenever there are call paths both ways between a pair of functions, they
belong to the same cycle. If a and b call each other
and b and c call each other, all three make one cycle.
Note that even if b only calls a if it was not called
from a, gprof cannot determine this, so a
and b are still considered a cycle.
The cycles are numbered with consecutive integers. When a function belongs to a cycle, each time the function name appears in the call graph it is followed by `<cycle number>'.
The reason cycles matter is that they make the time values in the call graph
paradoxical. The "time spent in children" of a should include the
time spent in its subroutine b and in b's
subroutines--but one of b's subroutines is a! How much
of a's time should be included in the children of a,
when a is indirectly recursive?
The way gprof resolves this paradox is by creating a single
entry for the cycle as a whole. The primary line of this entry describes the
total time spent directly in the functions of the cycle. The "subroutines" of
the cycle are the individual functions of the cycle, and all other functions
that were called directly by them. The "callers" of the cycle are the functions,
outside the cycle, that called functions in the cycle.
Here is an example portion of a call graph which shows a cycle containing
functions a and b. The cycle was entered by a call to
a from main; both a and b
called c.
index % time self children called name
----------------------------------------
1.77 0 1/1 main [2]
[3] 91.71 1.77 0 1+5 <cycle 1 as a whole> [3]
1.02 0 3 b <cycle 1> [4]
0.75 0 2 a <cycle 1> [5]
----------------------------------------
3 a <cycle 1> [5]
[4] 52.85 1.02 0 0 b <cycle 1> [4]
2 a <cycle 1> [5]
0 0 3/6 c [6]
----------------------------------------
1.77 0 1/1 main [2]
2 b <cycle 1> [4]
[5] 38.86 0.75 0 1 a <cycle 1> [5]
3 b <cycle 1> [4]
0 0 3/6 c [6]
----------------------------------------
(The entire call graph for this program contains in addition an entry for
main, which calls a, and an entry for c,
with callers a and b.)
index % time self children called name
<spontaneous>
[1] 100.00 0 1.93 0 start [1]
0.16 1.77 1/1 main [2]
----------------------------------------
0.16 1.77 1/1 start [1]
[2] 100.00 0.16 1.77 1 main [2]
1.77 0 1/1 a <cycle 1> [5]
----------------------------------------
1.77 0 1/1 main [2]
[3] 91.71 1.77 0 1+5 <cycle 1 as a whole> [3]
1.02 0 3 b <cycle 1> [4]
0.75 0 2 a <cycle 1> [5]
0 0 6/6 c [6]
----------------------------------------
3 a <cycle 1> [5]
[4] 52.85 1.02 0 0 b <cycle 1> [4]
2 a <cycle 1> [5]
0 0 3/6 c [6]
----------------------------------------
1.77 0 1/1 main [2]
2 b <cycle 1> [4]
[5] 38.86 0.75 0 1 a <cycle 1> [5]
3 b <cycle 1> [4]
0 0 3/6 c [6]
----------------------------------------
0 0 3/6 b <cycle 1> [4]
0 0 3/6 a <cycle 1> [5]
[6] 0.00 0 0 6 c [6]
----------------------------------------
The self field of the cycle's primary line is the total time
spent in all the functions of the cycle. It equals the sum of the
self fields for the individual functions in the cycle, found in the
entry in the subroutine lines for these functions.
The children fields of the cycle's primary line and subroutine
lines count only subroutines outside the cycle. Even though a calls
b, the time spent in those calls to b is not counted
in a's children time. Thus, we do not encounter the
problem of what to do when the time in those calls to b includes
indirect recursive calls back to a.
The children field of a caller-line in the cycle's entry
estimates the amount of time spent in the whole cycle, and its other
subroutines, on the times when that caller called a function in the cycle.
The calls field in the primary line for the cycle has two
numbers: first, the number of times functions in the cycle were called by
functions outside the cycle; second, the number of times they were called by
functions in the cycle (including times when a function in the cycle calls
itself). This is a generalization of the usual split into nonrecursive and
recursive calls.
