gprof: Implementation
9.1 Implementation of Profiling
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Profiling works by changing how every function in your program is
compiled so that when it is called, it will stash away some information
about where it was called from. From this, the profiler can figure out
what function called it, and can count how many times it was called.
This change is made by the compiler when your program is compiled with
the '-pg' option, which causes every function to call 'mcount' (or
'_mcount', or '__mcount', depending on the OS and compiler) as one of
its first operations.
The 'mcount' routine, included in the profiling library, is
responsible for recording in an in-memory call graph table both its
parent routine (the child) and its parent's parent. This is typically
done by examining the stack frame to find both the address of the child,
and the return address in the original parent. Since this is a very
machine-dependent operation, 'mcount' itself is typically a short
assembly-language stub routine that extracts the required information,
and then calls '__mcount_internal' (a normal C function) with two
arguments--'frompc' and 'selfpc'. '__mcount_internal' is responsible
for maintaining the in-memory call graph, which records 'frompc',
'selfpc', and the number of times each of these call arcs was traversed.
GCC Version 2 provides a magical function
('__builtin_return_address'), which allows a generic 'mcount' function
to extract the required information from the stack frame. However, on
some architectures, most notably the SPARC, using this builtin can be
very computationally expensive, and an assembly language version of
'mcount' is used for performance reasons.
Number-of-calls information for library routines is collected by
using a special version of the C library. The programs in it are the
same as in the usual C library, but they were compiled with '-pg'. If
you link your program with 'gcc ... -pg', it automatically uses the
profiling version of the library.
Profiling also involves watching your program as it runs, and keeping
a histogram of where the program counter happens to be every now and
then. Typically the program counter is looked at around 100 times per
second of run time, but the exact frequency may vary from system to
system.
This is done is one of two ways. Most UNIX-like operating systems
provide a 'profil()' system call, which registers a memory array with
the kernel, along with a scale factor that determines how the program's
address space maps into the array. Typical scaling values cause every 2
to 8 bytes of address space to map into a single array slot. On every
tick of the system clock (assuming the profiled program is running), the
value of the program counter is examined and the corresponding slot in
the memory array is incremented. Since this is done in the kernel,
which had to interrupt the process anyway to handle the clock interrupt,
very little additional system overhead is required.
However, some operating systems, most notably Linux 2.0 (and
earlier), do not provide a 'profil()' system call. On such a system,
arrangements are made for the kernel to periodically deliver a signal to
the process (typically via 'setitimer()'), which then performs the same
operation of examining the program counter and incrementing a slot in
the memory array. Since this method requires a signal to be delivered
to user space every time a sample is taken, it uses considerably more
overhead than kernel-based profiling. Also, due to the added delay
required to deliver the signal, this method is less accurate as well.
A special startup routine allocates memory for the histogram and
either calls 'profil()' or sets up a clock signal handler. This routine
('monstartup') can be invoked in several ways. On Linux systems, a
special profiling startup file 'gcrt0.o', which invokes 'monstartup'
before 'main', is used instead of the default 'crt0.o'. Use of this
special startup file is one of the effects of using 'gcc ... -pg' to
link. On SPARC systems, no special startup files are used. Rather, the
'mcount' routine, when it is invoked for the first time (typically when
'main' is called), calls 'monstartup'.
If the compiler's '-a' option was used, basic-block counting is also
enabled. Each object file is then compiled with a static array of
counts, initially zero. In the executable code, every time a new
basic-block begins (i.e., when an 'if' statement appears), an extra
instruction is inserted to increment the corresponding count in the
array. At compile time, a paired array was constructed that recorded
the starting address of each basic-block. Taken together, the two
arrays record the starting address of every basic-block, along with the
number of times it was executed.
The profiling library also includes a function ('mcleanup') which is
typically registered using 'atexit()' to be called as the program exits,
and is responsible for writing the file 'gmon.out'. Profiling is turned
off, various headers are output, and the histogram is written, followed
by the call-graph arcs and the basic-block counts.
The output from 'gprof' gives no indication of parts of your program
that are limited by I/O or swapping bandwidth. This is because samples
of the program counter are taken at fixed intervals of the program's run
time. Therefore, the time measurements in 'gprof' output say nothing
about time that your program was not running. For example, a part of
the program that creates so much data that it cannot all fit in physical
memory at once may run very slowly due to thrashing, but 'gprof' will
say it uses little time. On the other hand, sampling by run time has
the advantage that the amount of load due to other users won't directly
affect the output you get.
Info Catalog
gprof: Details
gprof: File Format