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630 lines
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<h1><a class="header" href="#why-futures" id="why-futures">Why Futures</a></h1>
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<p>Before we go into the details about Futures in Rust, let's take a quick look
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at the alternatives for handling concurrent programming in general and some
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pros and cons for each of them.</p>
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<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>
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<p>Now one way of accomplishing this is letting the OS take care of everything for
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us. We do this by simply spawning a new OS thread for each task we want to
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accomplish and write code like we normally would.</p>
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<p><strong>Pros:</strong></p>
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<ul>
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<li>Simple</li>
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<li>Easy to use</li>
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<li>Switching between tasks is reasonably fast</li>
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<li>You get parallelism for free</li>
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</ul>
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<p><strong>Cons:</strong></p>
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<ul>
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<li>OS level threads come with a rather large stack. If you have many tasks
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waiting simultaneously (like you would in a web-server under heavy load) you'll
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run out of memory pretty soon.</li>
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<li>There are a lot of syscalls involved. This can be pretty costly when the number
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of tasks is high.</li>
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<li>The OS has many things it needs to handle. It might not switch back to your
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thread as fast as you'd wish.</li>
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<li>Might not be an option on some systems</li>
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</ul>
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<p>Using OS threads in Rust looks like this:</p>
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<pre><pre class="playpen"><code class="language-rust">use std::thread;
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fn main() {
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println!("So we start the program here!");
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let t1 = thread::spawn(move || {
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thread::sleep(std::time::Duration::from_millis(200));
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println!("We create tasks which gets run when they're finished!");
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});
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let t2 = thread::spawn(move || {
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thread::sleep(std::time::Duration::from_millis(100));
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println!("We can even chain callbacks...");
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let t3 = thread::spawn(move || {
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thread::sleep(std::time::Duration::from_millis(50));
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println!("...like this!");
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});
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t3.join().unwrap();
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});
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println!("While our tasks are executing we can do other stuff here.");
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t1.join().unwrap();
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t2.join().unwrap();
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}
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</code></pre></pre>
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<h2><a class="header" href="#green-threads" id="green-threads">Green threads</a></h2>
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<p>Green threads has been popularized by GO in the recent years. Green threads
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uses the same basic technique as operating systems does to handle concurrency.</p>
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<p>Green threads are implemented by setting up a stack for each task you want to
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execute and make the CPU "jump" from one stack to another to switch between
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tasks.</p>
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<p>The typical flow will be like this:</p>
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<ol>
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<li>Run som non-blocking code</li>
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<li>Make a blocking call to some external resource</li>
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<li>CPU jumps to the "main" thread which schedules a different thread to run and
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"jumps" to that stack</li>
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<li>Run some non-blocking code on the new thread until a new blocking call or the
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task is finished</li>
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<li>"jumps" back to the "main" thread and so on</li>
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</ol>
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<p>These "jumps" are know as context switches. Your OS is doing it many times each
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second as you read this.</p>
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<p>The main advantages are:</p>
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<ol>
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<li>Simple to use. The code will look like it does when using OS threads.</li>
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<li>A "context switch" is reasonably fast</li>
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<li>Each stack only gets a little memory to start with so you can have hundred of
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thousands of green threads running.</li>
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<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>
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which puts a lot of control in the hands of the runtime implementors.</li>
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</ol>
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<p>The main cons are:</p>
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<ol>
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<li>The stacks might need to grow. Solving this is not easy and will have a cost.</li>
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<li>You need to save all the CPU state on every switch</li>
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<li>It's not a <em>zero cost abstraction</em> (which is one of the reasons Rust removed
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them early on).</li>
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<li>Complicated to implement correctly if you want to support many different
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platforms.</li>
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</ol>
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<p>If you were to implement green threads in Rust, it could look something like
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this:</p>
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<pre><code>The example presented below is from an earlier book I wrote about green
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threads called [Green Threads Explained in 200 lines of Rust.](https://cfsamson.gitbook.io/green-threads-explained-in-200-lines-of-rust/)
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If you want to know what's going on everything is explained in detail
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in that book.
