Debugging for Ray Developers
============================

This debugging guide is for contributors to the Ray project.

Starting processes in a debugger
--------------------------------
When processes are crashing, it is often useful to start them in a debugger.
Ray currently allows processes to be started in the following:

- valgrind
- the valgrind profiler
- the perftools profiler
- gdb
- tmux

To use any of these tools, please make sure that you have them installed on
your machine first (``gdb`` and ``valgrind`` on MacOS are known to have issues).
Then, you can launch a subset of ray processes by adding the environment
variable ``RAY_{PROCESS_NAME}_{DEBUGGER}=1``. For instance, if you wanted to
start the raylet in ``valgrind``, then you simply need to set the environment
variable ``RAY_RAYLET_VALGRIND=1``.

To start a process inside of ``gdb``, the process must also be started inside of
``tmux``. So if you want to start the raylet in ``gdb``, you would start your
Python script with the following:

.. code-block:: bash

 RAY_RAYLET_GDB=1 RAY_RAYLET_TMUX=1 python

You can then list the ``tmux`` sessions with ``tmux ls`` and attach to the
appropriate one.

You can also get a core dump of the ``raylet`` process, which is especially
useful when filing `issues`_. The process to obtain a core dump is OS-specific,
but usually involves running ``ulimit -c unlimited`` before starting Ray to
allow core dump files to be written.

.. _backend-logging:

Backend logging
---------------
The ``raylet`` process logs detailed information about events like task
execution and object transfers between nodes. To set the logging level at
runtime, you can set the ``RAY_BACKEND_LOG_LEVEL`` environment variable before
starting Ray. For example, you can do:

.. code-block:: shell

 export RAY_BACKEND_LOG_LEVEL=debug
 ray start

This will print any ``RAY_LOG(DEBUG)`` lines in the source code to the
``raylet.err`` file, which you can find in :ref:`temp-dir-log-files`.
If it worked, you should see as the first line in ``raylet.err``:

.. code-block:: shell

  logging.cc:270: Set ray log level from environment variable RAY_BACKEND_LOG_LEVEL to -1

(-1 is defined as RayLogLevel::DEBUG in logging.h.)

.. literalinclude:: /../../src/ray/util/logging.h
  :language: C
  :lines: 113,120

Backend event stats
-------------------
The ``raylet`` process also periodically dumps event stats to ``debug_state.txt`` and its log
file if ``RAY_event_stats=1`` environment variable is set. To alter the interval at which
Ray writes stats to log files, you can set ``RAY_event_stats_print_interval_ms``.

Event stats include ASIO event handlers, periodic timers, and RPC handlers. Here is a sample
of what the event stats look like:

.. code-block:: shell

  Event stats:
  Global stats: 739128 total (27 active)
  Queueing time: mean = 47.402 ms, max = 1372.219 s, min = -0.000 s, total = 35035.892 s
  Execution time:  mean = 36.943 us, total = 27.306 s
  Handler stats:
    ClientConnection.async_read.ReadBufferAsync - 241173 total (19 active), CPU time: mean = 9.999 us, total = 2.411 s
    ObjectManager.ObjectAdded - 61215 total (0 active), CPU time: mean = 43.953 us, total = 2.691 s
    CoreWorkerService.grpc_client.AddObjectLocationOwner - 61204 total (0 active), CPU time: mean = 3.860 us, total = 236.231 ms
    CoreWorkerService.grpc_client.GetObjectLocationsOwner - 51333 total (0 active), CPU time: mean = 25.166 us, total = 1.292 s
    ObjectManager.ObjectDeleted - 43188 total (0 active), CPU time: mean = 26.017 us, total = 1.124 s
    CoreWorkerService.grpc_client.RemoveObjectLocationOwner - 43177 total (0 active), CPU time: mean = 2.368 us, total = 102.252 ms
    NodeManagerService.grpc_server.PinObjectIDs - 40000 total (0 active), CPU time: mean = 194.860 us, total = 7.794 s

Callback latency injection
--------------------------
Sometimes, bugs are caused by RPC issues, for example, due to the delay of some requests, the system goes to a deadlock.
To debug and reproduce this kind of issue, we need to have a way to inject latency for the RPC request. To enable this,
``RAY_testing_asio_delay_us`` is introduced. If you'd like to make the callback of some RPC requests be executed after some time,
you can do it with this variable. For example:

.. code-block:: shell

  RAY_testing_asio_delay_us="NodeManagerService.grpc_client.PrepareBundleResources=2000000:2000000" ray start --head


The syntax for this is ``RAY_testing_asio_delay_us="method1=min_us:max_us,method2=min_us:max_us"``. Entries are comma separated.
There is a special method ``*`` which means all methods. It has a lower priority compared with other entries.

.. _`issues`: https://github.com/ray-project/ray/issues