Trc

Trc is a performant biased reference-counted smart pointer for Rust. It is a heap-allocated smart pointer for sharing data across threads is a thread-safe manner without putting locks on the data. Trc<T> stands for: Thread Reference Counted. Trc<T> provides a shared ownership of the data similar to Arc<T> and Rc<T>. It implements a custom version of biased reference counting, which is based on the observation that most objects are only used by one thread. This means that two reference counts can be created: one for thread-local use, and one atomic one for sharing between threads. This implementation of biased reference counting sets the atomic reference count to the number of threads using the data.

A cycle between Trc pointers cannot be deallocated as the reference counts will never reach zero. The solution is a Weak<T>. A Weak<T> is a non-owning reference to the data held by a Trc<T>. They break reference cycles by adding a layer of indirection and act as an observer. They cannot even access the data directly, and must be converted back into Trc<T>. Weak<T> does not keep the value alive (whcih can be dropped), and only keeps the backing allocation alive.

To soundly implement thread safety Trc<T> does not itself implement [Send] or [Sync]. However, SharedTrc<T> does, and it is the only way to safely send a Trc<T> across threads. See SharedTrc for it's API, which is similar to that of Weak.

Trc will automatically compile to use either locks or atomics, depending on the system. By default, Trc uses std. However, Trc can be compiled without std. Compilation with locks or atomics can be forced with a feature flag.

Examples

Example of Trc<T> in a single thread:

use trc::Trc;

let mut trc = Trc::new(100);
assert_eq!(*trc, 100);
*trc = 200;
assert_eq!(*trc, 200);

Example of Trc<T> with multiple threads:

use std::thread;
use trc::Trc;
use trc::SharedTrc;

let trc = Trc::new(100);
let shared = SharedTrc::from_trc(&thread_trc_main);
let handle = thread::spawn(move || {
    let mut trc = SharedTrc::to_trc(shared);
    *trc2 = 200;
});

handle.join().unwrap();
assert_eq!(*trc, 200);

Example of Weak<T> in a single thread:

use trc::Trc;
use trc::Weak;

let trc = Trc::new(100);
let weak = Weak::from_trc(&trc);
let mut new_trc = Weak::to_trc(&weak).unwrap();
println!("Deref test! {}", *new_trc);
println!("DerefMut test");
*new_trc = 200;
println!("Deref test! {}", *new_trc);

Example of Weak<T> with multiple threads:

use std::thread;
use trc::Trc;
use trc::Weak;

let trc = Trc::new(100);
let weak = Weak::from_trc(&trc);

let handle = thread::spawn(move || {
    let mut trc = Weak::to_trc(&weak).unwrap();
    println!("{:?}", *trc);
    *trc = 200;
});
handle.join().unwrap();
println!("{}", *trc);
assert_eq!(*trc, 200);

Benchmarks

Benchmarks via Criterion. As can be seen, Trc's performance realy shines when there are many Clones. The reason Trc does not do as well for fewer operations is because it needs to allocate n+1 blocks of memory for n threads, and so for 1 thread, there are 2 allocations. However, after allocations, Trc performs very well - 3.77x Arc for Clones.

Clone

Type	Mean time
Trc	40.927ms
Arc	40.204ns
Rc	14.939ns

Multiple Clone (100 times)

Type	Mean time
Trc	362.850ns
Arc	1366.500ns
Rc	324.830ns

Deref

Type	Mean time
Trc	27.284ns
Arc	26.012ns
Rc	12.554ns

Multiple Deref (100 times)

Type	Mean time
Trc	58.464ns
Arc	57.631ns
Rc	46.272ns