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1056 lines
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1056 lines
43 KiB
Plaintext
\input texinfo @c -*-texinfo-*-
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@setfilename gprof.info
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@settitle GNU gprof
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@setchapternewpage odd
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@ifinfo
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@c This is a dir.info fragment to support semi-automated addition of
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@c manuals to an info tree. zoo@cygnus.com is developing this facility.
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@format
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START-INFO-DIR-ENTRY
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* gprof: (gprof). Profiling your program's execution
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END-INFO-DIR-ENTRY
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@end format
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@end ifinfo
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@ifinfo
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This file documents the gprof profiler of the GNU system.
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Copyright (C) 1988, 1992 Free Software Foundation, Inc.
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Permission is granted to make and distribute verbatim copies of
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this manual provided the copyright notice and this permission notice
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are preserved on all copies.
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@ignore
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Permission is granted to process this file through Tex and print the
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results, provided the printed document carries copying permission
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notice identical to this one except for the removal of this paragraph
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(this paragraph not being relevant to the printed manual).
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@end ignore
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the entire
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resulting derived work is distributed under the terms of a permission
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notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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into another language, under the above conditions for modified versions.
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@end ifinfo
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@finalout
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@smallbook
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@titlepage
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@title GNU gprof
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@subtitle The @sc{gnu} Profiler
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@author Jay Fenlason and Richard Stallman
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@page
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This manual describes the @sc{gnu} profiler, @code{gprof}, and how you
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can use it to determine which parts of a program are taking most of the
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execution time. We assume that you know how to write, compile, and
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execute programs. @sc{gnu} @code{gprof} was written by Jay Fenlason.
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This manual was edited January 1993 by Jeffrey Osier.
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@vskip 0pt plus 1filll
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Copyright @copyright{} 1988, 1992 Free Software Foundation, Inc.
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Permission is granted to make and distribute verbatim copies of
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this manual provided the copyright notice and this permission notice
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are preserved on all copies.
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@ignore
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Permission is granted to process this file through TeX and print the
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|
results, provided the printed document carries copying permission
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notice identical to this one except for the removal of this paragraph
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(this paragraph not being relevant to the printed manual).
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@end ignore
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the entire
|
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resulting derived work is distributed under the terms of a permission
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notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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into another language, under the same conditions as for modified versions.
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@end titlepage
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@ifinfo
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@node Top
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@top Profiling a Program: Where Does It Spend Its Time?
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This manual describes the @sc{gnu} profiler, @code{gprof}, and how you
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can use it to determine which parts of a program are taking most of the
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execution time. We assume that you know how to write, compile, and
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execute programs. @sc{gnu} @code{gprof} was written by Jay Fenlason.
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This manual was updated January 1993.
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@menu
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* Why:: What profiling means, and why it is useful.
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* Compiling:: How to compile your program for profiling.
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* Executing:: How to execute your program to generate the
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profile data file @file{gmon.out}.
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* Invoking:: How to run @code{gprof}, and how to specify
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options for it.
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* Flat Profile:: The flat profile shows how much time was spent
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executing directly in each function.
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* Call Graph:: The call graph shows which functions called which
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others, and how much time each function used
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when its subroutine calls are included.
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* Implementation:: How the profile data is recorded and written.
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* Sampling Error:: Statistical margins of error.
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How to accumulate data from several runs
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to make it more accurate.
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* Assumptions:: Some of @code{gprof}'s measurements are based
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on assumptions about your program
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that could be very wrong.
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* Incompatibilities:: (between GNU @code{gprof} and Unix @code{gprof}.)
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@end menu
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@end ifinfo
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@node Why
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@chapter Why Profile
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Profiling allows you to learn where your program spent its time and which
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functions called which other functions while it was executing. This
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information can show you which pieces of your program are slower than you
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expected, and might be candidates for rewriting to make your program
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execute faster. It can also tell you which functions are being called more
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or less often than you expected. This may help you spot bugs that had
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otherwise been unnoticed.
