Rust: Systems Programming for Everyone

Rust: Systems
Programming for
Everyone
Felix Klock (@pnkfelix), Mozilla
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rust

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Highlights
- attended by more than 12,000 delegates since 2007
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Presented at QCon London
www.qconlondon.com

Why use Rust?
Fast code, low memory footprint
Go from bare metal (assembly; C FFI) ...
... to high-level (collections, closures, generic
containers) ...
with zero cost (no GC, unboxed closures,
monomorphization of generics)
Safety and Parallelism

Safety and Parallelism
Safety
No segmentation faults
No undefined behavior
No data races
(Multi-paradigm) Parallelism
msg passing via channels
shared state via Arcand atomics, Mutex, etc
use native threads... or scoped threads... or work-stealing...

Why would you (Felix) work
on Rust?
It's awesome!
(Were prior slides really not a sufficient answer?)
oh, maybe you meant ...

Why would Mozilla sponsor Rust?
Hard to prototype research-y browser changes atop C++ code base
Rust ⇒Servo, WebRender
Want Rust for next-gen infrastructure (services, IoT)
"Our mission is to ensure the Internet is a global
public resource, open and accessible to all. An
Internet that truly puts people first, where
individuals can shape their own experience and are
empowered, safe and independent."
"accessible to all"

Where is Rust now?
1.0 release was back in May 2015
Rolling release cycle (up to Rust 1.7 as of March 2nd 2016)
Open source from the begining
https://github.com/rust-lang/rust/
Open model for future change (RFC process)
https://github.com/rust-lang/rfcs/
Awesome developer community (~1,000 people in #rust, ~250
people in #rust-internals, ~1,300 unique commiters to rust.git)

Talk plan
"Why Rust" Demonstration
"Ownership is easy" (... or is it?)
Sharing Stuff
Sharing capabilities (Language stuff)
Sharing work (Parallelism stuff)
Sharing code (Open source distribution stuff)

Demo: sequential web page fetch
fn sequential_web_fetch() {
use hyper::{self, Client};
use std::io::Read; // pulls in `chars` method
let sites = &["http://www.eff.org/", "http://rust-lang.org/",
"http://imgur.com", "http://mozilla.org"];
for &site in sites { // step through the array...
let client = Client::new();
let res = client.get(site).send().unwrap();
assert_eq!(res.status, hyper::Ok);
let char_count = res.chars().count();
println!("site: {} chars: {}", site, char_count);
}
}
(lets get rid of the Rust-specific pattern binding in for; this is not a
tutorial)

Demo: sequential web page fetch
fn sequential_web_fetch() {
for site_ref in sites { // step through the array...
let site = *site_ref; // (separated for expository purposes)
{ // (and a separate block, again for expository purposes)
}
}
}

Demo: concurrent web page fetch
fn concurrent_web_fetch() -> Vec<::std::thread::JoinHandle<()>> {
let mut handles = Vec::new();
for site_ref in sites {
let site = *site_ref;
let handle = ::std::thread::spawn(move || {
// block code put in closure: ~~~~~~~
});
handles.push(handle);
}
return handles;
}

Print outs
Sequential version:
site: http://www.eff.org/ chars: 42425
site: http://rust-lang.org/ chars: 16748
site: http://imgur.com chars: 152384
site: http://mozilla.org chars: 63349
(on every run, when internet, and sites, available)
Concurrent version:
site: http://imgur.com chars: 152384
site: http://rust-lang.org/ chars: 16748
site: http://mozilla.org chars: 63349
site: http://www.eff.org/ chars: 42425
(on at least one run)

"what is this 'soundness' of which
you speak?"

Demo: soundness I
fn sequential_web_fetch_2() {
// ~~~~~ `sites`, an array (slice) of strings, is stack-local
// ~~~~~~~~ `site_ref` is a *reference to* elem of array.
let res = client.get(*site_ref).send().unwrap();
// moved deref here ~~~~~~~~~
println!("site: {} chars: {}", site_ref, char_count);
}
}

Demo: soundness II
fn concurrent_web_fetch_2() -> Vec<::std::thread::JoinHandle<()>> {
// ~~~~~ `sites`, an array (slice) of strings, is stack-local
// ~~~~~~~~ `site_ref` still a *reference* into an array
let res = client.get(*site_ref).send().unwrap();
// moved deref here ~~~~~~~~~
println!("site: {} chars: {}", site_ref, char_count);
// Q: will `sites` array still be around when above runs?
});
}
return handles;
}

some (white) lies:
"Rust is just about
ownership"

"Ownership is intuitive"
Let's buy a car
let money: Money = bank.withdraw_cash();
let my_new_car: Car = dealership.buy_car(money);
let second_car = dealership.buy_car(money); // <-- cannot reuse
money transferred into dealership, and car transferred to us.

