auto merge of #11501 : alexcrichton/rust/dox, r=brson

The official documentation sorely needs an explanation of the rust runtime and what it is exactly, and I want this guide to provide that information.

I'm unsure of whether I've been too light on some topics while too heavy on others. I also feel like a few things are still missing. As always, feedback is appreciated, especially about things you'd like to see written about!

configure

@@ -795,6 +795,7 @@ do
     make_dir $h/test/codegen
     make_dir $h/test/doc-tutorial
     make_dir $h/test/doc-guide-ffi
+    make_dir $h/test/doc-guide-runtime
     make_dir $h/test/doc-guide-macros
     make_dir $h/test/doc-guide-lifetimes
     make_dir $h/test/doc-guide-pointers

doc/guide-runtime.md

@@ -0,0 +1,263 @@
+% 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`][schedpool] 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 {
+    do 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

doc/index.md

@@ -20,6 +20,7 @@
 [Packaging](guide-rustpkg.html)  
 [Testing](guide-testing.html)  
 [Conditions](guide-conditions.html)
+[Rust's Runtime](guide-runtime.html)
 
 # Libraries
 

doc/tutorial.md

@@ -3282,6 +3282,7 @@ guides on individual topics.
 * [Packaging up Rust code][rustpkg]
 * [Documenting Rust code][rustdoc]
 * [Testing Rust code][testing]
+* [The Rust Runtime][runtime]
 
 There is further documentation on the [wiki], however those tend to be even more out of date as this document.
 
@@ -3294,6 +3295,7 @@ There is further documentation on the [wiki], however those tend to be even more
 [conditions]: guide-conditions.html
 [rustpkg]: guide-rustpkg.html
 [testing]: guide-testing.html
+[runtime]: guide-runtime.html
 [rustdoc]: rustdoc.html
 [wiki]: https://github.com/mozilla/rust/wiki/Docs
 

mk/docs.mk

@@ -224,6 +224,12 @@ doc/guide-pointers.html: $(S)doc/guide-pointers.md $(HTML_DEPS)
 	$(Q)$(CFG_NODE) $(S)doc/prep.js --highlight $< | \
 	$(CFG_PANDOC) $(HTML_OPTS) --output=$@
 
+DOCS += doc/guide-runtime.html
+doc/guide-runtime.html: $(S)doc/guide-runtime.md $(HTML_DEPS)
+	@$(call E, pandoc: $@)
+	$(Q)$(CFG_NODE) $(S)doc/prep.js --highlight $< | \
+	$(CFG_PANDOC) $(HTML_OPTS) --output=$@
+
   ifeq ($(CFG_PDFLATEX),)
     $(info cfg: no pdflatex found, omitting doc/rust.pdf)
   else

mk/tests.mk

@@ -22,7 +22,7 @@ TEST_CRATES = $(TEST_TARGET_CRATES) $(TEST_HOST_CRATES)
 # Markdown files under doc/ that should have their code extracted and run
 DOC_TEST_NAMES = tutorial guide-ffi guide-macros guide-lifetimes \
                  guide-tasks guide-conditions guide-container guide-pointers \
-                 complement-cheatsheet \
+                 complement-cheatsheet guide-runtime \
                  rust
 
 ######################################################################