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docs: More task tutorial

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Brian Anderson 2012-10-01 15:41:21 -07:00
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doc/tutorial-tasks.md
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@ -2,12 +2,16 @@
# Introduction
Rust supports concurrency and parallelism through lightweight tasks.
Rust tasks are significantly cheaper to create than traditional
threads, with a typical 32-bit system able to run hundreds of
thousands simultaneously. Tasks in Rust are what are often referred to
as _green threads_, cooperatively scheduled by the Rust runtime onto a
small number of operating system threads.
The Rust language is designed from the ground up to support pervasive
and safe concurrency through lightweight, memory-isolated tasks and
message passing.
Rust tasks are not the same as traditional threads - they are what are
often referred to as _green threads_, cooperatively scheduled by the
Rust runtime onto a small number of operating system threads. Being
significantly cheaper to create than traditional threads, Rust can
create hundreds of thousands of concurrent tasks on a typical 32-bit
system.
Tasks provide failure isolation and recovery. When an exception occurs
in rust code (either by calling `fail` explicitly or by otherwise performing
@ -16,11 +20,11 @@ to `catch` an exception as in other languages. Instead tasks may monitor
each other to detect when failure has occurred.
Rust tasks have dynamically sized stacks. When a task is first created
it starts off with a small amount of stack (in the hundreds to
low thousands of bytes, depending on plattform), and more stack is
added as needed. A Rust task will never run off the end of the stack as
is possible in many other languages, but they do have a stack budget,
and if a Rust task exceeds its stack budget then it will fail safely.
it starts off with a small amount of stack (currently in the low
thousands of bytes, depending on platform) and more stack is acquired as
needed. A Rust task will never run off the end of the stack as is
possible in many other languages, but they do have a stack budget, and
if a Rust task exceeds its stack budget then it will fail safely.
Tasks make use of Rust's type system to provide strong memory safety
guarantees, disallowing shared mutable state. Communication between
@ -32,12 +36,12 @@ explore some typical patterns in concurrent Rust code, and finally
discuss some of the more exotic synchronization types in the standard
library.
# A note about the libraries
## A note about the libraries
While Rust's type system provides the building blocks needed for safe
and efficient tasks, all of the task functionality itself is implemented
in the core and standard libraries, which are still under development
and do not always present a nice programming interface.
and do not always present a consistent interface.
In particular, there are currently two independent modules that provide
a message passing interface to Rust code: `core::comm` and `core::pipes`.
@ -66,43 +70,96 @@ concurrency at the moment.
[`std::arc`]: std/arc.html
[`std::par`]: std/par.html
# Spawning a task
# Basics
Spawning a task is done using the various spawn functions in the
module `task`. Let's begin with the simplest one, `task::spawn()`:
The programming interface for creating and managing tasks is contained
in the `task` module of the `core` library, making it available to all
Rust code by default. At it's simplest, creating a task is a matter of
calling the `spawn` function, passing a closure to run in the new
task.
~~~~
# use io::println;
use task::spawn;
use io::println;
let some_value = 22;
// Print something profound in a different task using a named function
fn print_message() { println("I am running in a different task!"); }
spawn(print_message);
// Print something more profound in a different task using a lambda expression
spawn( || println("I am also running in a different task!") );
// The canonical way to spawn is using `do` notation
do spawn {
println(~"This executes in the child task.");
println(fmt!("%d", some_value));
println("I too am running in a different task!");
}
~~~~
The argument to `task::spawn()` is a [unique
closure](#unique-closures) of type `fn~()`, meaning that it takes no
arguments and generates no return value. The effect of `task::spawn()`
is to fire up a child task that will execute the closure in parallel
with the creator.
In Rust, there is nothing special about creating tasks - the language
itself doesn't know what a 'task' is. Instead, Rust provides in the
type system all the tools necessary to implement safe concurrency,
_owned types_ in particular, and leaves the dirty work up to the
core library.
# Communication
The `spawn` function has a very simple type signature: `fn spawn(f:
~fn())`. Because it accepts only owned closures, and owned closures
contained only owned data, `spawn` can safely move the entire closure
and all its associated state into an entirely different task for
execution. Like any closure, the function passed to spawn may capture
an environment that it carries across tasks.
Now that we have spawned a child task, it would be nice if we could
communicate with it. This is done using *pipes*. Pipes are simply a
pair of endpoints, with one for sending messages and another for
receiving messages. The easiest way to create a pipe is to use
`pipes::stream`. Imagine we wish to perform two expensive
computations in parallel. We might write something like:
~~~
# use io::println;
# use task::spawn;
# fn generate_task_number() -> int { 0 }
// Generate some state locally
let child_task_number = generate_task_number();
do spawn {
// Capture it in the remote task
println(fmt!("I am child number %d", child_task_number));
}
~~~
By default tasks will be multiplexed across the available cores, running
in parallel, thus on a multicore machine, running the following code
should interleave the output in vaguely random order.
~~~
# use io::print;
# use task::spawn;
for int::range(0, 20) |child_task_number| {
do spawn {
print(fmt!("I am child number %d\n", child_task_number));
}
}
~~~
## Communication
Now that we have spawned a new task, it would be nice if we could
communicate with it. Recall that Rust does not have shared mutable
state, so one task may not manipulate variables owned by another task.