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Since the profiler uses information collected during the actual execution
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of your program, it can be used on programs that are too large or too
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complex to analyze by reading the source. However, how your program is run
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will affect the information that shows up in the profile data. If you
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don't use some feature of your program while it is being profiled, no
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profile information will be generated for that feature.
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Profiling has several steps:
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@itemize @bullet
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@item
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You must compile and link your program with profiling enabled.
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@xref{Compiling}.
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@item
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You must execute your program to generate a profile data file.
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@xref{Executing}.
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@item
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You must run @code{gprof} to analyze the profile data.
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@xref{Invoking}.
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@end itemize
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The next three chapters explain these steps in greater detail.
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The result of the analysis is a file containing two tables, the
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@dfn{flat profile} and the @dfn{call graph} (plus blurbs which briefly
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explain the contents of these tables).
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The flat profile shows how much time your program spent in each function,
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and how many times that function was called. If you simply want to know
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which functions burn most of the cycles, it is stated concisely here.
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@xref{Flat Profile}.
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The call graph shows, for each function, which functions called it, which
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other functions it called, and how many times. There is also an estimate
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of how much time was spent in the subroutines of each function. This can
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suggest places where you might try to eliminate function calls that use a
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lot of time. @xref{Call Graph}.
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@node Compiling
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@chapter Compiling a Program for Profiling
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The first step in generating profile information for your program is
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to compile and link it with profiling enabled.
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To compile a source file for profiling, specify the @samp{-pg} option when
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you run the compiler. (This is in addition to the options you normally
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use.)
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To link the program for profiling, if you use a compiler such as @code{cc}
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to do the linking, simply specify @samp{-pg} in addition to your usual
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options. The same option, @samp{-pg}, alters either compilation or linking
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to do what is necessary for profiling. Here are examples:
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@example
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cc -g -c myprog.c utils.c -pg
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cc -o myprog myprog.o utils.o -pg
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@end example
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The @samp{-pg} option also works with a command that both compiles and links:
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@example
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cc -o myprog myprog.c utils.c -g -pg
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@end example
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If you run the linker @code{ld} directly instead of through a compiler
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such as @code{cc}, you must specify the profiling startup file
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@file{/lib/gcrt0.o} as the first input file instead of the usual startup
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file @file{/lib/crt0.o}. In addition, you would probably want to
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specify the profiling C library, @file{/usr/lib/libc_p.a}, by writing
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@samp{-lc_p} instead of the usual @samp{-lc}. This is not absolutely
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necessary, but doing this gives you number-of-calls information for
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standard library functions such as @code{read} and @code{open}. For
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example:
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@example
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ld -o myprog /lib/gcrt0.o myprog.o utils.o -lc_p
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@end example
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If you compile only some of the modules of the program with @samp{-pg}, you
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can still profile the program, but you won't get complete information about
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the modules that were compiled without @samp{-pg}. The only information
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you get for the functions in those modules is the total time spent in them;
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there is no record of how many times they were called, or from where. This
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will not affect the flat profile (except that the @code{calls} field for
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the functions will be blank), but will greatly reduce the usefulness of the
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call graph.
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@node Executing
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@chapter Executing the Program to Generate Profile Data
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Once the program is compiled for profiling, you must run it in order to
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generate the information that @code{gprof} needs. Simply run the program
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as usual, using the normal arguments, file names, etc. The program should
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run normally, producing the same output as usual. It will, however, run
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somewhat slower than normal because of the time spent collecting and the
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writing the profile data.
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The way you run the program---the arguments and input that you give
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it---may have a dramatic effect on what the profile information shows. The
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profile data will describe the parts of the program that were activated for
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the particular input you use. For example, if the first command you give
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to your program is to quit, the profile data will show the time used in
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initialization and in cleanup, but not much else.
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You program will write the profile data into a file called @file{gmon.out}
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just before exiting. If there is already a file called @file{gmon.out},
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its contents are overwritten. There is currently no way to tell the
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program to write the profile data under a different name, but you can rename
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the file afterward if you are concerned that it may be overwritten.
