3.3. Rust에 다른 언어 포함하기 (Rust Inside Other Languages) - 0%
For our third project, we’re going to choose something that shows off one of
Rust’s greatest strengths: a lack of a substantial runtime.
As organizations grow, they increasingly rely on a multitude of programming
languages. Different programming languages have different strengths and
weaknesses, and a polyglot stack lets you use a particular language where
its strengths make sense, and use a different language where it’s weak.
A very common area where many programming languages are weak is in runtime
performance of programs. Often, using a language that is slower, but offers
greater programmer productivity is a worthwhile trade-off. To help mitigate
this, they provide a way to write some of your system in C, and then call
the C code as though it were written in the higher-level language. This is
called a ‘foreign function interface’, often shortened to ‘FFI’.
Rust has support for FFI in both directions: it can call into C code easily,
but crucially, it can also be called into as easily as C. Combined with
Rust’s lack of a garbage collector and low runtime requirements, this makes
Rust a great candidate to embed inside of other languages when you need
some extra oomph.
There is a whole chapter devoted to FFI and its specifics elsewhere in
the book, but in this chapter, we’ll examine this particular use-case of FFI,
with three examples, in Ruby, Python, and JavaScript.
The problem
There are many different projects we could choose here, but we’re going to
pick an example where Rust has a clear advantage over many other languages:
numeric computing and threading.
Many languages, for the sake of consistency, place numbers on the heap, rather
than on the stack. Especially in languages that focus on object-oriented
programming and use garbage collection, heap allocation is the default. Sometimes
optimizations can stack allocate particular numbers, but rather than relying
on an optimizer to do its job, we may want to ensure that we’re always using
primitive number types rather than some sort of object type.
Second, many languages have a ‘global interpreter lock’, which limits
concurrency in many situations. This is done in the name of safety, which is
a positive effect, but it limits the amount of work that can be done at the
same time, which is a big negative.
To emphasize these two aspects, we’re going to create a little project that
uses these two aspects heavily. Since the focus of the example is the embedding
of Rust into the languages, rather than the problem itself, we’ll just use a
toy example:
Start ten threads. Inside each thread, count from one to five million. After
All ten threads are finished, print out ‘done!’.
I chose five million based on my particular computer. Here’s an example of this
code in Ruby:
threads = []
10.times do
threads << Thread.new do
count = 0
5_000_000.times do
count += 1
end
end
end
threads.each {|t| t.join }
puts "done!"
Try running this example, and choose a number that runs for a few seconds.
Depending on your computer’s hardware, you may have to increase or decrease the
number.
On my system, running this program takes 2.156 seconds. And, if I use some
sort of process monitoring tool, like top, I can see that it only uses one
core on my machine. That’s the GIL kicking in.
While it’s true that this is a synthetic program, one can imagine many problems
that are similar to this in the real world. For our purposes, spinning up some
busy threads represents some sort of parallel, expensive computation.
A Rust library
Let’s re-write this problem in Rust. First, let’s make a new project with
Cargo:
$ cargo new embed
$ cd embed
This program is fairly easy to write in Rust:
use std::thread;
fn process() {
let handles: Vec<_> = (0..10).map(|_| {
thread::spawn(|| {
let mut _x = 0;
for _ in (0..5_000_001) {
_x += 1
}
})
}).collect();
for h in handles {
h.join().ok().expect("Could not join a thread!");
}
}
Some of this should look familiar from previous examples. We spin up ten
threads, collecting them into a handles vector. Inside of each thread, we
loop five million times, and add one to _x each time. Why the underscore?
Well, if we remove it and compile:
$ cargo build
Compiling embed v0.1.0 (file:///home/steve/src/embed)
src/lib.rs:3:1: 16:2 warning: function is never used: `process`, #[warn(dead_code)] on by default
src/lib.rs:3 fn process() {
src/lib.rs:4 let handles: Vec<_> = (0..10).map(|_| {
src/lib.rs:5 thread::spawn(|| {
src/lib.rs:6 let mut x = 0;
src/lib.rs:7 for _ in (0..5_000_001) {
src/lib.rs:8 x += 1
...
src/lib.rs:6:17: 6:22 warning: variable `x` is assigned to, but never used, #[warn(unused_variables)] on by default
src/lib.rs:6 let mut x = 0;
^~~~~
That first warning is because we are building a library. If we had a test
for this function, the warning would go away. But for now, it’s never
called.
The second is related to x versus _x. Because we never actually do
anything with x, we get a warning about it. In our case, that’s perfectly
okay, as we’re just trying to waste CPU cycles. Prefixing x with the
underscore removes the warning.
