9.1 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 ‘-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.