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In order to write the @file{gmon.out} file properly, your program must exit
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normally: by returning from @code{main} or by calling @code{exit}. Calling
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the low-level function @code{_exit} does not write the profile data, and
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neither does abnormal termination due to an unhandled signal.
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The @file{gmon.out} file is written in the program's @emph{current working
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directory} at the time it exits. This means that if your program calls
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@code{chdir}, the @file{gmon.out} file will be left in the last directory
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your program @code{chdir}'d to. If you don't have permission to write in
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this directory, the file is not written. You may get a confusing error
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message if this happens. (We have not yet replaced the part of Unix
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responsible for this; when we do, we will make the error message
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comprehensible.)
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@node Invoking
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@chapter @code{gprof} Command Summary
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After you have a profile data file @file{gmon.out}, you can run @code{gprof}
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to interpret the information in it. The @code{gprof} program prints a
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flat profile and a call graph on standard output. Typically you would
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redirect the output of @code{gprof} into a file with @samp{>}.
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You run @code{gprof} like this:
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@smallexample
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gprof @var{options} [@var{executable-file} [@var{profile-data-files}@dots{}]] [> @var{outfile}]
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@end smallexample
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@noindent
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Here square-brackets indicate optional arguments.
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If you omit the executable file name, the file @file{a.out} is used. If
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you give no profile data file name, the file @file{gmon.out} is used. If
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any file is not in the proper format, or if the profile data file does not
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appear to belong to the executable file, an error message is printed.
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You can give more than one profile data file by entering all their names
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after the executable file name; then the statistics in all the data files
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are summed together.
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The following options may be used to selectively include or exclude
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functions in the output:
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@table @code
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@item -a
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The @samp{-a} option causes @code{gprof} to suppress the printing of
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statically declared (private) functions. (These are functions whose
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names are not listed as global, and which are not visible outside the
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file/function/block where they were defined.) Time spent in these
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functions, calls to/from them, etc, will all be attributed to the
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function that was loaded directly before it in the executable file.
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@c This is compatible with Unix @code{gprof}, but a bad idea.
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This option affects both the flat profile and the call graph.
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@item -e @var{function_name}
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The @samp{-e @var{function}} option tells @code{gprof} to not print
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information about the function @var{function_name} (and its
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children@dots{}) in the call graph. The function will still be listed
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as a child of any functions that call it, but its index number will be
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shown as @samp{[not printed]}. More than one @samp{-e} option may be
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given; only one @var{function_name} may be indicated with each @samp{-e}
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option.
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@item -E @var{function_name}
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The @code{-E @var{function}} option works like the @code{-e} option, but
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time spent in the function (and children who were not called from
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anywhere else), will not be used to compute the percentages-of-time for
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the call graph. More than one @samp{-E} option may be given; only one
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@var{function_name} may be indicated with each @samp{-E} option.
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@item -f @var{function_name}
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The @samp{-f @var{function}} option causes @code{gprof} to limit the
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call graph to the function @var{function_name} and its children (and
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their children@dots{}). More than one @samp{-f} option may be given;
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only one @var{function_name} may be indicated with each @samp{-f}
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option.
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@item -F @var{function_name}
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The @samp{-F @var{function}} option works like the @code{-f} option, but
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only time spent in the function and its children (and their
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children@dots{}) will be used to determine total-time and
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percentages-of-time for the call graph. More than one @samp{-F} option
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may be given; only one @var{function_name} may be indicated with each
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@samp{-F} option. The @samp{-F} option overrides the @samp{-E} option.
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@item -k @var{from@dots{}} @var{to@dots{}}
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The @samp{-k} option allows you to delete from the profile any arcs from
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routine @var{from} to routine @var{to}.
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@item -v
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The @samp{-v} flag causes @code{gprof} to print the current version
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number, and then exit.
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@item -z
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If you give the @samp{-z} option, @code{gprof} will mention all
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functions in the flat profile, even those that were never called, and
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that had no time spent in them. This is useful in conjunction with the
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@samp{-c} option for discovering which routines were never called.
