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<ol class="chapter"><li class="affix"><a href="introduction.html">Introduction</a></li><li><a href="1_why_futures.html" class="active"><strong aria-hidden="true">1.</strong> Why Futures</a></li><li><a href="1_background_information.html"><strong aria-hidden="true">2.</strong> Some background information</a></li><li><a href="2_waker_context.html"><strong aria-hidden="true">3.</strong> Waker and Context</a></li><li><a href="3_generators_pin.html"><strong aria-hidden="true">4.</strong> Generators</a></li><li><a href="4_pin.html"><strong aria-hidden="true">5.</strong> Pin</a></li><li><a href="6_future_example.html"><strong aria-hidden="true">6.</strong> Futures - our main example</a></li><li><a href="8_finished_example.html"><strong aria-hidden="true">7.</strong> Finished example (editable)</a></li><li class="affix"><a href="conclusion.html">Conclusion and exercises</a></li></ol>
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<h1><a class="header" href="#why-futures" id="why-futures">Why Futures</a></h1>
<p>Before we go into the details about Futures in Rust, let's take a quick look
at the alternatives for handling concurrent programming in general and some
pros and cons for each of them.</p>
<h2><a class="header" href="#threads-provided-by-the-operating-system" id="threads-provided-by-the-operating-system">Threads provided by the operating system</a></h2>
<p>Now one way of accomplishing this is letting the OS take care of everything for
us. We do this by simply spawning a new OS thread for each task we want to
accomplish and write code like we normally would.</p>
<p><strong>Pros:</strong></p>
<ul>
<li>Simple</li>
<li>Easy to use</li>
<li>Switching between tasks is reasonably fast</li>
<li>You get parallelism for free</li>
</ul>
<p><strong>Cons:</strong></p>
<ul>
<li>OS level threads come with a rather large stack. If you have many tasks
waiting simultaneously (like you would in a web-server under heavy load) you'll
run out of memory pretty soon.</li>
<li>There are a lot of syscalls involved. This can be pretty costly when the number
of tasks is high.</li>
<li>The OS has many things it needs to handle. It might not switch back to your
thread as fast as you'd wish.</li>
<li>Might not be an option on some systems</li>
</ul>
<p>Using OS threads in Rust looks like this:</p>
<pre><pre class="playpen"><code class="language-rust">use std::thread;
fn main() {
println!(&quot;So we start the program here!&quot;);
let t1 = thread::spawn(move || {
thread::sleep(std::time::Duration::from_millis(200));
println!(&quot;We create tasks which gets run when they're finished!&quot;);
});
let t2 = thread::spawn(move || {
thread::sleep(std::time::Duration::from_millis(100));
println!(&quot;We can even chain callbacks...&quot;);
let t3 = thread::spawn(move || {
thread::sleep(std::time::Duration::from_millis(50));
println!(&quot;...like this!&quot;);
});
t3.join().unwrap();
});
println!(&quot;While our tasks are executing we can do other stuff here.&quot;);
t1.join().unwrap();
t2.join().unwrap();
}
</code></pre></pre>
<h2><a class="header" href="#green-threads" id="green-threads">Green threads</a></h2>
<p>Green threads has been popularized by GO in the recent years. Green threads
uses the same basic technique as operating systems does to handle concurrency.</p>
<p>Green threads are implemented by setting up a stack for each task you want to
execute and make the CPU &quot;jump&quot; from one stack to another to switch between
tasks.</p>
<p>The typical flow will be like this:</p>
<ol>
<li>Run som non-blocking code</li>
<li>Make a blocking call to some external resource</li>
<li>CPU jumps to the &quot;main&quot; thread which schedules a different thread to run and
&quot;jumps&quot; to that stack</li>
<li>Run some non-blocking code on the new thread until a new blocking call or the
task is finished</li>
<li>&quot;jumps&quot; back to the &quot;main&quot; thread and so on</li>
</ol>
<p>These &quot;jumps&quot; are know as context switches. Your OS is doing it many times each
second as you read this.</p>
<p>The main advantages are:</p>
<ol>
<li>Simple to use. The code will look like it does when using OS threads.</li>
<li>A &quot;context switch&quot; is reasonably fast</li>
<li>Each stack only gets a little memory to start with so you can have hundred of
thousands of green threads running.</li>
<li>It's easy to incorporate <a href="https://cfsamson.gitbook.io/green-threads-explained-in-200-lines-of-rust/green-threads#preemptive-multitasking"><em>preemtion</em></a>
which puts a lot of control in the hands of the runtime implementors.</li>
</ol>
<p>The main cons are:</p>
<ol>
<li>The stacks might need to grow. Solving this is not easy and will have a cost.</li>
<li>You need to save all the CPU state on every switch</li>
<li>It's not a <em>zero cost abstraction</em> (which is one of the reasons Rust removed
them early on).</li>
<li>Complicated to implement correctly if you want to support many different
platforms.</li>
</ol>
<p>If you were to implement green threads in Rust, it could look something like
this:</p>
<pre><code>The example presented below is from an earlier book I wrote about green
threads called [Green Threads Explained in 200 lines of Rust.](https://cfsamson.gitbook.io/green-threads-explained-in-200-lines-of-rust/)
If you want to know what's going on everything is explained in detail
in that book.
