doc/developer-guide/debugging/tracing.en.rst

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.. include:: ../../common.defs
.. default-domain:: cpp

.. _developer-tracing:

Tracing
*********

ATS includes statically defined tracepoints.  These are useful for debugging a running instance with minimal overhead.

USDT
====

A number of tools can be used to read USDT tracepoints.  Tools such as bpftrace, bcc, and perf use eBPF technology within the Linux kernel to provide tracing capabilities.

These tools can list available tracepoints, attach to a tracepoint, and some can process trace data in real time with eBPF programs.

Built-in tracepoints
====================

ATS includes a tracepoint at each HTTP state machine state.  The state machine id, sm_id, is included as an argument to help isolate each transaction.  These tracepoints
are named milestone_<state> and are located in the HttpSM.cc file.  See :func:`TSHttpTxnMilestoneGet` for a list of the states.

Example with bpftrace
=====================
.. code-block:: bash

   #!/usr/bin/env bpftrace

   BEGIN {
      @ = (uint64)0;
   }

   usdt:/opt/ats/bin/traffic_server:trafficserver:milestone_sm_start {
      $sm_id = arg0;
      if (@ == 0) {
         @ = $sm_id;
      }
      @start_time = nsecs;
   }

   usdt:/opt/ats/bin/traffic_server:trafficserver:milestone_* {
      $sm_id = arg0;
      if ($sm_id == @) {
         printf("%s %d\n", probe, nsecs - @start_time);
      }
   }

   usdt:/opt/ats/bin/traffic_server:trafficserver:milestone_sm_finish {
      $sm_id = arg0;
      if ($sm_id == @) {
         printf("End of state machine %d\n", $sm_id);
         exit();
      }
   }