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@end table
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The order of these options does not matter.
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Note that only one function can be specified with each @code{-e},
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@code{-E}, @code{-f} or @code{-F} option. To specify more than one
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function, use multiple options. For example, this command:
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@example
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gprof -e boring -f foo -f bar myprogram > gprof.output
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@end example
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@noindent
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lists in the call graph all functions that were reached from either
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@code{foo} or @code{bar} and were not reachable from @code{boring}.
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There are a few other useful @code{gprof} options:
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@table @code
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@item -b
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If the @samp{-b} option is given, @code{gprof} doesn't print the
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verbose blurbs that try to explain the meaning of all of the fields in
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the tables. This is useful if you intend to print out the output, or
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are tired of seeing the blurbs.
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@item -c
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The @samp{-c} option causes the static call-graph of the program to be
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discovered by a heuristic which examines the text space of the object
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file. Static-only parents or children are indicated with call counts of
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@samp{0}.
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@item -d @var{num}
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The @samp{-d @var{num}} option specifies debugging options.
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@c @xref{debugging}.
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@item -s
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The @samp{-s} option causes @code{gprof} to summarize the information
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in the profile data files it read in, and write out a profile data
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file called @file{gmon.sum}, which contains all the information from
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the profile data files that @code{gprof} read in. The file @file{gmon.sum}
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may be one of the specified input files; the effect of this is to
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merge the data in the other input files into @file{gmon.sum}.
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@xref{Sampling Error}.
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Eventually you can run @code{gprof} again without @samp{-s} to analyze the
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cumulative data in the file @file{gmon.sum}.
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@item -T
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The @samp{-T} option causes @code{gprof} to print its output in
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``traditional'' BSD style.
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@end table
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@node Flat Profile
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@chapter How to Understand the Flat Profile
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@cindex flat profile
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The @dfn{flat profile} shows the total amount of time your program
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spent executing each function. Unless the @samp{-z} option is given,
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functions with no apparent time spent in them, and no apparent calls
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to them, are not mentioned. Note that if a function was not compiled
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for profiling, and didn't run long enough to show up on the program
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counter histogram, it will be indistinguishable from a function that
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was never called.
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This is part of a flat profile for a small program:
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@smallexample
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@group
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Flat profile:
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Each sample counts as 0.01 seconds.
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% cumulative self self total
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time seconds seconds calls ms/call ms/call name
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33.34 0.02 0.02 7208 0.00 0.00 open
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16.67 0.03 0.01 244 0.04 0.12 offtime
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16.67 0.04 0.01 8 1.25 1.25 memccpy
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16.67 0.05 0.01 7 1.43 1.43 write
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16.67 0.06 0.01 mcount
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0.00 0.06 0.00 236 0.00 0.00 tzset
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0.00 0.06 0.00 192 0.00 0.00 tolower
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0.00 0.06 0.00 47 0.00 0.00 strlen
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0.00 0.06 0.00 45 0.00 0.00 strchr
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0.00 0.06 0.00 1 0.00 50.00 main
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0.00 0.06 0.00 1 0.00 0.00 memcpy
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0.00 0.06 0.00 1 0.00 10.11 print
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0.00 0.06 0.00 1 0.00 0.00 profil
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0.00 0.06 0.00 1 0.00 50.00 report
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@dots{}
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@end group
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@end smallexample
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@noindent
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The functions are sorted by decreasing run-time spent in them. The
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functions @samp{mcount} and @samp{profil} are part of the profiling
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|
aparatus and appear in every flat profile; their time gives a measure of
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|
the amount of overhead due to profiling.
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The sampling period estimates the margin of error in each of the time
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figures. A time figure that is not much larger than this is not
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reliable. In this example, the @samp{self seconds} field for
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@samp{mcount} might well be @samp{0} or @samp{0.04} in another run.
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@xref{Sampling Error}, for a complete discussion.
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|
Here is what the fields in each line mean:
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|
@table @code
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@item % time
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This is the percentage of the total execution time your program spent
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in this function. These should all add up to 100%.
