Crate biasedrc 

An implementation of biased reference counting for Rust.

This crate requires the standard library due to use of std::thread_local!.

Tradeoffs

Like many low-level optimizations, biased reference counting has tradeoffs.

The most significant advantage is performance. On the biased thread, Brc::clone and Brc::drop are 2-3x faster than the equivalent operations on Arc, nearing the performance of Rc. Except for the final drop operation, clones and drops are the same speed as Arc on non-biased threads.

Unfortunately, there are some performance downsides too: Allocating a Brc is currently 20%-100% slower than allocating an Arc, and dropping the last reference is about 20% slower than with Arc. The costs are paid once per object, so the performance difference is amortized away after just 1-3 biased clone/drop operations.

Other major downsides of using biasedrc include the need to maintain a thread-local queue and in some cases delayed memory reclamation.

Panics & Unwinding

This library aims to unwind only when an equivalent method on Arc would unwind.

I say unwind here rather than panic, because the library needs to deal with deferred destructors panicking during implicit collections. If a a panic happens during an implicit collection, it will abort the program. This is because unwinding from Brc::<u32>::clone or Brc::<u32>::drop would be highly unexpected, and make transitioning from Arc to Brc much more difficult. Unwinding from deferred destructors makes local reasoning about program behavior impossible, which is one of the main reasons Dijkstra criticised goto.

If unwinding panics during implicit collections are truly desired behavior, they can be emulated by creating a newtype wrapper for a Brc which calls collect instead of collect_nounwind.

Prior Art

• trc - Requires explicit choice of either SharedTrc or Trc, avoiding need for runtime checks but preventing use as a drop-in replacement for Arc
• hybrid_rc - Appears to require a similar choice as trc between shared and local references.

Structs

Brc

A thread-safe reference counted object, biased towards a particular thread.

ImpreciseRefCountError

An error returned by Brc::strong_count if the reference count cannot be precisely determined.

Weak

A weak-reference to a Brc.

Functions

collect

Perform thread-local cleanup operations if deemed necessary, potentially executing deferred destructors.

collect_force

Forcibly perform the collect operation, regardless of internal heuristics.

collect_nounwind

Equivalent to collect(), but panics in deferred destructors trigger aborts instead of unwinding.