1508b6e953
"How do I start in libX" is a common question that I've seen, so I figured putting the examples in as many places as possible is probably a good idea.
262 lines
12 KiB
Markdown
262 lines
12 KiB
Markdown
% A Guide to the Rust Runtime
|
|
|
|
Rust includes two runtime libraries in the standard distribution, which provide
|
|
a unified interface to primitives such as I/O, but the language itself does not
|
|
require a runtime. The compiler is capable of generating code that works in all
|
|
environments, even kernel environments. Neither does the Rust language need a
|
|
runtime to provide memory safety; the type system itself is sufficient to write
|
|
safe code, verified statically at compile time. The runtime merely uses the
|
|
safety features of the language to build a number of convenient and safe
|
|
high-level abstractions.
|
|
|
|
That being said, code without a runtime is often very limited in what it can do.
|
|
As a result, Rust's standard libraries supply a set of functionality that is
|
|
normally considered the Rust runtime. This guide will discuss Rust's user-space
|
|
runtime, how to use it, and what it can do.
|
|
|
|
# What is the runtime?
|
|
|
|
The Rust runtime can be viewed as a collection of code which enables services
|
|
like I/O, task spawning, TLS, etc. It's essentially an ephemeral collection of
|
|
objects which enable programs to perform common tasks more easily. The actual
|
|
implementation of the runtime itself is mostly a sparse set of opt-in primitives
|
|
that are all self-contained and avoid leaking their abstractions into libraries.
|
|
|
|
The current runtime is the engine behind these features (not a comprehensive
|
|
list):
|
|
|
|
* I/O
|
|
* Task spawning
|
|
* Message passing
|
|
* Task synchronization
|
|
* Task-local storage
|
|
* Logging
|
|
* Local heaps (GC heaps)
|
|
* Task unwinding
|
|
|
|
## What is the runtime accomplishing?
|
|
|
|
The runtime is designed with a few goals in mind:
|
|
|
|
* Rust libraries should work in a number of environments without having to worry
|
|
about the exact details of the environment itself. Two commonly referred to
|
|
environments are the M:N and 1:1 environments. Since the Rust runtime was
|
|
first designed, it has supported M:N threading, and it has since gained 1:1
|
|
support as well.
|
|
|
|
* The runtime should not enforce separate "modes of compilation" in order to
|
|
work in multiple circumstances. Is it an explicit goal that you compile a Rust
|
|
library once and use it forever (in all environments).
|
|
|
|
* The runtime should be fast. There should be no architectural design barrier
|
|
which is preventing programs from running at optimal speeds. It is not a goal
|
|
for the runtime to be written "as fast as can be" at every moment in time. For
|
|
example, no claims will be made that the current implementation of the runtime
|
|
is the fastest it will ever be. This goal is simply to prevent any
|
|
architectural roadblock from hindering performance.
|
|
|
|
* The runtime should be nearly invisible. The design of the runtime should not
|
|
encourage direct interaction with it, and using the runtime should be
|
|
essentially transparent to libraries. This does not mean it should be
|
|
impossible to query the runtime, but rather it should be unconventional.
|
|
|
|
# Architecture of the runtime
|
|
|
|
This section explains the current architecture of the Rust runtime. It has
|
|
evolved over the development of Rust through many iterations, and this is simply
|
|
the documentation of the current iteration.
|
|
|
|
## A local task
|
|
|
|
The core abstraction of the Rust runtime is the task. A task represents a
|
|
"thread" of execution of Rust code, but it does not necessarily correspond to an
|
|
OS thread. Most runtime services are accessed through the local task, allowing
|
|
for runtime policy decisions to be made on a per-task basis.
|
|
|
|
A consequence of this decision is to require all Rust code using the standard
|
|
library to have a local `Task` structure available to them. This `Task` is
|
|
stored in the OS's thread local storage (OS TLS) to allow for efficient access
|
|
to it.
|
|
|
|
It has also been decided that the presence or non-presence of a local `Task` is
|
|
essentially the *only* assumption that the runtime can make. Almost all runtime
|
|
services are routed through this local structure.
|
|
|
|
This requirement of a local task is a core assumption on behalf of *all* code
|
|
using the standard library, hence it is defined in the standard library itself.