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@item cumulative seconds
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|
This is the cumulative total number of seconds the computer spent
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|
executing this functions, plus the time spent in all the functions
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above this one in this table.
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@item self seconds
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|
This is the number of seconds accounted for by this function alone.
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|
The flat profile listing is sorted first by this number.
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@item calls
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|
This is the total number of times the function was called. If the
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function was never called, or the number of times it was called cannot
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be determined (probably because the function was not compiled with
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|
profiling enabled), the @dfn{calls} field is blank.
|
|
|
|
@item self ms/call
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|
This represents the average number of milliseconds spent in this
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|
function per call, if this function is profiled. Otherwise, this field
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|
is blank for this function.
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|
@item total ms/call
|
|
This represents the average number of milliseconds spent in this
|
|
function and its descendants per call, if this function is profiled.
|
|
Otherwise, this field is blank for this function.
|
|
|
|
@item name
|
|
This is the name of the function. The flat profile is sorted by this
|
|
field alphabetically after the @dfn{self seconds} field is sorted.
|
|
@end table
|
|
|
|
@node Call Graph
|
|
@chapter How to Read the Call Graph
|
|
@cindex call graph
|
|
|
|
The @dfn{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 @code{gprof} run as the flat profile example in the previous
|
|
chapter.
|
|
|
|
@smallexample
|
|
@group
|
|
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]
|
|
-----------------------------------------------
|
|
@end group
|
|
@end smallexample
|
|
|
|
The lines full of dashes divide this table into @dfn{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
|
|
@dfn{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 @code{mcount} (@pxref{Flat Profile})
|
|
is never mentioned in the call graph.
|
|
|
|
@menu
|
|
* Primary:: Details of the primary line's contents.
|
|
* Callers:: Details of caller-lines' contents.
|
|
* Subroutines:: Details of subroutine-lines' contents.
|
|
* Cycles:: When there are cycles of recursion,
|
|
such as @code{a} calls @code{b} calls @code{a}@dots{}
|
|
@end menu
|
|
|
|
@node Primary
|
|
@section The Primary Line
|
|
|
|
The @dfn{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
|
|
@code{report} in our main example, together with the heading line that
|
|
shows the names of the fields:
|
|
|
|
@smallexample
|
|
@group
|
|
index % time self children called name
|
|
@dots{}
|
|
[3] 100.0 0.00 0.05 1 report [3]
|
|
@end group
|
|
@end smallexample
|
|
|
|
Here is what the fields in the primary line mean:
|
|
|
|
@table @code
|
|
@item index
|
|
Entries are numbered with consecutive integers. Each function
|
|
therefore has an index number, which appears at the beginning of its
|
|
primary line.
|
|
|
|
Each cross-reference to a function, as a caller or subroutine of
|
|
another, gives its index number as well as its name. The index number
|
|
guides you if you wish to look for the entry for that function.
|
|
|
|
@item % time
|
|
This is the percentage of the total time that was spent in this
|
|
function, including time spent in subroutines called from this
|
|
function.
|
|
|
|
The time spent in this function is counted again for the callers of
|
|
this function. Therefore, adding up these percentages is meaningless.
|
|
|
|
@item self
|
|
This is the total amount of time spent in this function. This
|
|
should be identical to the number printed in the @code{seconds} field
|
|
for this function in the flat profile.
|
|
|
|
@item children
|
|
This is the total amount of time spent in the subroutine calls made by
|
|
this function. This should be equal to the sum of all the @code{self}
|
|
and @code{children} entries of the children listed directly below this
|
|
function.
|
|
|
|
@item called
|
|
This is the number of times the function was called.
|
|
|
|
If the function called itself recursively, there are two numbers,
|
|
separated by a @samp{+}. The first number counts non-recursive calls,
|
|
and the second counts recursive calls.
|
|
|
|
In the example above, the function @code{report} was called once from
|
|
@code{main}.
|
|
|
|
@item name
|
|
This is the name of the current function. The index number is
|
|
repeated after it.