The calls field of a subroutine-line for a cycle member in the
cycle's entry says how many time that function was called from functions in the
cycle. The total of all these is the second number in the primary line's
calls field.
In the individual entry for a function in a cycle, the other functions in the
same cycle can appear as subroutines and as callers. These lines show how many
times each function in the cycle called or was called from each other function
in the cycle. The self and children fields in these
lines are blank because of the difficulty of defining meanings for them when
recursion is going on.
gprof's `-l' option causes the program to perform
line-by-line profiling. In this mode, histogram samples are assigned
not to functions, but to individual lines of source code. The program usually
must be compiled with a `-g' option, in addition to
`-pg', in order to generate debugging symbols for tracking source
code lines.
The flat profile is the most useful output table in line-by-line mode. The
call graph isn't as useful as normal, since the current version of
gprof does not propagate call graph arcs from source code lines to
the enclosing function. The call graph does, however, show each line of code
that called each function, along with a count.
Here is a section of gprof's output, without line-by-line
profiling. Note that ct_init accounted for four histogram hits, and
13327 calls to init_block.
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls us/call us/call name
30.77 0.13 0.04 6335 6.31 6.31 ct_init
Call graph (explanation follows)
granularity: each sample hit covers 4 byte(s) for 7.69% of 0.13 seconds
index % time self children called name
0.00 0.00 1/13496 name_too_long
0.00 0.00 40/13496 deflate
0.00 0.00 128/13496 deflate_fast
0.00 0.00 13327/13496 ct_init
[7] 0.0 0.00 0.00 13496 init_block
Now let's look at some of gprof's output from the same program
run, this time with line-by-line profiling enabled. Note that
ct_init's four histogram hits are broken down into four lines of
source code - one hit occured on each of lines 349, 351, 382 and 385. In the
call graph, note how ct_init's 13327 calls to
init_block are broken down into one call from line 396, 3071 calls
from line 384, 3730 calls from line 385, and 6525 calls from 387.
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self
time seconds seconds calls name
7.69 0.10 0.01 ct_init (trees.c:349)
7.69 0.11 0.01 ct_init (trees.c:351)
7.69 0.12 0.01 ct_init (trees.c:382)
7.69 0.13 0.01 ct_init (trees.c:385)
Call graph (explanation follows)
granularity: each sample hit covers 4 byte(s) for 7.69% of 0.13 seconds
% time self children called name
0.00 0.00 1/13496 name_too_long (gzip.c:1440)
0.00 0.00 1/13496 deflate (deflate.c:763)
0.00 0.00 1/13496 ct_init (trees.c:396)
0.00 0.00 2/13496 deflate (deflate.c:727)
0.00 0.00 4/13496 deflate (deflate.c:686)
0.00 0.00 5/13496 deflate (deflate.c:675)
0.00 0.00 12/13496 deflate (deflate.c:679)
0.00 0.00 16/13496 deflate (deflate.c:730)
0.00 0.00 128/13496 deflate_fast (deflate.c:654)
0.00 0.00 3071/13496 ct_init (trees.c:384)
0.00 0.00 3730/13496 ct_init (trees.c:385)
0.00 0.00 6525/13496 ct_init (trees.c:387)
[6] 0.0 0.00 0.00 13496 init_block (trees.c:408)
gprof's `-A' option triggers an annotated source
listing, which lists the program's source code, each function labeled with the
number of times it was called. You may also need to specify the
`-I' option, if gprof can't find the source code
files.
Compiling with `gcc ... -g -pg -a' augments your program with
basic-block counting code, in addition to function counting code. This enables
gprof to determine how many times each line of code was exeucted.
