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Introduction

Charm4py's programming model is based on an actor model. Distributed objects in Charm4py are called Chares (pronounced char). A chare is essentially a Python object in the OOP sense with its own attributes (data) and methods. A chare lives on one process, and some chares can migrate between processes (e.g. for dynamic load balancing). Chares can call methods of any other chares in the system via remote method invocation, with the same syntax as calling regular Python methods. The runtime automatically takes care of location management and uses the most efficient technique for method invocation and message passing. Parallelism is achieved by having chares distributed across processes/cores.

You can create as many collections of distributed objects as you want, of the same or different types. There can be multiple chares on one process, each executing one or multiple tasks. Having many chares per core can help the runtime maximize resource utilization, dynamically balance load and overlap communication and computation.

In addition, Charm4py supports the following features to facilitate expression of concurrency: coroutines, channels and futures. These are seamlessly integrated into the actor model.

We will show some simple examples now to quickly illustrate these concepts. For a more step-by-step tutorial, you can check the :doc:`tutorial` which can be done in an interactive session.

Actor model

Chares are defined as regular Python classes that are a subclass of Chare:

from charm4py import charm, Chare, Array

# Define my own chare type A (instances will be distributed objects)
class A(Chare):

    def start(self):
        # call method 'sayHi' of element 1 in my Array
        self.thisProxy[1].sayHi('hello world')

    def sayHi(self, message):
        print(message, 'on process', charm.myPe())
        exit()

def main(args):
    # create a distributed Array of 2 objects of type A
    array_proxy = Array(A, 2)
    # call method 'start' of element 0 of the Array
    array_proxy[0].start()

# start the Charm runtime. after initialization, the runtime will call
# function 'main' on the first process
charm.start(main)

One important thing to note here is that in Charm4py every remote method invocation is asynchronous. This allows the runtime to maximize resource efficiency and overlap communication and computation. This also means that calls will return immediately. You can, however, request a future when calling remote methods, and use the future to suspend the current coroutine until the remote method completes, or to obtain a return value (more on this below).

Coroutines

Chare methods can act as coroutines, which simply means that they can suspend their execution to wait for events/messages, and continue where they left off when the event arrives. This can allow writing significant parts of your code in direct or sequential style. Simply decorate a method with @coro to allow it to work as a coroutine. When a coroutine suspends, the runtime is free to schedule other work on the same process, even for the same chare.

Coroutines are typically used in conjunction with channels and futures (described below).

Channels

Channels establish streamed connections between chares (currently one-to-one). Messages can be sent/received to/from the channel using the methods send() and recv(). The following example uses Channels and coroutines:

from charm4py import charm, Chare, Array, coro, Channel

class A(Chare):

    @coro
    def start(self):
        if self.thisIndex == (0,):
            # I am element 0, establish a Channel with element 1 of my Array
            ch = Channel(self, remote=self.thisProxy[1])
            # send msg on the channel (this is asynchronous)
            ch.send('hello world')
        else:
            # I am element 1, establish a Channel with element 0 of my Array
            ch = Channel(self, remote=self.thisProxy[0])
            # receive msg from the channel. coroutine suspends until the msg arrives
            print(ch.recv())
            exit()

def main(args):
    a = Array(A, 2)
    # call method 'start' of every element of the array (this is a broadcast)
    a.start()

charm.start(main)

Tip

Coroutine methods are currently implemented using greenlets, which are very lightweight. The amount of overhead they add is tiny, so don't hesitate to use them where appropiate. Also note that the runtime will tell you if @coro is needed.

Futures

Coroutines can also create futures and use them to wait for certain events/messages. A future can be sent to other chares in the system, and any chare can send a value to the future, which will resume the coroutine that was waiting on it. For example:

from charm4py import charm, Chare, Array, coro, Channel, Future

class A(Chare):

    @coro
    def start(self, done):
        neighbor = self.thisProxy[(self.thisIndex[0] + 1) % 2]
        # establish a channel with my neighbor
        ch = Channel(self, remote=neighbor)
        # each chare sends and receives a msg to/from its neighbor for 10 steps
        for i in range(10):
            ch.send(i)
            assert ch.recv() == i
        if self.thisIndex == (0,):
            # signal the future that we are done
            done()

def main(args):
    a = Array(A, 2)
    # create a Future
    done = Future()
    # call start method on both elements (broadcast), passing the future
    a.start(done)
    # ... do work ...
    # 'get' suspends the coroutine until the future receives a value
    # (note that the main function is always a coroutine)
    done.get()
    exit()

charm.start(main)

Awaitable remote method calls

As mentioned above, you can also obtain a future when invoking a remote method of any chare. This is done by using the keywords awaitable=True and ret=True when calling the method. The former specifies that the call is awaitable and allows waiting for completion. The latter specifies that the caller wants to receive the return value(s). Note that ret=True automatically implies that the call is awaitable (a return value can only be received after the call has completed).

Example:

from charm4py import charm, Chare, Array

class A(Chare):

    def work(self):
        result = # ... do some work ...
        return result

def main(args):
    a = Array(A, 2)
    future = a[1].work(ret=True)
    # ... can do other stuff while the remote chare works ...
    # query future now. will suspend 'main' if the value has not arrived yet
    value = future.get()
    print('Result is', value)
    exit()

charm.start(main)

Caution!

For broadcasts, ret=True will cause a list of return values to be sent to the caller. This is more expensive than simply waiting for completion of the broadcast with awaitable=True, and can also result in very long lists of return values if you are broadcasting to thousands of chares. In summary, only use ret=True for broadcasts if a list of return values is what you want.

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