Instead we use *pipes*.
Pipes are simply a pair of endpoints, with one for sending messages
and another for receiving messages. Pipes are low-level communication
building-blocks and so come in a variety of forms, appropriate for
different use cases, but there are just a few varieties that are most
commonly used, which we will cover presently.
The simplest way to create a pipe is to use the `pipes::stream`
function to create a `(Chan, Port)` pair. In Rust parlance a 'channel'
is a sending endpoint of a pipe, and a 'port' is the recieving
endpoint. Consider the following example of performing two calculations
concurrently.
~~~~
use task::spawn;
use pipes::{stream, Port, Chan};
let (chan, port) = stream();
let (chan, port): (Chan<int>, Port<int>) = stream();
do spawn {
let result = some_expensive_computation();
@ -116,17 +173,19 @@ let result = port.recv();
# fn some_other_expensive_computation() {}
~~~~
Let's walk through this code line-by-line. The first line creates a
stream for sending and receiving integers:
Let's examine this example in detail. The `let` statement first creates a
stream for sending and receiving integers (recall that `let` can be
used for destructuring patterns, in this case separating a tuple into
its component parts).
~~~~ {.ignore}
# use pipes::stream;
let (chan, port) = stream();
~~~~
# use pipes::{stream, Chan, Port};
let (chan, port): (Chan<int>, Port<int>) = stream();
~~~~
This port is where we will receive the message from the child task
once it is complete. The channel will be used by the child to send a
message to the port. The next statement actually spawns the child:
The channel will be used by the child task to send data to the parent task,
which will wait to recieve the data on the port. The next statement
spawns the child task.
~~~~
# use task::{spawn};
@ -140,14 +199,15 @@ do spawn {
}
~~~~
This child will perform the expensive computation send the result
over the channel. (Under the hood, `chan` was captured by the
closure that forms the body of the child task. This capture is
allowed because channels are sendable.)
Notice that `chan` was transferred to the child task implicitly by
capturing it in the task closure. Both `Chan` and `Port` are sendable
types and may be captured into tasks or otherwise transferred between
them. In the example, the child task performs an expensive computation
then sends the result over the captured channel.
Finally, the parent continues by performing
some other expensive computation and then waiting for the child's result
to arrive on the port:
Finally, the parent continues by performing some other expensive
computation and then waiting for the child's result to arrive on the
port:
~~~~
# use pipes::{stream, Port, Chan};
@ -158,7 +218,73 @@ some_other_expensive_computation();
let result = port.recv();
~~~~
# Creating a task with a bi-directional communication path
The `Port` and `Chan` pair created by `stream` enable efficient
communication between a single sender and a single receiver, but
multiple senders cannot use a single `Chan`, nor can multiple
receivers use a single `Port`. What if our example needed to
perform multiple computations across a number of tasks? In that
case we might use a `SharedChan`, a type that allows a single
`Chan` to be used by multiple senders.
~~~
# use task::spawn;
use pipes::{stream, SharedChan};
let (chan, port) = stream();
let chan = SharedChan(move chan);
for uint::range(0, 3) |init_val| {
// Create a new channel handle to distribute to the child task
let child_chan = chan.clone();
do spawn {
child_chan.send(some_expensive_computation(init_val));
}
}
let result = port.recv() + port.recv() + port.recv();
# fn some_expensive_computation(_i: uint) -> int { 42 }
~~~
Here we transfer ownership of the channel into a new `SharedChan`
value. Like `Chan`, `SharedChan` is a non-copyable, owned type
(sometimes also referred to as an 'affine' or 'linear' type). Unlike
`Chan` though, `SharedChan` may be duplicated with the `clone()`
method. A cloned `SharedChan` produces a new handle to the same
channel, allowing multiple tasks to send data to a single port.
Between `spawn`, `stream` and `SharedChan` we have enough tools
to implement many useful concurrency patterns.
Note that the above `SharedChan` example is somewhat contrived since
you could also simply use three `stream` pairs, but it serves to
illustrate the point. For reference, written with multiple streams it
might look like the example below.
~~~
# use task::spawn;
# use pipes::{stream, Port, Chan};
let ports = do vec::from_fn(3) |init_val| {
let (chan, port) = stream();
do spawn {
chan.send(some_expensive_computation(init_val));
}
port
};
// Wait on each port, accumulating the results
let result = ports.foldl(0, |accum, port| *accum + port.recv() );
# fn some_expensive_computation(_i: uint) -> int { 42 }
~~~
# Unfinished notes
## Actor patterns
## Linearity, option dancing, owned closures
## Creating a task with a bi-directional communication path
A very common thing to do is to spawn a child task where the parent
and child both need to exchange messages with each other. The
@ -227,3 +353,4 @@ assert from_child.recv() == ~"0";
The parent task first calls `DuplexStream` to create a pair of bidirectional endpoints. It then uses `task::spawn` to create the child task, which captures one end of the communication channel. As a result, both parent
and child can send and receive data to and from the other.