Further Reading
There is not an abundance of information on profiling tools in PHP. The individual profilers mentioned in this chapter all have some information on their respective Web sites but there is no comprehensive discussion on the art of profiling.
In addition to PHP-level profilers, there are a plethora of lower-level profilers you can use to profile a system. These tools are extremely useful if you are trying to improve the performance of the PHP language itself, but they're not terribly useful for improving an application's performance. The problem is that it is almost impossible to directly connect lower-level (that is, engine-internal) C function calls or kernel system calls to actions you take in PHP code. Here are some excellent C profiling tools:
gprof is the GNU profiler and is available on almost any system. It profiles C code well, but it can be difficult to interpret. valgrind, combined with its companion GUI kcachegrind, is an incredible memory debugger and profiler for Linux. If you write C code on Linux, you should learn to use valgrind. ooprofile is a kernel-level profiler for Linux. If you are doing low-level debugging where you need to profile an application's system calls, ooprofile is a good tool for the job.
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