For example, consider the following function, taken from gzip, with line numbers
added:
1 ulg updcrc(s, n)
2 uch *s;
3 unsigned n;
4 {
5 register ulg c;
6
7 static ulg crc = (ulg)0xffffffffL;
8
9 if (s == NULL) {
10 c = 0xffffffffL;
11 } else {
12 c = crc;
13 if (n) do {
14 c = crc_32_tab[...];
15 } while (--n);
16 }
17 crc = c;
18 return c ^ 0xffffffffL;
19 }
updcrc has at least five basic-blocks. One is the function
itself. The if statement on line 9 generates two more basic-blocks,
one for each branch of the if. A fourth basic-block results from
the if on line 13, and the contents of the do loop
form the fifth basic-block. The compiler may also generate additional
basic-blocks to handle various special cases.
A program augmented for basic-block counting can be analyzed with gprof
-l -A. I also suggest use of the `-x' option, which ensures
that each line of code is labeled at least once. Here is updcrc's
annotated source listing for a sample gzip run:
ulg updcrc(s, n)
uch *s;
unsigned n;
2 ->{
register ulg c;
static ulg crc = (ulg)0xffffffffL;
2 -> if (s == NULL) {
1 -> c = 0xffffffffL;
1 -> } else {
1 -> c = crc;
1 -> if (n) do {
26312 -> c = crc_32_tab[...];
26312,1,26311 -> } while (--n);
}
2 -> crc = c;
2 -> return c ^ 0xffffffffL;
2 ->}
In this example, the function was called twice, passing once through each
branch of the if statement. The body of the do loop
was executed a total of 26312 times. Note how the while statement
is annotated. It began execution 26312 times, once for each iteration through
the loop. One of those times (the last time) it exited, while it branched back
to the beginning of the loop 26311 times.
gprof OutputThe run-time figures that gprof gives you are based on a
sampling process, so they are subject to statistical inaccuracy. If a function
runs only a small amount of time, so that on the average the sampling process
ought to catch that function in the act only once, there is a pretty good chance
it will actually find that function zero times, or twice.
By contrast, the number-of-calls and basic-block figures are derived by counting, not sampling. They are completely accurate and will not vary from run to run if your program is deterministic.
The sampling period that is printed at the beginning of the flat profile says how often samples are taken. The rule of thumb is that a run-time figure is accurate if it is considerably bigger than the sampling period.
The actual amount of error can be predicted. For n samples, the
expected error is the square-root of n. For example, if the
sampling period is 0.01 seconds and foo's run-time is 1 second,
n is 100 samples (1 second/0.01 seconds), sqrt(n) is 10
samples, so the expected error in foo's run-time is 0.1 seconds
(10*0.01 seconds), or ten percent of the observed value. Again, if the sampling
period is 0.01 seconds and bar's run-time is 100 seconds,
n is 10000 samples, sqrt(n) is 100 samples, so the
expected error in bar's run-time is 1 second, or one percent of the
observed value. It is likely to vary this much on the average from one
profiling run to the next. (Sometimes it will vary more.)
This does not mean that a small run-time figure is devoid of information. If the program's total run-time is large, a small run-time for one function does tell you that that function used an insignificant fraction of the whole program's time. Usually this means it is not worth optimizing.
One way to get more accuracy is to give your program more (but similar) input
data so it will take longer. Another way is to combine the data from several
runs, using the `-s' option of gprof. Here is how:
gprof -s executable-file gmon.out gmon.sum
gprof executable-file gmon.sum > output-file
children TimesSome of the figures in the call graph are estimates--for example, the
children time values and all the the time figures in caller and
subroutine lines.
There is no direct information about these measurements in the profile data
itself. Instead, gprof estimates them by making an assumption about
your program that might or might not be true.
The assumption made is that the average time spent in each call to any
function foo is not correlated with who called foo. If
foo used 5 seconds in all, and 2/5 of the calls to foo
came from a, then foo contributes 2 seconds to
a's children time, by assumption.
This assumption is usually true enough, but for some programs it is far from
true. Suppose that foo returns very quickly when its argument is
zero; suppose that a always passes zero as an argument, while other
callers of foo pass other arguments. In this program, all the time
spent in foo is in the calls from callers other than
a. But gprof has no way of knowing this; it will
blindly and incorrectly charge 2 seconds of time in foo to the
children of a.