Finally, we join on each thread.
Right now, however, this is a Rust library, and it doesn’t expose anything
that’s callable from C. If we tried to hook this up to another language right
now, it wouldn’t work. We only need to make two small changes to fix this,
though. The first is modify the beginning of our code:
#[no_mangle]
pub extern fn process() {
We have to add a new attribute, no_mangle. When you create a Rust library, it
changes the name of the function in the compiled output. The reasons for this
are outside the scope of this tutorial, but in order for other languages to
know how to call the function, we need to not do that. This attribute turns
that behavior off.
The other change is the pub extern. The pub means that this function should
be callable from outside of this module, and the extern says that it should
be able to be called from C. That’s it! Not a whole lot of change.
The second thing we need to do is to change a setting in our Cargo.toml. Add
this at the bottom:
[lib]
name = "embed"
crate-type = ["dylib"]
This tells Rust that we want to compile our library into a standard dynamic
library. By default, Rust compiles into an ‘rlib’, a Rust-specific format.
Let’s build the project now:
$ cargo build --release
Compiling embed v0.1.0 (file:///home/steve/src/embed)
We’ve chosen cargo build --release, which builds with optimizations on. We
want this to be as fast as possible! You can find the output of the library in
target/release:
$ ls target/release/
build deps examples libembed.so native
That libembed.so is our ‘shared object’ library. We can use this file
just like any shared object library written in C! As an aside, this may be
embed.dll or libembed.dylib, depending on the platform.
Now that we’ve got our Rust library built, let’s use it from our Ruby.
Ruby
Open up a embed.rb file inside of our project, and do this:
require 'ffi'
module Hello
extend FFI::Library
ffi_lib 'target/release/libembed.so'
attach_function :process, [], :void
end
Hello.process
puts "done!”
Before we can run this, we need to install the ffi gem:
$ gem install ffi # this may need sudo
Fetching: ffi-1.9.8.gem (100%)
Building native extensions. This could take a while...
Successfully installed ffi-1.9.8
Parsing documentation for ffi-1.9.8
Installing ri documentation for ffi-1.9.8
Done installing documentation for ffi after 0 seconds
1 gem installed
And finally, we can try running it:
$ ruby embed.rb
done!
$
Whoah, that was fast! On my system, this took 0.086 seconds, rather than
the two seconds the pure Ruby version took. Let’s break down this Ruby
code:
require 'ffi'
We first need to require the ffi gem. This lets us interface with our
Rust library like a C library.
module Hello
extend FFI::Library
ffi_lib 'target/release/libembed.so'
The ffi gem’s authors recommend using a module to scope the functions
we’ll import from the shared library. Inside, we extend the necessary
FFI::Library module, and then call ffi_lib to load up our shared
object library. We just pass it the path that our library is stored,
which as we saw before, is target/release/libembed.so.
attach_function :process, [], :void
The attach_function method is provided by the FFI gem. It’s what
connects our process() function in Rust to a Ruby function of the
same name. Since process() takes no arguments, the second parameter
is an empty array, and since it returns nothing, we pass :void as
the final argument.
Hello.process
This is the actual call into Rust. The combination of our module
and the call to attach_function sets this all up. It looks like
a Ruby function, but is actually Rust!
puts "done!"
Finally, as per our project’s requirements, we print out done!.
That’s it! As we’ve seen, bridging between the two languages is really easy,
and buys us a lot of performance.
Next, let’s try Python!
Python
Create an embed.py file in this directory, and put this in it:
from ctypes import cdll
lib = cdll.LoadLibrary("target/release/libembed.so")
lib.process()
print("done!")
Even easier! We use cdll from the ctypes module. A quick call
to LoadLibrary later, and we can call process().
On my system, this takes 0.017 seconds. Speedy!
Node.js
Node isn’t a language, but it’s currently the dominant implementation of
server-side JavaScript.
In order to do FFI with Node, we first need to install the library:
$ npm install ffi
After that installs, we can use it:
var ffi = require('ffi');
var lib = ffi.Library('target/release/libembed', {
'process': [ 'void', [] ]
});
lib.process();
console.log("done!");
It looks more like the Ruby example than the Python example. We use
the ffi module to get access to ffi.Library(), which loads up
our shared object. We need to annotate the return type and argument
types of the function, which are 'void' for return, and an empty
array to signify no arguments. From there, we just call it and
print the result.
On my system, this takes a quick 0.092 seconds.
Conclusion
As you can see, the basics of doing this are very easy. Of course,
there's a lot more that we could do here. Check out the FFI
chapter for more details.