thingbuf

"I'm at the buffer pool. I'm at the MPSC channel. I'm at the combination MPSC channel and buffer pool."

What Is It?

thingbuf is a lock-free array-based concurrent ring buffer that allows access to slots in the buffer by reference. It's also asynchronous and blocking bounded MPSC channels implemented using the ring buffer.

When Should I Use It?

• If you want a high-throughput bounded MPSC channel that allocates only on channel creation. Some MPSC channels have good throughput. Some other MPSC channels won't allocate memory per-waiter. thingbuf::mpsc has both. thingbuf::mpsc is a competitive choice for a general-purpose MPSC channel in most use cases.

  Both asynchronous and blocking MPSC channels are available, so thingbuf can be used in place of asynchronous channels like futures::channel::mpsc and blocking channels like std::sync::mpsc::sync_channel.

• If you can't allocate or you need to build with #![no_std] because you're working on embedded systems or other bare-metal software. Thingbuf provides a statically-allocated MPSC channel and a statically-allocated lock-free queue. These can be placed in a static initializer and used without requiring any runtime allocations.

• You want to use the same MPSC channel with and without std . Thingbuf's asynchronous MPSC channel provides an identical API and feature set regardless of whether or not the "std" feature flag is enabled. If you're writing a library that needs to conditionally support #![no_std], and you need an asynchronous MPSC channel, it might be easier to use thingbuf::mpsc in both cases, rather than switching between separate std and #![no_std] channel implementations.

When Shouldn't I Use It?

It's equally important to discuss when thingbuf should not be used. Here are some cases where you might be better off considering other options:

• You need a really, really, absurdly high bound and you're not going to be near it most of the time. If you want to set a very, very high bound on a bounded MPSC channel, and the channel will typically never be anywhere near that full, thingbuf::mpsc might not be the best choice.

  Thingbuf's channels will allocate an array with length equal to the capacity as soon as they're constructed. This improves performance by avoiding additional allocations, but if you need to set very high bounds, you might prefer a channel implementation that only allocates memory for messages as it's needed (such as tokio::sync::mpsc).

• You need a blocking channel with a select operation. I'm probably not going to implement it. I may accept a PR if you raise it.

  If you need a synchronous channel with this kind of functionality, crossbeam-channel is probably a good choice.

• You want an unbounded channel. I'm not going to write an unbounded channel. Unbounded channels are evil.

Terminology

This crate's API and documentation makes a distinction between the terms "queue" and "channel". The term queue will refer to the queue abstract data type in general — any first-in, first-out data structure is a queue.

The term channel will refer to a subtype of concurrent queue that also functions as a synchronization primitive. A channel is a queue which can be shared between multiple threads or asynchronous tasks, and which allows those threads or tasks to wait for elements to be added or removed from the queue.

In the Rust standard library, the std::collections::VecDeque type is an example of a queue that is not a channel: it is a first-in, first-out data structure, but it cannot be concurrently enqueued to and dequeued from by multiple threads or tasks. In comparison, the types in the std::sync::mpsc module provide a prototypical example of channels, as they serve as synchronization primitives for cross-thread communication.

Usage

To get started using thingbuf, add the following to your Cargo.toml:

[dependencies]
thingbuf = "0.1"

By default, thingbuf depends on the Rust standard library, in order to implement APIs such as synchronous (blocking) channels. In #![no_std] projects, the std feature flag must be disabled:

[dependencies]
thingbuf = { version = "0.1", default-features = false }

With the std feature disabled, thingbuf will depend only on libcore. This means that APIs that require dynamic memory allocation will not be enabled. Statically allocated channels and queues are available for code without a memory allocator, if the static feature flag is enabled:

[dependencies]
thingbuf = { version = "0.1", default-features = false, features = ["static"] }

However, if a memory allocator is available, #![no_std] code can also enable the alloc feature flag to depend on liballoc:

[dependencies]
thingbuf = { version = "0.1", default-features = false, features = ["alloc"] }

Crate Feature Flags

• std (Enabled by default): Enables features that require the Rust standard library, such as synchronous (blocking) channels. This implicitly enables the "alloc" feature flag.
• alloc: Enables features that require liballoc (but not libstd). This enables thingbuf queues and asynchronous channels where the size of the channel is determined at runtime.
• static (Disabled by default, requires Rust 1.59+): Enables the static (const-generic-based) thingbuf queues and channels. These can be used without dynamic memory allocation when the size of a queue or channel is known at compile-time.

Compiler Support

thingbuf is built against the latest stable release. The minimum supported version is Rust 1.57. The current thingbuf version is not guaranteed to build on Rust versions earlier than the minimum supported version.

Some feature flags may require newer Rust releases. For example, the "static" feature flag requries Rust 1.60+.

FAQs

• Q: Why did you make this?

  A: For tracing, I wanted to be able to send formatted log lines to a dedicated worker thread that writes them to a file. Right now, we do this using crossbeam-channel. However, this has the sad disadvantage that we have to allocate Strings, send them through the channel to the writer, and immediately drop them. It would be nice to do this while reusing those allocations. Thus...StringBuf.

• Q: Is it lock-free?

  A: Extremely.

• Q: Why is there only a bounded variant?

  A: Because unbounded queues are of the Devil.

• Q: Isn't this just a giant memory leak?

  A: If you use it wrong, yes.

• Q: Why is it called that?

  A: Originally, I imagined it as a kind of ring buffer, so (as a pun on "ringbuf"), I called it "stringbuf". Then, I realized you could do this with more than just strings. In fact, it can be generalized to arbitrary...things. So, "thingbuf".