|
|
|
|
If the function is part of a cycle of recursion, the cycle number is
|
|
printed between the function's name and the index number
|
|
(@pxref{Cycles}). For example, if function @code{gnurr} is part of
|
|
cycle number one, and has index number twelve, its primary line would
|
|
be end like this:
|
|
|
|
@example
|
|
gnurr <cycle 1> [12]
|
|
@end example
|
|
@end table
|
|
|
|
@node Callers, Subroutines, Primary, Call Graph
|
|
@section Lines for a Function's Callers
|
|
|
|
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
|
|
@code{report}, the primary line and one caller-line preceding it, together
|
|
with the heading line that shows the names of the fields:
|
|
|
|
@smallexample
|
|
index % time self children called name
|
|
@dots{}
|
|
0.00 0.05 1/1 main [2]
|
|
[3] 100.0 0.00 0.05 1 report [3]
|
|
@end smallexample
|
|
|
|
Here are the meanings of the fields in the caller-line for @code{report}
|
|
called from @code{main}:
|
|
|
|
@table @code
|
|
@item self
|
|
An estimate of the amount of time spent in @code{report} itself when it was
|
|
called from @code{main}.
|
|
|
|
@item children
|
|
An estimate of the amount of time spent in subroutines of @code{report}
|
|
when @code{report} was called from @code{main}.
|
|
|
|
The sum of the @code{self} and @code{children} fields is an estimate
|
|
of the amount of time spent within calls to @code{report} from @code{main}.
|
|
|
|
@item called
|
|
Two numbers: the number of times @code{report} was called from @code{main},
|
|
followed by the total number of nonrecursive calls to @code{report} from
|
|
all its callers.
|
|
|
|
@item name and index number
|
|
The name of the caller of @code{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 @code{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.
|
|
@end table
|
|
|
|
If the identity of the callers of a function cannot be determined, a
|
|
dummy caller-line is printed which has @samp{<spontaneous>} as the
|
|
``caller's name'' and all other fields blank. This can happen for
|
|
signal handlers.
|
|
@c What if some calls have determinable callers' names but not all?
|
|
@c FIXME - still relevant?
|
|
|
|
@node Subroutines, Cycles, Callers, Call Graph
|
|
@section Lines for a Function's Subroutines
|
|
|
|
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
|
|
@code{main}, the primary line and a line for a subroutine, together
|
|
with the heading line that shows the names of the fields:
|
|
|
|
@smallexample
|
|
index % time self children called name
|
|
@dots{}
|
|
[2] 100.0 0.00 0.05 1 main [2]
|
|
0.00 0.05 1/1 report [3]
|
|
@end smallexample
|
|
|
|
Here are the meanings of the fields in the subroutine-line for @code{main}
|
|
calling @code{report}:
|
|
|
|
@table @code
|
|
@item self
|
|
An estimate of the amount of time spent directly within @code{report}
|
|
when @code{report} was called from @code{main}.
|
|
|
|
@item children
|
|
An estimate of the amount of time spent in subroutines of @code{report}
|
|
when @code{report} was called from @code{main}.
|
|
|
|
The sum of the @code{self} and @code{children} fields is an estimate
|
|
of the total time spent in calls to @code{report} from @code{main}.
|
|
|
|
@item called
|
|
Two numbers, the number of calls to @code{report} from @code{main}
|
|
followed by the total number of nonrecursive calls to @code{report}.
|
|
|
|
@item name
|
|
The name of the subroutine of @code{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.
|
|
@end table
|
|
|
|
@node Cycles,, Subroutines, Call Graph
|
|
@section How Mutually Recursive Functions Are Described
|
|
@cindex cycle
|
|
@cindex recursion cycle
|
|
|
|
The graph may be complicated by the presence of @dfn{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 @code{a} calls @code{b},
|
|
and @code{b} calls @code{a}, then @code{a} and @code{b} form a cycle.
