Dynamically TracingKDE

Manish ChakravartyThoughtWorks Studios

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Agenda

• Learn about DTrace

• Learn what and where to Trace

• Trace!

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I. What is DTrace?

While I go through the slides, please start copying the VM!

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DTrace is..

• a comprehensive Dynamic Tracing framework created by Sun Microsystems for troubleshooting kernel and application problems in real time.

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Excuse me?

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• Dtrace enables us to “look into” the internals of a running production application and allows us to instrument it

• It can do very neat stuff in the kernel as well, but that’s not the point of this session

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So how does DTrace work?

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• Dtrace allows you to dynamically modify the operating system kernel and user processes to record additional data that you specify at locations of interest, called probes

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• Dtrace probes come from a set of kernel modules called providers, each of which performs a particular type of instrumentation to create probes

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Provider:Module:Function:Name

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ProviderThe name of the DTrace provider that is publishing this

probe

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ModuleIf this probe corresponds to a specific program

location, the name of the module which the probe is located

The name is either the name of a kernel module or a user library

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FunctionIf this probe corresponds to a specific program

location, the name of the program function in which the probe is located

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NameThe final component of the probe name is a name that gives you some idea of the probe’s semantic meaning

such as BEGIN or END

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Thus:Provider:Module:Function:Name

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DTrace Architecture16

Let’s start Tracing!

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manish@belenix-KDE4:~/WorkOut/DTraceTutorial$ pfexec dtrace -n BEGIN

dtrace: description 'BEGIN' matched 1 probe

CPU ID FUNCTION:NAME

0 1 :BEGIN

^C

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manish@belenix-KDE4:~/WorkOut/DTraceTutorial$ pfexec dtrace -n BEGIN -n END

dtrace: description 'BEGIN' matched 1 probe

dtrace: description 'END' matched 1 probe

CPU ID FUNCTION:NAME

0 1 :BEGIN

^C

0 2 :END 19

• The probe BEGIN fires every time you start a new tracing request

• The probe END fires every time you exit DTrace. In this case hitting Ctrl-C triggered it.

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Hello, World!

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BEGIN

{

trace("hello, world");

exit(0);

}

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manish@belenix-KDE4:~/WorkOut/DTraceTutorial/1-HelloWorld$ pfexec dtrace -s hello.d

dtrace: script 'hello.d' matched 1 probe

CPU ID FUNCTION:NAME

0 1 :BEGIN hello, world

Output..

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61,809DTrace probes on the system!

pfexec dtrace -l | wc -l

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DTrace ArchitectureSource: http://wikis.sun.com/download/attachments/10390716/architecture.gif

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II. What / Where / How

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• Navigate to the directory having the Qt source code

• Find out a good place in QObject to put one

• Start writing probes!

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Hack!

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