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LeftRight: a concurrency technique for wait-free read access

LeftRight.jl is a Julia package implementing the left-right technique (Ramalhete and Correia, 2015) to provide an efficient mechanism for sharing mutable objects where the majority of accesses is read-only.

The left-right technique works by maintaining two equivalent copies of objects. The high-level idea is that the writer mutates one of the object while the reader is accessing another object. The writer repeats the identical operations to maintain the equivalence of two objects. For more explanations, see Ramalhete's CppCon 2015 talk.

Examples

julia> using LeftRight

julia> guard = LeftRight.Guard() do
           Dict{Symbol,Int}()  # create a guarded object
       end;

julia> guarding(guard) do dict  # mutate the guarded object `dict`
           dict[:a] = 111
       end;

julia> guarding_read(guard) do dict  # read from the guarded object `dict`
           dict[:a]
       end
111

Limitation

The function f! used as in guarding(f!, guard) is executed twice. Its side-effects on the argument must be "repeatable" in the sense that two equivalent objects must stay equivalent after the mutation. Schematically,

# Precondition:
@assert o1 !== o2  # this also applies to any internal mutable objects
@assert o1 ==′ o2  # e.g., ==′ = isequal

f!(o1)
f!(o2)

# Postcondition:
@assert o1 ==′ o2

The same caveat applies to the factory function passed to LeftRight.Guard constructor; i.e., it has to create objects that are equivalent.

Notes

The left-right technique itself is wait-free population oblivious for read accesses (Ramalhete and Correia, 2015). Like many nonblocking algorithm, the wait-free property does not apply directly to LeftRight.jl because Julia does not have wait-free garbage collector. However, LeftRight.jl is still useful for low-latency and high-throughput read operations. This is especially relevant when the shared object has to maintain complex invariance (i.e., there is no out-of-the-box concurrent data structure implementation). Furthermore, it is safe to read the shared objects using LeftRight.jl inside of a finalizer because the read is wait-free.

For write accesses, LeftRight.jl is starvation-free (Ramalhete and Correia, 2015). In particular, if the critical section of the read accesses have no yield points, the writer only needs to wait for up to 2 * nthreads() tasks. In LeftRight.jl implementation, the writer waits for the readers in the Julia scheduler (after spinning for a while) so that other tasks can be scheduled.

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