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- <title>LTTng User-space Tracing Design</title>
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-<h1>LTTng User-space Tracing Design</h1>
-
-<p>
-Earlier versions of LTTng contained simple, system call based, tracepoints and fast,
-user-space buffered, tracepoints for user-space tracing. During the kernel inclusion phase
-of LTTng, extensive rework and modularization of the kernel tracing portion was undertaken.
-This phase is well under way and several portions have been included already in the mainline kernel.
-The rework of the kernel tracing infrastructure will shortly thereafter be ported to fast user-space
-tracing.
-This fast user-space tracing scheme uses a direct function call to write events into buffers
-mapped in user-space. This should be an order of magnitude faster than the
-current Dtrace implementation (c.f.
-<a href="http://ltt.polymtl.ca/tracingwiki/index.php/DTrace">Dtrace information
-on the TracingWiki</a>) which uses a breakpoint to
-perform both dynamic and static user-space tracing. Performance comparison of
-a function call vs the int3 approach is available at
-<a href="http://sourceware.org/ml/systemtap/2006-q3/msg00793.html">Markers vs
-int3 performance comparison</a> (see "Conclusion").
-
-</p><p>
-Libmarkers will provide applications with user-space Markers and Tracepoints
-declarations, such that programmers will be able to insert Markers and Tracepoints in their
-libraries and applications. User-space Tracepoints and Markers, analogous to Kernel Tracepoints and Markers,
-define program locations and arguments provided. Libmarkers will also
-provide utility functions to enumerate, activate and deactivate tracepoints and markers in the
-process, and to associate probes with any tracepoint or marker.
-<ul><li> Linux Kernel Tracepoints documentation: <a href="http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=Documentation/tracepoints.txt" class="external text" title="http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=Documentation/tracepoints.txt" rel="nofollow">Documentation/tracepoints.txt</a>
-</li><li> Linux Kernel Tracepoints example: <a href="http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=samples/tracepoints" class="external text" title="http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=samples/tracepoints" rel="nofollow">samples/tracepoints</a>
-</li><li> Linux Kernel Markers documentation: <a href="http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=Documentation/markers.txt" class="external text" title="http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=Documentation/markers.txt" rel="nofollow">Documentation/markers.txt</a>
-</li><li> Linux Kernel Markers example: <a href="http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=samples/markers" class="external text" title="http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=samples/markers" rel="nofollow">samples/markers</a>
-</ul>
-
-
-</p><p>
-Libtracing will provide the infrastructure to allocate buffers, define event types, write event
-metadata and data to the buffers, and get notification when buffers are full. The initial implementation
-will simply use one set of buffers per process. Subsequent more optimized versions will allocate one
-set of buffers per thread; one set of buffers per CPU would be desirable but user-space programs cannot check
-or control CPU migration without resorting to more costly bus locking operations or system calls. The library
-provides a generic probe for markers which, when connected, generates an event in the buffer each time the marker
-is encountered.
-
-<UL>
-</li><li> LTTng Linux Kernel Markers Efficicient API: <a href="http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=ltt/probes/ltt-type-serializer.h" class="external text" title="http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=ltt/probes/ltt-type-serializer.h" rel="nofollow">ltt/probes/ltt-type-serializer.h</a>
-</li><li> LTTng Linux Kernel Markers Efficicient API usage example: <a href="http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=ltt/probes/kernel-trace.c" class="external text" title="http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=ltt/probes/kernel-trace.c" rel="nofollow">ltt/probes/kernel-trace.c</a>
-</li>
-</UL>
-</p><p>
-Finally, libtracingcontrol opens a connection allowing a separate process (e.g. LTTng daemon, Eclipse, GDB) to control the
-tracing within the application. Through this connection, the remote process can:
-
-<UL>
-<li> list the available Markers and Tracepoints;
-</li><li> dynamically load a library (presumably containing probes to connect);
-</li><li> connect a probe to a Tracepoint or Marker;
-</li><li> activate a Tracepoint or Marker;
-</li><li> initialize the tracing buffers;
-</li><li> retrieve the content of the tracing buffers;
-</li><li> flush the tracing buffers;
-</li><li> finalize the tracing buffers;
-</li>
-</UL>
-
-In the first version, libtracingcontrol will simply contain the basic functionality
-required to generate a trace of the Markers encountered. Subsequent versions will interact with
-other probing mechanisms like GDB Tracepoints, user-space SystemTap and those defined by virtual
-machines (e.g. Java Virtual Machine, Python, Errlang...).
-</p><p>
-Tracing of Java application is planned to be done through a JNI interface.
-Linking standard low-level C tracing library to the application within a JNI
-adaptation class will be required to trace Java events. This has been prototyped
-in the past. The work is available
-<a href="http://ltt.polymtl.ca/svn/trunk/obsolete/ltt-usertrace/java/">here</a>
-for older LTTng versions.
-<p></p>
-The principle of operation of libtracingcontrol is that
-when the instrumented application starts, a pipe is opened to allow external
-tracing control. Asynchronous notification is requested when commands arrive in
-the pipe, and a signal handler is installed for SIGIO (or a carefully chosen
-chainable signal number). Every time such signal is received, the runtime
-library checks for commands received on the external tracing control pipe.
-The application may also spontaneously provide information to the remote control process
-through the pipe:
-
-</p><ul></li><li> a buffer is full;
-</li><li> new tracepoints or markers are appearing or disappearing, because modules are
- dynamically loaded or unloaded;
-</li></ul>
-<p>
-In addition, the tracing control application should be notified when the
-application exits (to save the content of buffers if the application is
-crashing) or forks (to trace the child as well if needed).
-Such notification may be obtained through utrace.
-
-</p><p>
-In summary, a user-space application linking with libmarkers may contain static
-instrumentation, Tracepoints and Markers, just like the kernel with Kernel Markers and Tracepoints.
-The application can exploit this instrumentation itself or link with libtracing and have tracing probes
-connected to each Marker. Other instrumentation mechanisms, like the GCC instrument-function option, or hooks
-inserted by a JIT compiler, can also use libtracing to define and write event to the trace buffers.
-Finally, libtracingcontrol, analogous to GDB stubs, allows the remote control of the tracing by remote
-monitoring or debugging frameworks.
-
-</p></body></html>
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