Let's buy a car
// let second_car = dealership.buy_car(money); // <-- cannot reuse
my_new_car.drive_to(home);
garage.park(my_new_car);
my_new_car.drive_to(...) // now doesn't work
(can't drive car without access to it, e.g. taking it out of the garage)

Let's buy a car
// let second_car = dealership.buy_car(money); // <-- cannot reuse
garage.park(my_new_car);
// my_new_car.drive_to(...) // now doesn't work
(can't drive car without access to it, e.g. taking it out of the garage)
let my_car = garage.unpark();
my_car.drive_to(work);
...reflection time...

Correction: Ownership is intuitive,
except for programmers ...
(copying data like integers, and characters, and .mp3's, is "free")
... and anyone else who names things

Über Sinn und Bedeutung
("On sense and reference" -- Gottlob Frege, 1892)
If ownership were all we had, car-purchase slide seems nonsensical
Does this transfer homeinto the car?
Do I lose access to my home, just because I drive to it?
We must distinguish an object itself from ways to name that object
Above, homecannot be (an owned) Home
homemust instead be some kind of reference to a Home

So we will need references
We can solve any problem by introducing an extra
level of indirection
-- David J. Wheeler

a truth: Ownership is important

Ownership is important
Ownership enables: which removes:
RAII-style destructors a source of memory leaks (or fd leaks, etc)
no dangling pointers many resource management bugs
no data races many multithreading heisenbugs
Do I need to take ownership here, accepting the
associated resource management responsibility?
Would temporary access suffice?
Good developers ask this already!
Rust forces function signatures to encode the answers
(and they are checked by the compiler)

Sharing Data:
Ownership and
References

Rust types
Move Copy Copy if T:Copy
Vec<T>, String, ... i32, char, ... [T; n], (T1,T2,T3), ...
struct Car { color: Color, engine: Engine }
fn demo_ownership() {
let mut used_car: Car = Car { color: Color::Red,
engine: Engine::BrokenV8 };
let apartments = ApartmentBuilding::new();
references to data (&mut T, &T):
let my_home: &Home; // <-- an "immutable" borrow
let christine: &mut Car; // <-- a "mutable" borrow
my_home = &apartments[6]; // (read `mut` as "exclusive")
let neighbors_home = &apartments[5];
christine = &mut used_car;
christine.engine = Engine::VintageV8;
}

Why multiple &-reference types?
Distinguish exclusive access from shared access
Enables safe, parallel API's

(reminder: metaphors
never work 100%)

let christine = Car::new();
This is "Christine"
pristine unborrowed car
(apologies to Stephen King)

let read_only_borrow = &christine;
single inspector (immutable borrow)
(apologies to Randall Munroe)

read_only_borrows[2] = &christine;
many inspectors (immutable borrows)

When inspectors are finished, we are left again with:
pristine unborrowed car

let mutable_borrow = &mut christine; // like taking keys ...
give_arnie(mutable_borrow); // ... and giving them to someone
driven car (mutably borrowed)

Can't mix the two in safe code!
Otherwise: (data) races!

let mutable_borrow = &mut christine;
// ⇒ CHAOS!
mixing mutable and immutable is illegal

Ownership T
Exclusive access &mut T ("mutable")
Shared access &T ("read-only")

&mut: can I borrow the car?
fn borrow_the_car_1() {
let mut christine = Car::new();
{
let car_keys = &mut christine;
let arnie = invite_friend_over();
arnie.lend(car_keys);
} // end of scope for `arnie` and `car_keys`
christine.drive_to(work); // I still own the car!
}
But when her keys are elsewhere, I cannot drive christine!
fn borrow_the_car_2() {
let mut christine = Car::new();
{
let car_keys = &mut christine;
let arnie = invite_friend_over();
arnie.lend(car_keys);
christine.drive_to(work); // <-- compile error
} // end of scope for `arnie` and `car_keys`
}

Extending the metaphor
Possessing the keys, Arnie could take the car for a new paint job.
fn lend_1(arnie: &Arnie, k: &mut Car) { k.color = arnie.fav_color; }
Or lend keys to someone else (reborrowing) before paint job
fn lend_2(arnie: &Arnie, k: &mut Car) {
arnie.partner.lend(k); k.color = arnie.fav_color;
}
Owner loses capabilities attached to &mut-borrows only temporarily (*)
(*): "Car keys" return guaranteed by Rust; sadly, not by physical world

