System information

SystemTap—Filtering and Analyzing System Data 71

SystemTap—Filtering and

Analyzing System Data

SystemTap provides a command line interface and a scripting language to examine

the activities of a running Linux system, particularly the kernel, in fine detail. Sys-

temTap scripts are written in the SystemTap scripting language, are then compiled

to C-code kernel modules and inserted into the kernel. The scripts can be designed

to extract, filter and summarize data, thus allowing the diagnosis of complex perfor-

mance problems or functional problems. SystemTap provides information similar to

the output of tools like netstat, ps, top, and iostat. However, more filtering

and analysis options can be used for the collected information.

5.1 Conceptual Overview

Each time you run a SystemTap script, a SystemTap session is started. A number of

passes are done on the script before it is allowed to run, at which point the script is

compiled into a kernel module and loaded. In case the script has already been exe-

cuted before and no changes regarding any components have occurred (for exam-

ple, regarding compiler version, kernel version, library path, script contents), Sys-

temTap does not compile the script again, but uses the *.c and *.ko data stored

in the SystemTap cache (~/.systemtap). The module is unloaded when the tap

has finished running. For an example, see the test run in Section5.2, “Installation and

Setup” (page74) and the respective explanation.