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:


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#

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:

ray start

This will print any RAY_LOG(DEBUG) lines in the source code to the raylet.err file, which you can find in Logging and Debugging. If it worked, you should see as the first line in raylet.err: Set ray log level from environment variable RAY_BACKEND_LOG_LEVEL to -1

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

#include <chrono>

Backend event stats#

The raylet process also periodically dumps event stats to the debug_state.txt log file if the RAY_event_stats=1 environment variable is set. To also enable regular printing of the stats to log files, you can additional set RAY_event_stats_print_interval_ms=1000.

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

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:

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.