|
|
|
|
## I/O
|
|
|
|
When dealing with I/O in general, there are a few flavors by which it can be
|
|
dealt with, and not all flavors are right for all situations. I/O is also a
|
|
tricky topic that is nearly impossible to get consistent across all
|
|
environments. As a result, a Rust task is not guaranteed to have access to I/O,
|
|
and it is not even guaranteed what the implementation of the I/O will be.
|
|
|
|
This conclusion implies that I/O *cannot* be defined in the standard library.
|
|
The standard library does, however, provide the interface to I/O that all Rust
|
|
tasks are able to consume.
|
|
|
|
This interface is implemented differently for various flavors of tasks, and is
|
|
designed with a focus around synchronous I/O calls. This architecture does not
|
|
fundamentally prevent other forms of I/O from being defined, but it is not done
|
|
at this time.
|
|
|
|
The I/O interface that the runtime must provide can be found in the
|
|
[std::rt::rtio](std/rt/rtio/trait.IoFactory.html) module. Note that this
|
|
interface is *unstable*, and likely always will be.
|
|
|
|
## Task Spawning
|
|
|
|
A frequent operation performed by tasks is to spawn a child task to perform some
|
|
work. This is the means by which parallelism is enabled in Rust. This decision
|
|
of how to spawn a task is not a general decision, and is hence a local decision
|
|
to the task (not defined in the standard library).
|
|
|
|
Task spawning is interpreted as "spawning a sibling" and is enabled through the
|
|
high level interface in `std::task`. The child task can be configured
|
|
accordingly, and runtime implementations must respect these options when
|
|
spawning a new task.
|
|
|
|
Another local task operation is dealing with the runnable state of the task
|
|
itself. This frequently comes up when the question is "how do I block a task?"
|
|
or "how do I wake up a task?". These decisions are inherently local to the task
|
|
itself, yet again implying that they are not defined in the standard library.
|
|
|
|
## The `Runtime` trait and the `Task` structure
|
|
|
|
The full complement of runtime features is defined by the [`Runtime`
|
|
trait](std/rt/trait.Runtime.html) and the [`Task`
|
|
struct](std/rt/task/struct.Task.html). A `Task` is constant among all runtime
|
|
implementations, but each runtime implements has its own implementation of the
|
|
`Runtime` trait.
|
|
|
|
The local `Task` stores the runtime value inside of itself, and then ownership
|
|
dances ensue to invoke methods on the runtime.
|
|
|
|
# Implementations of the runtime
|
|
|
|
The Rust distribution provides two implementations of the runtime. These two
|
|
implementations are generally known as 1:1 threading and M:N threading.
|
|
|
|
As with many problems in computer science, there is no right answer in this
|
|
question of which implementation of the runtime to choose. Each implementation
|
|
has its benefits and each has its drawbacks. The descriptions below are meant to
|
|
inform programmers about what the implementation provides and what it doesn't
|
|
provide in order to make an informed decision about which to choose.
|
|
|
|
## 1:1 - using `libnative`
|
|
|
|
The library `libnative` is an implementation of the runtime built upon native OS
|
|
threads plus libc blocking I/O calls. This is called 1:1 threading because each
|
|
user-space thread corresponds to exactly one kernel thread.
|
|
|
|
In this model, each Rust task corresponds to one OS thread, and each I/O object
|
|
essentially corresponds to a file descriptor (or the equivalent of the platform
|
|
you're running on).
|
|
|
|
Some benefits to using libnative are:
|
|
|
|
* Guaranteed interop with FFI bindings. If a C library you are using blocks the
|
|
thread to do I/O (such as a database driver), then this will not interfere
|
|
with other Rust tasks (because only the OS thread will be blocked).
|
|
* Less I/O overhead as opposed to M:N in some cases. Not all M:N I/O is
|
|
guaranteed to be "as fast as can be", and some things (like filesystem APIs)
|
|
are not truly asynchronous on all platforms, meaning that the M:N
|
|
implementation may incur more overhead than a 1:1 implementation.