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</code></pre>
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<pre><pre class="playpen"><code class="language-rust">#![feature(asm)]
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#![feature(naked_functions)]
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use std::ptr;
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const DEFAULT_STACK_SIZE: usize = 1024 * 1024 * 2;
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const MAX_THREADS: usize = 4;
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static mut RUNTIME: usize = 0;
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pub struct Runtime {
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threads: Vec<Thread>,
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current: usize,
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}
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#[derive(PartialEq, Eq, Debug)]
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enum State {
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Available,
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Running,
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Ready,
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}
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struct Thread {
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id: usize,
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stack: Vec<u8>,
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ctx: ThreadContext,
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state: State,
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}
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#[derive(Debug, Default)]
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#[repr(C)]
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struct ThreadContext {
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rsp: u64,
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r15: u64,
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r14: u64,
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r13: u64,
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r12: u64,
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rbx: u64,
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rbp: u64,
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}
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impl Thread {
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fn new(id: usize) -> Self {
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Thread {
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id,
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stack: vec![0_u8; DEFAULT_STACK_SIZE],
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ctx: ThreadContext::default(),
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state: State::Available,
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}
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}
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}
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impl Runtime {
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pub fn new() -> Self {
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let base_thread = Thread {
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id: 0,
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stack: vec![0_u8; DEFAULT_STACK_SIZE],
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ctx: ThreadContext::default(),
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state: State::Running,
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};
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let mut threads = vec![base_thread];
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let mut available_threads: Vec<Thread> = (1..MAX_THREADS).map(|i| Thread::new(i)).collect();
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threads.append(&mut available_threads);
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Runtime {
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threads,
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current: 0,
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}
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||
}
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pub fn init(&self) {
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unsafe {
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let r_ptr: *const Runtime = self;
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RUNTIME = r_ptr as usize;
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}
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}
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pub fn run(&mut self) -> ! {
|
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while self.t_yield() {}
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std::process::exit(0);
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||
}
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||
|
||
fn t_return(&mut self) {
|
||
if self.current != 0 {
|
||
self.threads[self.current].state = State::Available;
|
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self.t_yield();
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}
|
||
}
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||
|
||
fn t_yield(&mut self) -> bool {
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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 {
|
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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;
|
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unsafe {
|
||
switch(&mut self.threads[old_pos].ctx, &self.threads[pos].ctx);
|
||
}
|
||
self.threads.len() > 0
|
||
}
|
||
|
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pub fn spawn(&mut self, f: fn()) {
|
||
let available = self
|
||
.threads
|
||
.iter_mut()
|
||
.find(|t| t.state == State::Available)
|
||
.expect("no available thread.");
|
||
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 & !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!("
|
||
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
|
||
"
|
||
:
|
||
:"r"(old), "r"(new)
|
||
:
|
||
: "volatile", "alignstack"
|
||
);
|
||
}
|
||
|
||
fn main() {
|
||
let mut runtime = Runtime::new();
|
||
runtime.init();
|
||
runtime.spawn(|| {
|
||
println!("THREAD 1 STARTING");
|
||
let id = 1;
|
||
for i in 0..10 {
|
||
println!("thread: {} counter: {}", id, i);
|
||
yield_thread();
|
||
}
|
||
println!("THREAD 1 FINISHED");
|
||
});
|
||
runtime.spawn(|| {
|
||
println!("THREAD 2 STARTING");
|
||
let id = 2;
|
||
for i in 0..15 {
|
||
println!("thread: {} counter: {}", id, i);
|
||
yield_thread();
|
||
}
|
||
println!("THREAD 2 FINISHED");
|
||
});
|
||
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 "callback hell".</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!("So we start the program here!");
|
||
set_timeout(200, || {
|
||
println!("We create tasks which gets run when they're finished!");
|
||
});
|
||
set_timeout(100, || {
|
||
println!("We can even chain callbacks...");
|
||
set_timeout(50, || {
|
||
println!("...like this!");
|
||
})
|
||
});
|
||
println!("While our tasks are executing we can do other stuff here.");
|
||
}
|
||
|
||
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<HashMap<usize, Box<dyn FnOnce() -> ()>>>,
|
||
next_id: RefCell<usize>,
|
||
evt_sender: Sender<usize>,
|
||
evt_reciever: Receiver<usize>,
|
||
}
|
||
|
||
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() -> Self {
|
||
let (evt_sender, evt_reciever) = channel();
|
||
Runtime {
|
||
callbacks: RefCell::new(HashMap::new()),
|
||
next_id: RefCell::new(1),
|
||
evt_sender,
|
||
evt_reciever,
|
||
}
|
||
}
|
||
|
||
fn run(&self, program: fn()) {
|
||
program();
|
||
for evt_id in &self.evt_reciever {
|
||
let cb = self.callbacks.borrow_mut().remove(&evt_id).unwrap();
|
||
cb();
|
||
if self.callbacks.borrow().is_empty() {
|
||
break;
|
||
}
|
||
}
|
||
}
|
||
}
|
||
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