|
|
|
|
Whenever there are call-paths both ways between a pair of functions, they
|
|
belong to the same cycle. If @code{a} and @code{b} call each other and
|
|
@code{b} and @code{c} call each other, all three make one cycle. Note that
|
|
even if @code{b} only calls @code{a} if it was not called from @code{a},
|
|
@code{gprof} cannot determine this, so @code{a} and @code{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 @samp{<cycle @var{number}>}.
|
|
|
|
The reason cycles matter is that they make the time values in the call
|
|
graph paradoxical. The ``time spent in children'' of @code{a} should
|
|
include the time spent in its subroutine @code{b} and in @code{b}'s
|
|
subroutines---but one of @code{b}'s subroutines is @code{a}! How much of
|
|
@code{a}'s time should be included in the children of @code{a}, when
|
|
@code{a} is indirectly recursive?
|
|
|
|
The way @code{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 @code{a} and @code{b}. The cycle was entered by a call to
|
|
@code{a} from @code{main}; both @code{a} and @code{b} called @code{c}.
|
|
|
|
@smallexample
|
|
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]
|
|
----------------------------------------
|
|
@end smallexample
|
|
|
|
@noindent
|
|
(The entire call graph for this program contains in addition an entry for
|
|
@code{main}, which calls @code{a}, and an entry for @code{c}, with callers
|
|
@code{a} and @code{b}.)
|
|
|
|
@smallexample
|
|
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]
|
|
----------------------------------------
|
|
@end smallexample
|
|
|
|
The @code{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
|
|
@code{self} fields for the individual functions in the cycle, found
|
|
in the entry in the subroutine lines for these functions.
|
|
|
|
The @code{children} fields of the cycle's primary line and subroutine lines
|
|
count only subroutines outside the cycle. Even though @code{a} calls
|
|
@code{b}, the time spent in those calls to @code{b} is not counted in
|
|
@code{a}'s @code{children} time. Thus, we do not encounter the problem of
|
|
what to do when the time in those calls to @code{b} includes indirect
|
|
recursive calls back to @code{a}.
|
|
|
|
The @code{children} field of a caller-line in the cycle's entry estimates
|
|
the amount of time spent @emph{in the whole cycle}, and its other
|
|
subroutines, on the times when that caller called a function in the cycle.
|
|
|
|
The @code{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 @code{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
|
|
@code{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 @code{self} and @code{children} fields in these
|
|
lines are blank because of the difficulty of defining meanings for them
|
|
when recursion is going on.
|
|
|
|
@node Implementation, Sampling Error, Call Graph, Top
|
|
@chapter Implementation of Profiling
|
|
|
|
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 @samp{-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 @samp{gcc @dots{} -pg} to
|
|
link. The startup file also includes an @samp{exit} function which is
|
|
responsible for writing the file @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 @samp{-pg}. If you
|
|
link your program with @samp{gcc @dots{} -pg}, it automatically uses the
|
|
profiling version of the library.
|
|
|
|
The output from @code{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 @code{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 @code{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.
|
|
|
|
@node Sampling Error, Assumptions, Implementation, Top
|
|
@chapter Statistical Inaccuracy of @code{gprof} Output
|
|
|
|
The run-time figures that @code{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 @dfn{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 @var{n} times the sampling period, the @emph{expected}
|
|
error in it is the square-root of @var{n} sampling periods. If the
|
|
sampling period is 0.01 seconds and @code{foo}'s run-time is 1 second, the
|
|
expected error in @code{foo}'s run-time is 0.1 seconds. It is likely to
|
|
vary this much @emph{on the average} from one profiling run to the next.
|
|
(@emph{Sometimes} it will vary more.)
|
|
|
|
This does not mean that a small run-time figure is devoid of information.
|
|
If the program's @emph{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 @samp{-s} option of @code{gprof}. Here is how:
|
|
|
|
@enumerate
|
|
@item
|
|
Run your program once.
|
|
|
|
@item
|
|
Issue the command @samp{mv gmon.out gmon.sum}.
|
|
|
|
@item
|
|
Run your program again, the same as before.