End of metaphor
(on to models)

Stack allocation
let b = B::new();
stack allocation

let b = B::new();
let r1: &B = &b;
let r2: &B = &b;
stack allocation and immutable borrows
(bhas lost write capability)

let mut b = B::new();
let w: &mut B = &mut b;
stack allocation and mutable borrows
(bhas temporarily lost both read and write capabilities)

Heap allocation: Box
let a = Box::new(B::new());
pristine boxed B
a(as owner) has both read and write capabilities

Immutably borrowing a box
let a = Box::new(B::new());
let r_of_box: &Box = &a; // (not directly a ref of B)
let r1: &B = &*a;
let r2: &B = &a; // <-- coercion!
immutable borrows of heap-allocated B
aretains read capabilities (has temporarily lost write)

Mutably borrowing a box
let mut a = Box::new(B::new());
let w: &mut B = &mut a; // (again, coercion happening here)
mutable borrow of heap-allocated B
ahas temporarily lost both read and write capabilities

Heap allocation: Vec
let mut a = Vec::new();
for i in 0..n { a.push(B::new()); }

Vec Reallocation
...
a.push(B::new());
before after

Slices: borrowing parts of an array

Basic Vec
pristine unborrowed vec
(ahas read and write capabilities)

Immutable borrowed slices
let r1 = &a[0..3];
let r2 = &a[7..n-4];
mutiple borrowed slices vec
(ahas only read capability now; shares it with r1and r2)

Safe overlap between &[..]
let r1 = &a[0..7];
let r2 = &a[3..n-4];
overlapping slices

Basic Vecagain
pristine unborrowed vec
(ahas read and write capabilities)

Mutable slice of whole vec
let w = &mut a[0..n];
mutable slice of vec
(ahas no capabilities; wnow has read and write capability)

Mutable disjoint slices
let (w1,w2) = a.split_at_mut(n-4);
disjoint mutable borrows
(w1and w2share read and write capabilities for disjoint portions)

Shared Ownership
let rc1 = Rc::new(B::new());
let rc2 = rc1.clone(); // increments ref-count on heap-alloc'd value
shared ownership via ref counting
(rc1and rc2each have read access; but neither can statically assume
exclusive (mut) access, nor can they provide &mutborrows without
assistance.)

RefCell<T>: Dynamic Exclusivity
let b = Box::new(RefCell::new(B::new()));
let r1: &RefCell = &b;
box of refcell

RefCell<T>: Dynamic Exclusivity
let b = Box::new(RefCell::new(B::new()));
let w = r2.borrow_mut(); // if successful, `w` acts like `&mut B`
fallible mutable borrow

// below panics if `w` still in scope
let w2 = b.borrow_mut();

Previous generalizes to
shared ownership

Rc<RefCell<T>>
let rc1 = Rc::new(RefCell::new(B::new()));
let rc2 = rc1.clone(); // increments ref-count on heap-alloc'd value
shared ownership of refcell

Rc<RefCell<T>>
let rc2 = rc1.clone();
let r1: &RefCell = &rc1;
let r2: &RefCell = &rc2; // (or even just `r1`)
borrows of refcell can alias

Rc<RefCell<T>>
let rc2 = rc1.clone();
let w = rc2.borrow_mut();
there can be only one!

What static guarantees does
Rc<RefCell<T>>have?
Not much!
If you want to port an existing imperative algorithm with all sorts of
sharing, you could try using Rc<RefCell<T>>.
You then might spend much less time wrestling with Rust's type
(+borrow) checker.
The point: Rc<RefCell<T>>is nearly an anti-pattern. It limits static
reasoning. You should avoid it if you can.

Other kinds of shared ownership
TypedArena<T>
Cow<T>
Rc<T>vs Arc<T>

Sharing Work:
Parallelism /
Concurrency

Threading APIs (plural!)
std::thread
dispatch: OS X-specific "Grand Central Dispatch"
crossbeam: Lock-Free Abstractions, Scoped "Must-be" Concurrency
rayon: Scoped Fork-join "Maybe" Parallelism (inspired by Cilk)
(Only the first comes with Rust out of the box)

std::thread
fn concurrent_web_fetch() -> Vec<::std::thread::JoinHandle<()>> {
// block code put in closure: ~~~~~~~
});
}
return handles;
}