We hope some day to put more complete data into `gmon.out', so that this assumption is no longer needed, if we can figure out how. For the nonce, the estimated figures are usually more useful than misleading.
gprof -l -C objfile | sort -k 3 -n -rThis listing will show you the lines in your code executed most often, but not necessarily those that consumed the most time.
gprof -l and lookup the function in the call graph. The
callers will be broken down by function and line number.
for i in `seq 1 100`; do fastprog mv gmon.out gmon.out.$i done gprof -s fastprog gmon.out.* gprof fastprog gmon.sumIf your program is completely deterministic, all the call counts will be simple multiples of 100 (i.e. a function called once in each run will appear with a call count of 100).
gprofGNU gprof and Berkeley Unix gprof use the same data
file `gmon.out', and provide essentially the same information. But
there are a few differences.
gprof uses a new, generalized file format with support
for basic-block execution counts and non-realtime histograms. A magic cookie
and version number allows gprof to easily identify new style
files. Old BSD-style files can still be read. See section Profiling
Data File Format.
gprof lists the function as a
parent and as a child, with a calls field that lists the number
of recursive calls. GNU gprof omits these lines and puts the
number of recursive calls in the primary line.
gprof still lists it as a subroutine of functions that call
it.
gprof accepts the `-k' with its argument in
the form `from/to', instead of `from to'.
gprof prints all of their counts, seperated by
commas.
gprof prints blurbs after the tables, so that you can see the
tables without skipping the blurbs. Profiling works by changing how every function in your program is compiled so
that when it is called, it will stash away some information about where it was
called from. From this, the profiler can figure out what function called it, and
can count how many times it was called. This change is made by the compiler when
your program is compiled with the `-pg' option, which causes every
function to call mcount (or _mcount, or
__mcount, depending on the OS and compiler) as one of its first
operations.
The mcount routine, included in the profiling library, is
responsible for recording in an in-memory call graph table both its parent
routine (the child) and its parent's parent. This is typically done by examining
the stack frame to find both the address of the child, and the return address in
the original parent. Since this is a very machine-dependant operation,
mcount itself is typically a short assembly-language stub routine
that extracts the required information, and then calls
__mcount_internal (a normal C function) with two arguments -
frompc and selfpc. __mcount_internal is
responsible for maintaining the in-memory call graph, which records
frompc, selfpc, and the number of times each of these
call arcs was transversed.
GCC Version 2 provides a magical function
(__builtin_return_address), which allows a generic
mcount function to extract the required information from the stack
frame. However, on some architectures, most notably the SPARC, using this
builtin can be very computationally expensive, and an assembly language version
of mcount is used for performance reasons.
Number-of-calls information for library routines is collected by using a special version of the C library. The programs in it are the same as in the usual C library, but they were compiled with `-pg'. If you link your program with `gcc ... -pg', it automatically uses the profiling version of the library.
Profiling also involves watching your program as it runs, and keeping a histogram of where the program counter happens to be every now and then. Typically the program counter is looked at around 100 times per second of run time, but the exact frequency may vary from system to system.
This is done is one of two ways. Most UNIX-like operating systems provide a
profil() system call, which registers a memory array with the
kernel, along with a scale factor that determines how the program's address
space maps into the array. Typical scaling values cause every 2 to 8 bytes of
address space to map into a single array slot. On every tick of the system clock
(assuming the profiled program is running), the value of the program counter is
examined and the corresponding slot in the memory array is incremented. Since
this is done in the kernel, which had to interrupt the process anyway to handle
the clock interrupt, very little additional system overhead is required.
However, some operating systems, most notably Linux 2.0 (and earlier), do not
provide a profil() system call. On such a system, arrangements are
made for the kernel to periodically deliver a signal to the process (typically
via setitimer()), which then performs the same operation of
examining the program counter and incrementing a slot in the memory array. Since
this method requires a signal to be delivered to user space every time a sample
is taken, it uses considerably more overhead than kernel-based profiling. Also,
due to the added delay required to deliver the signal, this method is less
accurate as well.