|
|
|
|
@item
|
|
Merge the new data in @file{gmon.out} into @file{gmon.sum} with this command:
|
|
|
|
@example
|
|
gprof -s @var{executable-file} gmon.out gmon.sum
|
|
@end example
|
|
|
|
@item
|
|
Repeat the last two steps as often as you wish.
|
|
|
|
@item
|
|
Analyze the cumulative data using this command:
|
|
|
|
@example
|
|
gprof @var{executable-file} gmon.sum > @var{output-file}
|
|
@end example
|
|
@end enumerate
|
|
|
|
@node Assumptions, Incompatibilities, Sampling Error, Top
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|
@chapter Estimating @code{children} Times Uses an Assumption
|
|
|
|
Some of the figures in the call graph are estimates---for example, the
|
|
@code{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, @code{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 @code{foo} is not correlated with who called @code{foo}. If
|
|
@code{foo} used 5 seconds in all, and 2/5 of the calls to @code{foo} came
|
|
from @code{a}, then @code{foo} contributes 2 seconds to @code{a}'s
|
|
@code{children} time, by assumption.
|
|
|
|
This assumption is usually true enough, but for some programs it is far
|
|
from true. Suppose that @code{foo} returns very quickly when its argument
|
|
is zero; suppose that @code{a} always passes zero as an argument, while
|
|
other callers of @code{foo} pass other arguments. In this program, all the
|
|
time spent in @code{foo} is in the calls from callers other than @code{a}.
|
|
But @code{gprof} has no way of knowing this; it will blindly and
|
|
incorrectly charge 2 seconds of time in @code{foo} to the children of
|
|
@code{a}.
|
|
|
|
@c FIXME - has this been fixed?
|
|
We hope some day to put more complete data into @file{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.
|
|
|
|
@node Incompatibilities, , Assumptions, Top
|
|
@chapter Incompatibilities with Unix @code{gprof}
|
|
|
|
@sc{gnu} @code{gprof} and Berkeley Unix @code{gprof} use the same data
|
|
file @file{gmon.out}, and provide essentially the same information. But
|
|
there are a few differences.
|
|
|
|
@itemize @bullet
|
|
@item
|
|
For a recursive function, Unix @code{gprof} lists the function as a
|
|
parent and as a child, with a @code{calls} field that lists the number
|
|
of recursive calls. @sc{gnu} @code{gprof} omits these lines and puts
|
|
the number of recursive calls in the primary line.
|
|
|
|
@item
|
|
When a function is suppressed from the call graph with @samp{-e}, @sc{gnu}
|
|
@code{gprof} still lists it as a subroutine of functions that call it.
|
|
|
|
@ignore - it does this now
|
|
@item
|
|
The function names printed in @sc{gnu} @code{gprof} output do not include
|
|
the leading underscores that are added internally to the front of all
|
|
C identifiers on many operating systems.
|
|
@end ignore
|
|
|
|
@item
|
|
The blurbs, field widths, and output formats are different. @sc{gnu}
|
|
@code{gprof} prints blurbs after the tables, so that you can see the
|
|
tables without skipping the blurbs.
|
|
|
|
@contents
|
|
@bye
|
|
|
|
NEEDS AN INDEX
|
|
|
|
Still relevant?
|
|
The @file{gmon.out} file is written in the program's @emph{current working
|
|
directory} at the time it exits. This means that if your program calls
|
|
@code{chdir}, the @file{gmon.out} file will be left in the last directory
|
|
your program @code{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.)
|
|
|
|
-k from to...?
|
|
|
|
-d debugging...? should this be documented?
|
|
|
|
-T - "traditional BSD style": How is it different? Should the
|
|
differences be documented?
|
|
|
|
what is this about? (and to think, I *wrote* it...)
|
|
@item -c
|
|
The @samp{-c} option causes the static call-graph of the program to be
|
|
discovered by a heuristic which examines the text space of the object
|
|
file. Static-only parents or children are indicated with call counts of
|
|
@samp{0}.
|
|
|
|
example flat file adds up to 100.01%...
|
|
|
|
note: time estimates now only go out to one decimal place (0.0), where
|
|
they used to extend two (78.67).
|