</code></pre>
<pre><pre class="playpen"><code class="language-rust">#![feature(asm)]
#![feature(naked_functions)]
use std::ptr;
const DEFAULT_STACK_SIZE: usize = 1024 * 1024 * 2;
const MAX_THREADS: usize = 4;
static mut RUNTIME: usize = 0;
pub struct Runtime {
threads: Vec&lt;Thread&gt;,
current: usize,
}
#[derive(PartialEq, Eq, Debug)]
enum State {
Available,
Running,
Ready,
}
struct Thread {
id: usize,
stack: Vec&lt;u8&gt;,
ctx: ThreadContext,
state: State,
}
#[derive(Debug, Default)]
#[repr(C)]
struct ThreadContext {
rsp: u64,
r15: u64,
r14: u64,
r13: u64,
r12: u64,
rbx: u64,
rbp: u64,
}
impl Thread {
fn new(id: usize) -&gt; Self {
Thread {
id,
stack: vec![0_u8; DEFAULT_STACK_SIZE],
ctx: ThreadContext::default(),
state: State::Available,
}
}
}
impl Runtime {
pub fn new() -&gt; Self {
let base_thread = Thread {
id: 0,
stack: vec![0_u8; DEFAULT_STACK_SIZE],
ctx: ThreadContext::default(),
state: State::Running,
};
let mut threads = vec![base_thread];
let mut available_threads: Vec&lt;Thread&gt; = (1..MAX_THREADS).map(|i| Thread::new(i)).collect();
threads.append(&amp;mut available_threads);
Runtime {
threads,
current: 0,
}
}
pub fn init(&amp;self) {
unsafe {
let r_ptr: *const Runtime = self;
RUNTIME = r_ptr as usize;
}
}
pub fn run(&amp;mut self) -&gt; ! {
while self.t_yield() {}
std::process::exit(0);
}
fn t_return(&amp;mut self) {
if self.current != 0 {
self.threads[self.current].state = State::Available;
self.t_yield();
}
}
fn t_yield(&amp;mut self) -&gt; bool {
let mut pos = self.current;
while self.threads[pos].state != State::Ready {
pos += 1;
if pos == self.threads.len() {
pos = 0;
}
if pos == self.current {
return false;
}
}
if self.threads[self.current].state != State::Available {
self.threads[self.current].state = State::Ready;
}
self.threads[pos].state = State::Running;
let old_pos = self.current;
self.current = pos;
unsafe {
switch(&amp;mut self.threads[old_pos].ctx, &amp;self.threads[pos].ctx);
}
self.threads.len() &gt; 0
}
pub fn spawn(&amp;mut self, f: fn()) {
let available = self
.threads
.iter_mut()
.find(|t| t.state == State::Available)
.expect(&quot;no available thread.&quot;);
let size = available.stack.len();
unsafe {
let s_ptr = available.stack.as_mut_ptr().offset(size as isize);
let s_ptr = (s_ptr as usize &amp; !15) as *mut u8;
ptr::write(s_ptr.offset(-24) as *mut u64, guard as u64);
ptr::write(s_ptr.offset(-32) as *mut u64, f as u64);
available.ctx.rsp = s_ptr.offset(-32) as u64;
}
available.state = State::Ready;
}
}
fn guard() {
unsafe {
let rt_ptr = RUNTIME as *mut Runtime;
(*rt_ptr).t_return();
};
}
pub fn yield_thread() {
unsafe {
let rt_ptr = RUNTIME as *mut Runtime;
(*rt_ptr).t_yield();
};
}
#[naked]
#[inline(never)]
unsafe fn switch(old: *mut ThreadContext, new: *const ThreadContext) {
asm!(&quot;
mov %rsp, 0x00($0)
mov %r15, 0x08($0)
mov %r14, 0x10($0)
mov %r13, 0x18($0)
mov %r12, 0x20($0)
mov %rbx, 0x28($0)
mov %rbp, 0x30($0)
mov 0x00($1), %rsp
mov 0x08($1), %r15
mov 0x10($1), %r14
mov 0x18($1), %r13
mov 0x20($1), %r12
mov 0x28($1), %rbx
mov 0x30($1), %rbp
ret
&quot;
:
:&quot;r&quot;(old), &quot;r&quot;(new)
:
: &quot;volatile&quot;, &quot;alignstack&quot;
);
}
fn main() {
let mut runtime = Runtime::new();
runtime.init();
runtime.spawn(|| {