|
|
|
|
## M:N - using `libgreen`
|
|
|
|
The library `libgreen` implements the runtime with "green threads" on top of the
|
|
asynchronous I/O framework [libuv][libuv]. The M in M:N threading is the number
|
|
of OS threads that a process has, and the N is the number of Rust tasks. In this
|
|
model, N Rust tasks are multiplexed among M OS threads, and context switching is
|
|
implemented in user-space.
|
|
|
|
The primary concern of an M:N runtime is that a Rust task cannot block itself in
|
|
a syscall. If this happens, then the entire OS thread is frozen and unavailable
|
|
for running more Rust tasks, making this a (M-1):N runtime (and you can see how
|
|
this can reach 0/deadlock). By using asynchronous I/O under the hood (all I/O
|
|
still looks synchronous in terms of code), OS threads are never blocked until
|
|
the appropriate time comes.
|
|
|
|
Upon reading `libgreen`, you may notice that there is no I/O implementation
|
|
inside of the library, but rather just the infrastructure for maintaining a set
|
|
of green schedulers which switch among Rust tasks. The actual I/O implementation
|
|
is found in `librustuv` which are the Rust bindings to libuv. This distinction
|
|
is made to allow for other I/O implementations not built on libuv (but none
|
|
exist at this time).
|
|
|
|
Some benefits of using libgreen are:
|
|
|
|
* Fast task spawning. When using M:N threading, spawning a new task can avoid
|
|
executing a syscall entirely, which can lead to more efficient task spawning
|
|
times.
|
|
* Fast task switching. Because context switching is implemented in user-space,
|
|
all task contention operations (mutexes, channels, etc) never execute
|
|
syscalls, leading to much faster implementations and runtimes. An efficient
|
|
context switch also leads to higher throughput servers than 1:1 threading
|
|
because tasks can be switched out much more efficiently.
|
|
|
|
### Pools of Schedulers
|
|
|
|
M:N threading is built upon the concept of a pool of M OS threads (which
|
|
libgreen refers to as schedulers), able to run N Rust tasks. This abstraction is
|
|
encompassed in libgreen's [`SchedPool`](green/struct.SchedPool.html) type. This type allows for
|
|
fine-grained control over the pool of schedulers which will be used to run Rust
|
|
tasks.
|
|
|
|
In addition the `SchedPool` type is the *only* way through which a new M:N task
|
|
can be spawned. Sibling tasks to Rust tasks themselves (created through
|
|
`std::task::spawn`) will be spawned into the same pool of schedulers that the
|
|
original task was home to. New tasks must previously have some form of handle
|
|
into the pool of schedulers in order to spawn a new task.
|
|
|
|
## Which to choose?
|
|
|
|
With two implementations of the runtime available, a choice obviously needs to
|
|
be made to see which will be used. The compiler itself will always by-default
|
|
link to one of these runtimes. At the time of this writing, the default runtime
|
|
is `libgreen` but in the future this will become `libnative`.
|
|
|
|
Having a default decision made in the compiler is done out of necessity and
|
|
convenience. The compiler's decision of runtime to link to is *not* an
|
|
endorsement of one over the other. As always, this decision can be overridden.
|
|
|
|
For example, this program will be linked to "the default runtime"
|
|
|
|
~~~{.rust}
|
|
fn main() {}
|
|
~~~
|
|
|
|
Whereas this program explicitly opts into using a particular runtime
|
|
|
|
~~~{.rust}
|
|
extern mod green;
|
|
|
|
#[start]
|
|
fn start(argc: int, argv: **u8) -> int {
|
|
green::start(argc, argv, main)
|
|
}
|
|
|
|
fn main() {}
|
|
~~~
|
|
|
|
Both libgreen/libnative provide a top-level `start` function which is used to
|
|
boot an initial Rust task in that specified runtime.
|
|
|
|
# Finding the runtime
|
|
|
|
The actual code for the runtime is spread out among a few locations:
|
|
|
|
* [std::rt][stdrt]
|
|
* [libnative][libnative]
|
|
* [libgreen][libgreen]
|
|
* [librustuv][librustuv]
|
|
|
|
[libuv]: https://github.com/joyent/libuv/
|
|
[stdrt]: std/rt/index.html
|
|
[libnative]: native/index.html
|
|
[libgreen]: green/index.html
|
|
[librustuv]: rustuv/index.html
|