This document discusses some common problems that people run into when using Ray as well as some known problems. If you encounter other problems, please let us know.

No Speedup

You just ran an application using Ray, but it wasn’t as fast as you expected it to be. Or worse, perhaps it was slower than the serial version of the application! The most common reasons are the following.

  • Number of cores: How many cores is Ray using? When you start Ray, it will determine the number of CPUs on each machine with psutil.cpu_count(). Ray usually will not schedule more tasks in parallel than the number of CPUs. So if the number of CPUs is 4, the most you should expect is a 4x speedup.

  • Physical versus logical CPUs: Do the machines you’re running on have fewer physical cores than logical cores? You can check the number of logical cores with psutil.cpu_count() and the number of physical cores with psutil.cpu_count(logical=False). This is common on a lot of machines and especially on EC2. For many workloads (especially numerical workloads), you often cannot expect a greater speedup than the number of physical CPUs.

  • Small tasks: Are your tasks very small? Ray introduces some overhead for each task (the amount of overhead depends on the arguments that are passed in). You will be unlikely to see speedups if your tasks take less than ten milliseconds. For many workloads, you can easily increase the sizes of your tasks by batching them together.

  • Variable durations: Do your tasks have variable duration? If you run 10 tasks with variable duration in parallel, you shouldn’t expect an N-fold speedup (because you’ll end up waiting for the slowest task). In this case, consider using ray.wait to begin processing tasks that finish first.

  • Multi-threaded libraries: Are all of your tasks attempting to use all of the cores on the machine? If so, they are likely to experience contention and prevent your application from achieving a speedup. This is very common with some versions of numpy, and in that case can usually be setting an environment variable like MKL_NUM_THREADS (or the equivalent depending on your installation) to 1.

    For many - but not all - libraries, you can diagnose this by opening top while your application is running. If one process is using most of the CPUs, and the others are using a small amount, this may be the problem. The most common exception is PyTorch, which will appear to be using all the cores despite needing torch.set_num_threads(1) to be called to avoid contention.

If you are still experiencing a slowdown, but none of the above problems apply, we’d really like to know! Please create a GitHub issue and consider submitting a minimal code example that demonstrates the problem.


If Ray crashed, you may wonder what happened. Currently, this can occur for some of the following reasons.

  • Stressful workloads: Workloads that create many many tasks in a short amount of time can sometimes interfere with the heartbeat mechanism that we use to check that processes are still alive. On the head node in the cluster, you can check the files /tmp/ray/session_*/logs/monitor*. They will indicate which processes Ray has marked as dead (due to a lack of heartbeats). However, it is currently possible for a process to get marked as dead without actually having died.

  • Starting many actors: Workloads that start a large number of actors all at once may exhibit problems when the processes (or libraries that they use) contend for resources. Similarly, a script that starts many actors over the lifetime of the application will eventually cause the system to run out of file descriptors. This is addressable, but currently we do not garbage collect actor processes until the script finishes.

  • Running out of file descriptors: As a workaround, you may be able to increase the maximum number of file descriptors with a command like ulimit -n 65536. If that fails, double check that the hard limit is sufficiently large by running ulimit -Hn. If it is too small, you can increase the hard limit as follows (these instructions work on EC2).

    • Increase the hard ulimit for open file descriptors system-wide by running the following.

      sudo bash -c "echo $USER hard nofile 65536 >> /etc/security/limits.conf"
    • Logout and log back in.



You can run ray stack to dump the stack traces of all Ray workers on the current node. This requires py-spy to be installed.

If a workload is hanging and not progressing, the problem may be one of the following.

  • Reconstructing an object created with put: When an object that is needed has been evicted or lost, Ray will attempt to rerun the task that created the object. However, there are some cases that currently are not handled. For example, if the object was created by a call to ray.put on the driver process, then the argument that was passed into ray.put is no longer available and so the call to ray.put cannot be rerun (without rerunning the driver).
  • Reconstructing an object created by actor task: Ray currently does not reconstruct objects created by actor methods.

Serialization Problems

Ray’s serialization is currently imperfect. If you encounter an object that Ray does not serialize/deserialize correctly, please let us know. For example, you may want to bring it up on this thread.

Outdated Function Definitions

Due to subtleties of Python, if you redefine a remote function, you may not always get the expected behavior. In this case, it may be that Ray is not running the newest version of the function.

Suppose you define a remote function f and then redefine it. Ray should use the newest version.

def f():
    return 1

def f():
    return 2

ray.get(f.remote())  # This should be 2.

However, the following are cases where modifying the remote function will not update Ray to the new version (at least without stopping and restarting Ray).

  • The function is imported from an external file: In this case, f is defined in some external file If you import file, change the definition of f in, then re-import file, the function f will not be updated.

    This is because the second import gets ignored as a no-op, so f is still defined by the first import.

    A solution to this problem is to use reload(file) instead of a second import file. Reloading causes the new definition of f to be re-executed, and exports it to the other machines. Note that in Python 3, you need to do from importlib import reload.

  • The function relies on a helper function from an external file: In this case, f can be defined within your Ray application, but relies on a helper function h defined in some external file If the definition of h gets changed in, redefining f will not update Ray to use the new version of h.

    This is because when f first gets defined, its definition is shipped to all of the workers, and is unpickled. During unpickling, gets imported in the workers. Then when f gets redefined, its definition is again shipped and unpickled in all of the workers. But since has been imported in the workers already, it is treated as a second import and is ignored as a no-op.

    Unfortunately, reloading on the driver does not update h, as the reload needs to happen on the worker.

    A solution to this problem is to redefine f to reload before it calls h. For example, if inside you have

    def h():
        return 1

    And you define remote function f as

    def f():
        return file.h()

    You can redefine f as follows.

    def f():
        return file.h()

    This forces the reload to happen on the workers as needed. Note that in Python 3, you need to do from importlib import reload.