tutorial.md

Tutorial

To use Ray, you need to understand the following:

• How Ray uses object IDs to represent immutable remote objects.
• How Ray constructs a computation graph using remote functions.

Overview

Ray is a distributed extension of Python. When using Ray, several processes are involved.

• A scheduler: The scheduler assigns tasks to workers. It is its own process.
• Multiple workers: Workers execute tasks and store the results in object stores. Each worker is a separate process.
• One object store per node: The object store enables the sharing of Python objects between worker processes so each worker does not have to have a separate copy.
• A driver: The driver is the Python process that the user controls and which submits tasks to the scheduler. For example, if the user is running a script or using a Python shell, then the driver is the process that runs the script or the shell.

Starting Ray

To start Ray, start Python, and run the following commands.

import ray
ray.init(start_ray_local=True, num_workers=10)

That command starts a scheduler, one object store, and ten workers. Each of these are distinct processes. They will be killed when you exit the Python interpreter. They can also be killed manually with the following command.

killall scheduler objstore python

Immutable remote objects

In Ray, we can create and manipulate 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

The commands ray.get and ray.put can be used to convert between Python objects and object IDs, as shown in the example below.

x = [1, 2, 3]
ray.put(x)  # prints <ray.ObjectID at 0x1031baef0>

The command 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 changed.

In addition, 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 object.

The command 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 currently copies the object from the object store into the memory of the worker process. 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 copied from an object store that has it to the object store that needs it.

x_id = ray.put([1, 2, 3])
ray.get(x_id)  # prints [1, 2, 3]

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 created.

A very common use case of ray.get is to get a list of object IDs. In this case, you can call ray.get(object_ids) where object_ids is a list of object IDs.

result_ids = [ray.put(i) for i in range(10)]
ray.get(result_ids)  # prints [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Computation graphs in Ray

Ray represents computation with a directed acyclic graph of tasks. Tasks are added to this graph by calling remote functions.

For example, a normal Python function looks like this.

def add(a, b):
  return a + b

A remote function in Ray looks like this.

@ray.remote
def add(a, b):
  return a + b

Remote functions

Whereas in regular Python, calling add(1, 2) would return 3, in Ray, calling add.remote(1, 2) does not actually execute the task. Instead, it adds a task to the computation graph and immediately returns the object ID for the output of the computation.

x_id = add.remote(1, 2)
ray.get(x_id)  # prints 3

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 the scheduler, and the scheduler immediately returns object IDs for the outputs of the task. However, the task will not be executed until the scheduler actually schedules the task on a worker.

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.

add.remote(1, 2)
add.remote(1, ray.put(2))
add.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 Python objects corresponding to those object IDs will be retrieved and passed into the actual execution of the remote function.

Note that a remote function can return multiple object IDs.

@ray.remote(num_return_vals=3)
def return_multiple():
  return 0, 0.0, "zero"

a_id, b_id, c_id = return_multiple.remote()

Blocking computation

In a regular Python script, the specification of a computation is intimately linked to the actual execution of the code. For example, consider the following code.

import time

# This takes 20 seconds.
for i in range(10):
  time.sleep(2)

At the core of the above script, there are ten separate tasks, each of which sleeps for two seconds (this is a toy example, but you could imagine replacing the call to sleep with some computationally intensive task). These tasks do not depend on each other, so in principle, they could be executed in parallel. However, in the above implementation, they will be executed serially, which will take twenty seconds.

Ray gets around this by representing computation as a graph of tasks, where some tasks depend on the outputs of other tasks and where tasks can be executed once their dependencies are done.

For example, suppose we define the remote function sleep to be a wrapper around time.sleep.

import time

@ray.remote
def sleep(n):
  time.sleep(n)
  return 0

Then we can write

# Submit ten tasks to the scheduler. This finishes almost immediately.
result_ids = []
for i in range(10):
  result_ids.append(sleep.remote(2))

# Wait for the results. If we have at least ten workers, this takes 2 seconds.
ray.get(result_ids)  # prints [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

The for loop simply adds ten tasks to the computation graph, with no dependencies between the tasks. It executes almost instantaneously. Afterwards, we use ray.get to wait for the tasks to finish. If we have at least ten workers, then all ten tasks can be executed in parallel, and so the overall script should take two seconds.

Visualizing the Computation Graph

The computation graph can be viewed as follows.

ray.visualize_computation_graph(view=True)

In this figure, boxes are tasks and ovals are objects.

The box that says op-root in it just refers to the overall script itself. The dotted lines indicate that the script launched 10 tasks (tasks are denoted by rectangular boxes). The solid lines indicate that each task produces one output (represented by an oval).

It is clear from the computation graph that these ten tasks can be executed in parallel.

Computation graphs encode dependencies. For example, suppose we define

import numpy as np

@ray.remote
def zeros(shape):
  return np.zeros(shape)

@ray.remote
def dot(a, b):
  return np.dot(a, b)

Then we run

a_id = zeros.remote([10, 10])
b_id = zeros.remote([10, 10])
c_id = dot.remote(a_id, b_id)

The corresponding computation graph looks like this.

The three dashed lines indicate that the script launched three tasks (the two zeros tasks and the one dot task). Each task produces a single output, and the dot task depends on the outputs of the two zeros tasks.

This makes it clear that the two zeros tasks can execute in parallel but that the dot task must wait until the two zeros tasks have finished.

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) # prints [45, 55, 65, 75, 85]

When the remote function run_experiment is executed on a worker, it calls the remote function 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.