A special startup routine allocates memory for the histogram and either calls
profil() or sets up a clock signal handler. This routine
(monstartup) can be invoked in several ways. On Linux systems, a
special profiling startup file gcrt0.o, which invokes
monstartup before main, is used instead of the default
crt0.o. Use of this special startup file is one of the effects of
using `gcc ... -pg' to link. On SPARC systems, no special startup
files are used. Rather, the mcount routine, when it is invoked for
the first time (typically when main is called), calls
monstartup.
If the compiler's `-a' option was used, basic-block counting is
also enabled. Each object file is then compiled with a static array of counts,
initially zero. In the executable code, every time a new basic-block begins
(i.e. when an if statement appears), an extra instruction is
inserted to increment the corresponding count in the array. At compile time, a
paired array was constructed that recorded the starting address of each
basic-block. Taken together, the two arrays record the starting address of every
basic-block, along with the number of times it was executed.
The profiling library also includes a function (mcleanup) which
is typically registered using atexit() to be called as the program
exits, and is responsible for writing the file `gmon.out'. Profiling is
turned off, various headers are output, and the histogram is written, followed
by the call-graph arcs and the basic-block counts.
The output from gprof gives no indication of parts of your
program that are limited by I/O or swapping bandwidth. This is because samples
of the program counter are taken at fixed intervals of the program's run time.
Therefore, the time measurements in gprof output say nothing about
time that your program was not running. For example, a part of the program that
creates so much data that it cannot all fit in physical memory at once may run
very slowly due to thrashing, but gprof will say it uses little
time. On the other hand, sampling by run time has the advantage that the amount
of load due to other users won't directly affect the output you get.
The old BSD-derived file format used for profile data does not contain a
magic cookie that allows to check whether a data file really is a gprof file.
Furthermore, it does not provide a version number, thus rendering changes to the
file format almost impossible. GNU gprof uses a new file format
that provides these features. For backward compatibility, GNU gprof
continues to support the old BSD-derived format, but not all features are
supported with it. For example, basic-block execution counts cannot be
accommodated by the old file format.
The new file format is defined in header file `gmon_out.h'. It
consists of a header containing the magic cookie and a version number, as well
as some spare bytes available for future extensions. All data in a profile data
file is in the native format of the host on which the profile was collected. GNU
gprof adapts automatically to the byte-order in use.
In the new file format, the header is followed by a sequence of records.
Currently, there are three different record types: histogram records, call-graph
arc records, and basic-block execution count records. Each file can contain any
number of each record type. When reading a file, GNU gprof will
ensure records of the same type are compatible with each other and compute the
union of all records. For example, for basic-block execution counts, the union
is simply the sum of all execution counts for each basic-block.
Histogram records consist of a header that is followed by an array of bins. The header contains the text-segment range that the histogram spans, the size of the histogram in bytes (unlike in the old BSD format, this does not include the size of the header), the rate of the profiling clock, and the physical dimension that the bin counts represent after being scaled by the profiling clock rate. The physical dimension is specified in two parts: a long name of up to 15 characters and a single character abbreviation. For example, a histogram representing real-time would specify the long name as "seconds" and the abbreviation as "s". This feature is useful for architectures that support performance monitor hardware (which, fortunately, is becoming increasingly common). For example, under DEC OSF/1, the "uprofile" command can be used to produce a histogram of, say, instruction cache misses. In this case, the dimension in the histogram header could be set to "i-cache misses" and the abbreviation could be set to "1" (because it is simply a count, not a physical dimension). Also, the profiling rate would have to be set to 1 in this case.
Histogram bins are 16-bit numbers and each bin represent an equal amount of text-space. For example, if the text-segment is one thousand bytes long and if there are ten bins in the histogram, each bin represents one hundred bytes.
