Application
Development
Michael K. Johnson
Erik W. Troan
|
|
Linux
Application Development |
Michael K. Johnson Erik W. Troan |
gprof Command Summary
gprof Output
children Times Uses an Assumption
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.
Copyright (C) 1988, 1992 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.
The result of the analysis is a file containing two tables, the flat profile and the call graph (plus blurbs which briefly explain the contents of these tables).
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 How to Understand 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 How to Read the Call Graph.
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 must specify the profiling startup file
`/lib/gcrt0.o' as the first input file instead of the usual startup
file `/lib/crt0.o'. In addition, you would probably want to
specify the profiling C library, `/usr/lib/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.
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.
You 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. You may get a confusing error
message if this happens. (We have not yet replaced the part of Unix
responsible for this; when we do, we will make the error message
comprehensible.)
gprof Command Summary
After 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 following options may be used to selectively include or exclude functions in the output:
-a
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.
-D
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).
-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.
-k from... to...
-v
gprof to print the current version
number, and then exit.
-z
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.
The order of these options does not matter.
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.
There are a few other useful gprof options:
-b
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
-d num
-s
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'.
See section Statistical Inaccuracy of gprof Output.
Eventually you can run gprof again without `-s' to analyze the
cumulative data in the file `gmon.sum'.
-T
gprof to print its output in
"traditional" BSD style.
--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.
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 decreasing run-time spent in them. 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.
The 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, the `self seconds' field for
`mcount' might well be `0' or `0.04' in another run.
See section Statistical Inaccuracy of gprof Output, for a complete discussion.
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 How to Understand 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.
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.
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.
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.
A special startup routine allocates memory for the histogram and sets up a clock signal handler to make entries in it. Use of this special startup routine is one of the effects of using `gcc ... -pg' to link. The startup file also includes an `exit' function which is responsible for writing the file `gmon.out'.
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.
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 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.
gprof Output
The 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 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 is usually more than one sampling period. In
fact, if a value is n times the sampling period, the expected
error in it is the square-root of n sampling periods. If the
sampling period is 0.01 seconds and foo's run-time is 1 second, the
expected error in foo's run-time is 0.1 seconds. 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 Times Uses an Assumption
Some 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
GNU 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 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 prints blurbs after the tables, so that you can see the
tables without skipping the blurbs.
This document was generated on 20 November 1997 using the texi2html translator version 1.51.