To use Ray, you need to understand the following:
- How Ray executes tasks asynchronously to achieve parallelism.
- How Ray uses object IDs to represent immutable remote objects.
Ray is a Python-based distributed execution engine. The same code can be run on a single machine to achieve efficient multiprocessing, and it can be used on a cluster for large computations.
When using Ray, several processes are involved.
- Multiple worker processes execute tasks and store results in object stores. Each worker is a separate process.
- One object store per node stores immutable objects in shared memory and allows workers to efficiently share objects on the same node with minimal copying and deserialization.
- One local scheduler per node assigns tasks to workers on the same node.
- A global scheduler receives tasks from local schedulers and assigns them to other local schedulers.
- A driver is the Python process that the user controls. For example, if the user is running a script or using a Python shell, then the driver is the Python process that runs the script or the shell. A driver is similar to a worker in that it can submit tasks to its local scheduler and get objects from the object store, but it is different in that the local scheduler will not assign tasks to the driver to be executed.
- A Redis server maintains much of the system’s state. For example, it keeps track of which objects live on which machines and of the task specifications (but not data). It can also be queried directly for debugging purposes.
To start Ray, start Python and run the following commands.
import ray ray.init()
This starts Ray.
Immutable remote objects¶
In Ray, we can create and compute on objects. We refer to these objects as remote objects, and we use object IDs to refer to them. Remote objects are stored in object stores, and there is one object store per node in the cluster. In the cluster setting, we may not actually know which machine each object lives on.
An object ID is essentially a unique ID that can be used to refer to a remote object. If you’re familiar with Futures, our object IDs are conceptually similar.
We assume that remote objects are immutable. That is, their values cannot be changed after creation. This allows remote objects to be replicated in multiple object stores without needing to synchronize the copies.
Put and Get¶
ray.put can be used to convert between Python
objects and object IDs, as shown in the example below.
x = "example" ray.put(x) # ObjectID(b49a32d72057bdcfc4dda35584b3d838aad89f5d)
ray.put(x) would be run by a worker process or by the driver
process (the driver process is the one running your script). It takes a Python
object and copies it to the local object store (here local means on the same
node). Once the object has been stored in the object store, its value cannot be
ray.put(x) returns an object ID, which is essentially an ID that
can be used to refer to the newly created remote object. If we save the object
ID in a variable with
x_id = ray.put(x), then we can pass
x_id into remote
functions, and those remote functions will operate on the corresponding remote
ray.get(x_id) takes an object ID and creates a Python object from
the corresponding remote object. For some objects like arrays, we can use shared
memory and avoid copying the object. For other objects, this copies the object
from the object store to the worker process’s heap. If the remote object
corresponding to the object ID
x_id does not live on the same node as the
worker that calls
ray.get(x_id), then the remote object will first be
transferred from an object store that has it to the object store that needs it.
x_id = ray.put("example") ray.get(x_id) # "example"
If the remote object corresponding to the object ID
x_id has not been created
yet, the command
ray.get(x_id) will wait until the remote object has been
A very common use case of
ray.get is to get a list of object IDs. In this
case, you can call
object_ids is a list of object
result_ids = [ray.put(i) for i in range(10)] ray.get(result_ids) # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Asynchronous Computation in Ray¶
Ray enables arbitrary Python functions to be executed asynchronously. This is done by designating a Python function as a remote function.
For example, a normal Python function looks like this.
def add1(a, b): return a + b
A remote function looks like this.
@ray.remote def add2(a, b): return a + b
add1(1, 2) returns
3 and causes the Python interpreter to
block until the computation has finished, calling
immediately returns an object ID and creates a task. The task will be
scheduled by the system and executed asynchronously (potentially on a different
machine). When the task finishes executing, its return value will be stored in
the object store.
x_id = add2.remote(1, 2) ray.get(x_id) # 3
The following simple example demonstrates how asynchronous tasks can be used to parallelize computation.
import time def f1(): time.sleep(1) @ray.remote def f2(): time.sleep(1) # The following takes ten seconds. [f1() for _ in range(10)] # The following takes one second (assuming the system has at least ten CPUs). ray.get([f2.remote() for _ in range(10)])
There is a sharp distinction between submitting a task and executing the task. When a remote function is called, the task of executing that function is submitted to a local scheduler, and object IDs for the outputs of the task are immediately returned. However, the task will not be executed until the system actually schedules the task on a worker. Task execution is not done lazily. The system moves the input data to the task, and the task will execute as soon as its input dependencies are available and there are enough resources for the computation.
When a task is submitted, each argument may be passed in by value or by object ID. For example, these lines have the same behavior.
add2.remote(1, 2) add2.remote(1, ray.put(2)) add2.remote(ray.put(1), ray.put(2))
Remote functions never return actual values, they always return object IDs.
When the remote function is actually executed, it operates on Python objects. That is, if the remote function was called with any object IDs, the system will retrieve the corresponding objects from the object store.
Note that a remote function can return multiple object IDs.
@ray.remote(num_return_vals=3) def return_multiple(): return 1, 2, 3 a_id, b_id, c_id = return_multiple.remote()
Expressing dependencies between tasks¶
Programmers can express dependencies between tasks by passing the object ID output of one task as an argument to another task. For example, we can launch three tasks as follows, each of which depends on the previous task.
@ray.remote def f(x): return x + 1 x = f.remote(0) y = f.remote(x) z = f.remote(y) ray.get(z) # 3
The second task above will not execute until the first has finished, and the third will not execute until the second has finished. In this example, there are no opportunities for parallelism.
The ability to compose tasks makes it easy to express interesting dependencies. Consider the following implementation of a tree reduce.
import numpy as np @ray.remote def generate_data(): return np.random.normal(size=1000) @ray.remote def aggregate_data(x, y): return x + y # Generate some random data. This launches 100 tasks that will be scheduled on # various nodes. The resulting data will be distributed around the cluster. data = [generate_data.remote() for _ in range(100)] # Perform a tree reduce. while len(data) > 1: data.append(aggregate_data.remote(data.pop(0), data.pop(0))) # Fetch the result. ray.get(data)
Remote Functions Within Remote Functions¶
So far, we have been calling remote functions only from the driver. But worker processes can also call remote functions. To illustrate this, consider the following example.
@ray.remote def sub_experiment(i, j): # Run the jth sub-experiment for the ith experiment. return i + j @ray.remote def run_experiment(i): sub_results =  # Launch tasks to perform 10 sub-experiments in parallel. for j in range(10): sub_results.append(sub_experiment.remote(i, j)) # Return the sum of the results of the sub-experiments. return sum(ray.get(sub_results)) results = [run_experiment.remote(i) for i in range(5)] ray.get(results) # [45, 55, 65, 75, 85]
When the remote function
run_experiment is executed on a worker, it calls the
sub_experiment a number of times. This is an example of how
multiple experiments, each of which takes advantage of parallelism internally,
can all be run in parallel.