Call-graph records have a format that is identical to the one used in the BSD-derived file format. It consists of an arc in the call graph and a count indicating the number of times the arc was traversed during program execution. Arcs are specified by a pair of addresses: the first must be within caller's function and the second must be within the callee's function. When performing profiling at the function level, these addresses can point anywhere within the respective function. However, when profiling at the line-level, it is better if the addresses are as close to the call-site/entry-point as possible. This will ensure that the line-level call-graph is able to identify exactly which line of source code performed calls to a function.
Basic-block execution count records consist of a header followed by a sequence of address/count pairs. The header simply specifies the length of the sequence. In an address/count pair, the address identifies a basic-block and the count specifies the number of times that basic-block was executed. Any address within the basic-address can be used.
gprof's Internal OperationLike most programs, gprof begins by processing its options.
During this stage, it may building its symspec list
(sym_ids.c:sym_id_add), if options are specified which use
symspecs. gprof maintains a single linked list of symspecs, which
will eventually get turned into 12 symbol tables, organized into six
include/exclude pairs - one pair each for the flat profile
(INCL_FLAT/EXCL_FLAT), the call graph arcs (INCL_ARCS/EXCL_ARCS), printing in
the call graph (INCL_GRAPH/EXCL_GRAPH), timing propagation in the call graph
(INCL_TIME/EXCL_TIME), the annotated source listing (INCL_ANNO/EXCL_ANNO), and
the execution count listing (INCL_EXEC/EXCL_EXEC).
After option processing, gprof finishes building the symspec
list by adding all the symspecs in default_excluded_list to the
exclude lists EXCL_TIME and EXCL_GRAPH, and if line-by-line profiling is
specified, EXCL_FLAT as well. These default excludes are not added to EXCL_ANNO,
EXCL_ARCS, and EXCL_EXEC.
Next, the BFD library is called to open the object file, verify that it is an
object file, and read its symbol table (core.c:core_init), using
bfd_canonicalize_symtab after mallocing an appropiate sized array
of asymbols. At this point, function mappings are read (if the
`--file-ordering' option has been specified), and the core text
space is read into memory (if the `-c' option was given).
gprof's own symbol table, an array of Sym structures, is now
built. This is done in one of two ways, by one of two routines, depending on
whether line-by-line profiling (`-l' option) has been enabled. For
normal profiling, the BFD canonical symbol table is scanned. For line-by-line
profiling, every text space address is examined, and a new symbol table entry
gets created every time the line number changes. In either case, two passes are
made through the symbol table - one to count the size of the symbol table
required, and the other to actually read the symbols. In between the two passes,
a single array of type Sym is created of the appropiate length.
Finally, symtab.c:symtab_finalize is called to sort the symbol
table and remove duplicate entries (entries with the same memory address).
The symbol table must be a contiguous array for two reasons. First, the
qsort library function (which sorts an array) will be used to sort
the symbol table. Also, the symbol lookup routine
(symtab.c:sym_lookup), which finds symbols based on memory address,
uses a binary search algorithm which requires the symbol table to be a sorted
array. Function symbols are indicated with an is_func flag. Line
number symbols have no special flags set. Additionally, a symbol can have an
is_static flag to indicate that it is a local symbol.
With the symbol table read, the symspecs can now be translated into Syms
(sym_ids.c:sym_id_parse). Remember that a single symspec can match
multiple symbols. An array of symbol tables (syms) is created, each
entry of which is a symbol table of Syms to be included or excluded from a
particular listing. The master symbol table and the symspecs are examined by
nested loops, and every symbol that matches a symspec is inserted into the
appropriate syms table. This is done twice, once to count the size of each
required symbol table, and again to build the tables, which have been malloced
between passes. From now on, to determine whether a symbol is on an include or
exclude symspec list, gprof simply uses its standard symbol lookup
routine on the appropriate table in the syms array.
Now the profile data file(s) themselves are read
(gmon_io.c:gmon_out_read), first by checking for a new-style
`gmon.out' header, then assuming this is an old-style BSD
`gmon.out' if the magic number test failed.