println!(&quot;THREAD 1 STARTING&quot;);
let id = 1;
for i in 0..10 {
println!(&quot;thread: {} counter: {}&quot;, id, i);
yield_thread();
}
println!(&quot;THREAD 1 FINISHED&quot;);
});
runtime.spawn(|| {
println!(&quot;THREAD 2 STARTING&quot;);
let id = 2;
for i in 0..15 {
println!(&quot;thread: {} counter: {}&quot;, id, i);
yield_thread();
}
println!(&quot;THREAD 2 FINISHED&quot;);
});
runtime.run();
}
</code></pre></pre>
<h3><a class="header" href="#callback-based-approach" id="callback-based-approach">Callback based approach</a></h3>
<p>You probably already know this from Javascript since it's extremely common.
The whole idea behind a callback based approach is to save a pointer to a
set of instructions we want to run later on.</p>
<p>The basic idea of not involving threads as a primary way to achieve concurrency
is the common denominator for the rest of the approaches. Including the one
Rust uses today which we'll soon get to.</p>
<p><strong>Advantages:</strong></p>
<ul>
<li>Easy to implement in most languages</li>
<li>No context switching</li>
<li>Low memory overhead (in most cases)</li>
</ul>
<p><strong>Drawbacks:</strong></p>
<ul>
<li>Each task must save the state it needs for later, the memory usage will grow
linearly with the number of tasks i .</li>
<li>Can be hard to reason about, also known as &quot;callback hell&quot;.</li>
<li>Sharing state between tasks is a hard problem in Rust using this approach due
to it's ownership model.</li>
</ul>
<p>The</p>
<p>If we did that in Rust it could look something like this:</p>
<pre><pre class="playpen"><code class="language-rust">fn program_main() {
println!(&quot;So we start the program here!&quot;);
set_timeout(200, || {
println!(&quot;We create tasks which gets run when they're finished!&quot;);
});
set_timeout(100, || {
println!(&quot;We can even chain callbacks...&quot;);
set_timeout(50, || {
println!(&quot;...like this!&quot;);
})
});
println!(&quot;While our tasks are executing we can do other stuff here.&quot;);
}
fn main() {
RT.with(|rt| rt.run(program_main));
}
use std::sync::mpsc::{channel, Receiver, Sender};
use std::{cell::RefCell, collections::HashMap, thread};
thread_local! {
static RT: Runtime = Runtime::new();
}
struct Runtime {
callbacks: RefCell&lt;HashMap&lt;usize, Box&lt;dyn FnOnce() -&gt; ()&gt;&gt;&gt;,
next_id: RefCell&lt;usize&gt;,
evt_sender: Sender&lt;usize&gt;,
evt_reciever: Receiver&lt;usize&gt;,
}
fn set_timeout(ms: u64, cb: impl FnOnce() + 'static) {
RT.with(|rt| {
let id = *rt.next_id.borrow();
*rt.next_id.borrow_mut() += 1;
rt.callbacks.borrow_mut().insert(id, Box::new(cb));
let evt_sender = rt.evt_sender.clone();
thread::spawn(move || {
thread::sleep(std::time::Duration::from_millis(ms));
evt_sender.send(id).unwrap();
});
});
}
impl Runtime {
fn new() -&gt; Self {
let (evt_sender, evt_reciever) = channel();
Runtime {
callbacks: RefCell::new(HashMap::new()),
next_id: RefCell::new(1),
evt_sender,
evt_reciever,
}
}
fn run(&amp;self, program: fn()) {
program();
for evt_id in &amp;self.evt_reciever {
let cb = self.callbacks.borrow_mut().remove(&amp;evt_id).unwrap();
cb();
if self.callbacks.borrow().is_empty() {
break;
}
}
}
}
</code></pre></pre>
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