dispatch
fn concurrent_gcd_fetch() -> Vec<::dispatch::Queue> {
use dispatch::{Queue, QueueAttribute};
let mut queues = Vec::new();
let q = Queue::create("qcon2016", QueueAttribute::Serial);
q.async(move || {
});
queues.push(q);
}
return queues;
}

crossbeam
lock-free data structures
scoped threading abstraction
upholds Rust's safety (data-race freedom)
guarantees

crossbeamMPSC benchmark
mean ns/msg (2 producers, 1 consumer; msg count 10e6; 1G heap)
Rust
channel
crossbeam
MSQ
crossbeam
SegQueue
Scala
MSQ
Java
ConcurrentLinkedQue
108ns 98ns
53ns
461ns
192ns

crossbeamMPMC benchmark
mean ns/msg (2 producers, 2 consumers; msg count 10e6; 1G heap)
Rust
channel
(N/A)
crossbeam
MSQ
crossbeam
SegQueue
Scala
MSQ
Java
ConcurrentLinkedQue
102ns
58ns
239ns
204ns
See "Lock-freedom without garbage collection"
https://aturon.github.io/blog/2015/08/27/epoch/

scoped threading?
std::theaddoes not allow sharing stack-local data
fn std_thread_fail() {
let array: [u32; 3] = [1, 2, 3];
for i in &array {
::std::thread::spawn(|| {
println!("element: {}", i);
});
}
}
error: `array` does not live long enough

crossbeamscoped threading
fn crossbeam_demo() {
let array = [1, 2, 3];
::crossbeam::scope(|scope| {
for i in &array {
scope.spawn(move || {
println!("element: {}", i);
});
}
});
}
::crossbeam::scopeenforces parent thread joins on all spawned
children before returning
ensures that it is sound for children to access local references
passed into them.

crossbeam scope: "must-
be concurrency"
Each scope.spawn(..)invocation fires up a fresh
thread
(Literally just a wrapper around std::thread)

rayondemo 1: map reduce
Sequential
fn demo_map_reduce_seq(stores: &[Store], list: Groceries) -> u32 {
let total_price = stores.iter()
.map(|store| store.compute_price(&list))
.sum();
return total_price;
}
Parallel (potentially)
fn demo_map_reduce_par(stores: &[Store], list: Groceries) -> u32 {
let total_price = stores.par_iter()
.map(|store| store.compute_price(&list))
.sum();
return total_price;
}

Rayon's Rule
the decision of whether or not to use parallel threads
is made dynamically, based on whether idle cores
are available
i.e., solely for offloading work, not for when concurrent operation is
necessary for correctness
(uses work-stealing under the hood to distribute work among a fixed
set of threads)

rayondemo 2: quicksort
fn quick_sort<T:PartialOrd+Send>(v: &mut [T]) {
if v.len() > 1 {
let mid = partition(v);
let (lo, hi) = v.split_at_mut(mid);
rayon::join(|| quick_sort(lo),
|| quick_sort(hi));
}
}
fn partition<T:PartialOrd+Send>(v: &mut [T]) -> usize {
// see https://en.wikipedia.org/wiki/
// Quicksort#Lomuto_partition_scheme
...
}

rayondemo 3: buggy quicksort
if v.len() > 1 {
|| quick_sort(hi));
}
}
if v.len() > 1 {
|| quick_sort(lo));
// ~~ data race!
}
}
(See blog post "Rayon: Data Parallelism in Rust" bit.ly/1IZcku4)

Big Idea
3rd parties identify (and provide) new abstractions for
concurrency and parallelism unanticipated in std lib.

Soundness and 3rd
Party Concurrency

The Secret Sauce
Send
Sync
lifetime bounds

Send and Sync
T: Sendmeans an instance of Tcan be transferred between threads
(i.e. move or copied as appropriate)
T: Syncmeans two threads can safely share a reference to an
instance of T

Examples
T: Send: Tcan be transferred between threads
T: Sync: two threads can share refs to a T
Stringis Send
Vec<T>is Send(if Tis Send)
(double-check: why not require T: Syncfor Vec<T>: Send?)
Rc<T>is not Send(for any T)
but Arc<T>is Send(if Tis Sendand Sync)
(to ponder: why require T:Sendfor Arc<T>?)
&Tis Sendif T: Sync
&mut Tis Sendif T: Send

Send and Sync are only
half the story
other half is lifetime bounds; come
see me if curious

Sharing Code
std::threadis provided with std lib
But dispatch, crossbeam, and rayonare 3rd party
(not to mention hyperand a host of other crates used in this talk's
construction)
What is Rust's code distribution story?

Rust: Systems Programming for Everyone

Rust: Systems Programming for Everyone

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