New-style histogram records are read by hist.c:hist_read_rec.
For the first histogram record, allocate a memory array to hold all the bins,
and read them in. When multiple profile data files (or files with multiple
histogram records) are read, the starting address, ending address, number of
bins and sampling rate must match between the various histograms, or a fatal
error will result. If everything matches, just sum the additional histograms
into the existing in-memory array.
As each call graph record is read (call_graph.c:cg_read_rec),
the parent and child addresses are matched to symbol table entries, and a call
graph arc is created by cg_arcs.c:arc_add, unless the arc fails a
symspec check against INCL_ARCS/EXCL_ARCS. As each arc is added, a linked list
is maintained of the parent's child arcs, and of the child's parent arcs. Both
the child's call count and the arc's call count are incremented by the record's
call count.
Basic-block records are read (basic_blocks.c:bb_read_rec), but
only if line-by-line profiling has been selected. Each basic-block address is
matched to a corresponding line symbol in the symbol table, and an entry made in
the symbol's bb_addr and bb_calls arrays. Again, if multiple basic-block records
are present for the same address, the call counts are cumulative.
A gmon.sum file is dumped, if requested
(gmon_io.c:gmon_out_write).
If histograms were present in the data files, assign them to symbols
(hist.c:hist_assign_samples) by iterating over all the sample bins
and assigning them to symbols. Since the symbol table is sorted in order of
ascending memory addresses, we can simple follow along in the symbol table as we
make our pass over the sample bins. This step includes a symspec check against
INCL_FLAT/EXCL_FLAT. Depending on the histogram scale factor, a sample bin may
span multiple symbols, in which case a fraction of the sample count is allocated
to each symbol, proportional to the degree of overlap. This effect is rare for
normal profiling, but overlaps are more common during line-by-line profiling,
and can cause each of two adjacent lines to be credited with half a hit, for
example.
If call graph data is present, cg_arcs.c:cg_assemble is called.
First, if `-c' was specified, a machine-dependant routine
(find_call) scans through each symbol's machine code, looking for
subroutine call instructions, and adding them to the call graph with a zero call
count. A topological sort is performed by depth-first numbering all the symbols
(cg_dfn.c:cg_dfn), so that children are always numbered less than
their parents, then making a array of pointers into the symbol table and sorting
it into numerical order, which is reverse topological order (children appear
before parents). Cycles are also detected at this point, all members of which
are assigned the same topological number. Two passes are now made through this
sorted array of symbol pointers. The first pass, from end to beginning (parents
to children), computes the fraction of child time to propogate to each parent
and a print flag. The print flag reflects symspec handling of
INCL_GRAPH/EXCL_GRAPH, with a parent's include or exclude (print or no print)
property being propagated to its children, unless they themselves explicitly
appear in INCL_GRAPH or EXCL_GRAPH. A second pass, from beginning to end
(children to parents) actually propogates the timings along the call graph,
subject to a check against INCL_TIME/EXCL_TIME. With the print flag, fractions,
and timings now stored in the symbol structures, the topological sort array is
now discarded, and a new array of pointers is assembled, this time sorted by
propagated time.
Finally, print the various outputs the user requested, which is now fairly
straightforward. The call graph (cg_print.c:cg_print) and flat
profile (hist.c:hist_print) are regurgitations of values already
computed. The annotated source listing
(basic_blocks.c:print_annotated_source) uses basic-block
information, if present, to label each line of code with call counts, otherwise
only the function call counts are presented.
The function ordering code is marginally well documented in the source code
itself (cg_print.c). Basically, the functions with the most use and
the most parents are placed first, followed by other functions with the most
use, followed by lower use functions, followed by unused functions at the end.
gprofIf gprof was compiled with debugging enabled, the
`-d' option triggers debugging output (to stdout) which can be
helpful in understanding its operation. The debugging number specified is
interpreted as a sum of the following options:
This document was generated on 7 November 1998 using the texi2html translator version 1.52.