trunk/lttv/doc/developer/ust.txt

   1 LTTng User-space Tracing Design
   2
   3 A user-space application may contain static instrumentation, and be linked with
   4 an associated runtime library, in order to produce an execution trace. User
   5 space Tracepoints and Markers, analogous to Kernel Tracepoints and Markers,
   6 define program locations and arguments provided. Probes may then be connected to
   7 each Tracepoint and Marker before they are activated. The probes will typically
   8 call LTTng user-space event writing functions.
   9
  10 When the instrumented application starts, a pipe is opened to allow external
  11 tracing control, asynchronous notification is requested when commands arrive in
  12 the pipe, and a signal handler is installed for SIGIO (or a carefully chosen
  13 chainable signal number). Every time such signal is received, the runtime
  14 library checks for commands received on the external tracing control pipe. The
  15 recognized commands are:
  16
  17 - list the available Markers and Tracepoints;
  18 - dynamically load a library (presumably containing probes to connect);
  19 - connect a probe to a Tracepoint or Marker;
  20 - activate a Tracepoint or Marker;
  21 - initialize the tracing buffers;
  22 - flush the tracing buffers;
  23 - finalize the tracing buffers;
  24
  25 The application may also spontaneously provide information to the pipe:
  26
  27 - a buffer is full;
  28 - new tracepoints or markers are appearing or disappearing, because modules are
  29   dynamically loaded or unloaded;
  30
  31 In addition, the tracing control application should be notified when the
  32 application exits (to save the content of buffers if the application is
  33 crashing). Such notification may be obtained through utrace.
  34
  35 This tracing scheme plans to use a direct function call to tracing into buffers
  36 mapped in user-space. This should be an order of magnitude faster than the
  37 current Dtrace implementation (c.f.
  38 https://ltt.polymtl.ca/tracingwiki/index.php/DTrace) which uses a breakpoint to
  39 perform both dynamic and static user-space tracing. Performance comparison of
  40 a function call vs the int3 approach is available at
  41 http://sourceware.org/ml/systemtap/2006-q3/msg00793.html (see "Conclusion").
  42