+++ /dev/null
-% 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
-* 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. It is 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 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.
-
-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". The current
-default runtime is to use libnative.
-
-~~~{.rust}
-fn main() {}
-~~~
-
-### Force booting with libgreen
-
-In this example, the `main` function will be booted with I/O support powered by
-libuv. This is done by linking to the `rustuv` crate and specifying the
-`rustuv::event_loop` function as the event loop factory.
-
-To create a pool of green tasks which have no I/O support, you may shed the
-`rustuv` dependency and use the `green::basic::event_loop` function instead of
-`rustuv::event_loop`. All tasks will have no I/O support, but they will still be
-able to deschedule/reschedule (use channels, locks, etc).
-
-~~~{.ignore}
-extern crate green;
-extern crate rustuv;
-
-#[start]
-fn start(argc: int, argv: *const *const u8) -> int {
- green::start(argc, argv, rustuv::event_loop, main)
-}
-
-fn main() {}
-~~~
-
-### Force booting with libnative
-
-This program's `main` function will always be booted with libnative, running
-inside of an OS thread.
-
-~~~{.rust}
-extern crate native;
-
-#[start]
-fn start(argc: int, argv: *const *const u8) -> int {
- native::start(argc, argv, main)
-}
-
-fn main() {}
-~~~
-
-# 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
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